Hard fork of the Deno stdlib 'expect' module + BDD testing tools (from https://deno.land/std/expect/mod.ts)

expect.ts

// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
// Copyright 2019 Allain Lalonde. All rights reserved. ISC License.

import {
  type AnyConstructor,
  type Matcher,
  matchers,
  type TypeNames,
} from "./matchers.ts";
import { AssertionError, type Async, type Fn, isPromiseLike } from "./utils.ts";

export interface Expected {
  not: Omit<this, "not">;
  resolves: Async<Omit<this, "resolves" | "rejects">>;
  rejects: Async<Omit<this, "resolves" | "rejects">>;
  lastCalledWith<const A extends readonly unknown[]>(...expected: A): void;
  lastReturnedWith<const T>(expected: T): void;
  nthCalledWith<const A extends readonly unknown[]>(
    nth: number,
    ...expected: A
  ): void;
  nthReturnedWith<const T>(nth: number, expected: T): void;
  toBeCalled(): void;
  toBeCalledTimes(expected: number): void;
  toBeCalledWith<const A extends readonly unknown[]>(
    ...expected: A
  ): void;
  toBeCloseTo(candidate: number, tolerance?: number): void;
  toBeDefined(): void;
  toBeFalsy(): void;
  toBeGreaterThan(expected: number): void;
  toBeGreaterThanOrEqual(expected: number): void;
  toBeInstanceOf<T extends AnyConstructor>(expected: T): void;
  toBeLessThan(expected: number): void;
  toBeLessThanOrEqual(expected: number): void;
  toBeNaN(): void;
  toBeNull(): void;
  toBeTruthy(): void;
  toBeUndefined(): void;
  toBe:
    & {
      <const T>(expected: T): void;
      truthy(): void;
      falsy(): void;
      null(): void;
      undefined(): void;
      defined(): void;
      NaN(): void;
      type<T extends TypeNames>(expected: T): void;
      instanceof<T extends AnyConstructor>(expected: T): void;
      instanceOf<T extends AnyConstructor>(expected: T): void;
      closeTo(candidate: number, tolerance?: number): void;
      greaterThan(expected: number): void;
      greaterThanOrEqual(expected: number): void;
      lessThan(expected: number): void;
      lessThanOrEqual(expected: number): void;
      iterable(): void;
      asyncIterable(): void;
    }
    & {
      [K in "a" | "an"]: {
        <const T>(expected: T): void;
        function(): void;
        asyncFunction(): void;
        generatorFunction(): void;
        generator(): void;
        asyncGeneratorFunction(): void;
        asyncGenerator(): void;
        iterable(): void;
        asyncIterable(): void;
        array(): void;
        object(): void;
        string(): void;
        number(): void;
        bigint(): void;
        boolean(): void;
        symbol(): void;
      };
    };
  toBeFunction(): void;
  toBeAsyncFunction(): void;
  toBeGeneratorFunction(): void;
  toBeGeneratorObject(): void;
  toBeAsyncGeneratorFunction(): void;
  toBeAsyncGeneratorObject(): void;
  toBeIterable(): void;
  toBeAsyncIterable(): void;
  toBeType<T extends TypeNames>(expected: T): void;
  toContainEqual<const T>(expected: T): void;
  toContain<const T>(expected: T): void;
  toEqual<const T>(expected: T): void;
  toHaveBeenCalledTimes(expected: number): void;
  toHaveBeenCalledWith<
    const A extends readonly unknown[],
  >(...expected: A): void;
  toHaveBeenCalled(): void;
  toHaveBeenLastCalledWith<
    const A extends readonly unknown[],
  >(...expected: A): void;
  toHaveBeenNthCalledWith(nth: number, ...expected: unknown[]): void;
  toHaveLength(expected: number): void;
  toHaveLastReturnedWith<const T>(expected: T): void;
  toHaveNthReturnedWith<const T>(nth: number, expected: T): void;
  toHaveProperty<const T, const K extends PropertyKey | readonly PropertyKey[]>(
    propName: K,
    value?: T,
  ): void;
  toHaveReturnedTimes(expected: number): void;
  toHaveReturnedWith<const T>(expected: T): void;
  toHaveReturned(): void;
  toMatch(expected: RegExp): void;
  toMatchObject<const T extends Record<PropertyKey, unknown>>(
    expected: T,
  ): void;
  toReturn(): void;
  toReturnTimes(expected: number): void;
  toReturnWith(expected: unknown): void;
  toStrictEqual(candidate: unknown): void;
  toThrow<E extends Error = Error>(
    // deno-lint-ignore no-explicit-any
    expected?: string | RegExp | E | (new (...args: any[]) => E),
  ): void;
}

type MatcherKey = keyof Omit<Expected, "not" | "resolves" | "rejects">;

export function expect<const T>(value: T, customMessage?: string): Expected;
export function expect(value: unknown, customMessage?: string): Expected {
  let isNot = false;
  let isPromised = false;
  const self: Expected = new Proxy<Expected>(
    <Expected> {},
    {
      get(_, name) {
        if (name === "not") {
          isNot = !isNot;
          return self;
        }
        if (name === "resolves") {
          if (!isPromiseLike(value)) {
            throw new AssertionError("expected value must be Promiselike");
          }
          isPromised = true;
          return self;
        }
        if (name === "rejects") {
          if (!isPromiseLike(value)) {
            throw new AssertionError("expected value must be a PromiseLike");
          }
          value = value.then(
            (value) => {
              throw new AssertionError(
                `Promise did not reject. resolved to ${value}`,
              );
            },
            (err) => err,
          );
          isPromised = true;
          return self;
        }
        const matcher: Matcher = matchers[name as MatcherKey];
        if (!matcher) {
          throw new TypeError(
            typeof name === "string"
              ? `matcher not found: ${name}`
              : "matcher not found",
          );
        }
        function applyMatcher(value: unknown, args: unknown[]) {
          runMatcher(matcher, value, args);
        }
        function runMatcher(fn: Matcher, value: unknown, args: unknown[]) {
          const context = { value, isNot, customMessage };
          fn(context, ...args);
        }
        if (name === "toBe") {
          return new Proxy(matcher as typeof matchers.toBe, {
            apply(target, _thisArg, args) {
              if (args.length !== 1) {
                throw new TypeError(
                  `toBe expects exactly one argument, ${args.length} given`,
                );
              }
              if (isPromised) {
                return (value as Promise<unknown>).then((v) =>
                  runMatcher(target, v, args)
                );
              }
              return runMatcher(target, value, args);
            },
            get(target, p) {
              if (typeof p === "string") {
                switch (p) {
                  case "truthy": //fallthrough
                  case "falsy": //fallthrough
                  case "null": //fallthrough
                  case "undefined": //fallthrough
                  case "defined": //fallthrough
                  case "NaN": {
                    return () => {
                      if (isPromised) {
                        return (value as Promise<unknown>).then((v) =>
                          runMatcher(Reflect.get(target, p), v, [])
                        );
                      }
                      return runMatcher(Reflect.get(target, p), value, []);
                    };
                  }
                  case "instanceof": //fallthrough
                  case "instanceOf": {
                    return (expected: unknown) => {
                      if (isPromised) {
                        return (value as Promise<unknown>).then((v) =>
                          runMatcher(Reflect.get(target, p), v, [expected])
                        );
                      }
                      return runMatcher(Reflect.get(target, p), value, [
                        expected,
                      ]);
                    };
                  }
                  case "type": {
                    return (expected: TypeNames) => {
                      if (isPromised) {
                        return (value as Promise<unknown>).then((v) =>
                          runMatcher(Reflect.get(target, p), v, [expected])
                        );
                      }
                      return runMatcher(Reflect.get(target, p), value, [
                        expected,
                      ]);
                    };
                  }
                  case "closeTo": {
                    return (expected: number, tolerance?: number) => {
                      if (isPromised) {
                        return (value as Promise<unknown>).then((v) =>
                          runMatcher(Reflect.get(target, p), v, [
                            expected,
                            tolerance,
                          ])
                        );
                      }
                      return runMatcher(Reflect.get(target, p), value, [
                        expected,
                        tolerance,
                      ]);
                    };
                  }
                  case "greaterThan": //fallthrough
                  case "greaterThanOrEqual": //fallthrough
                  case "lessThan": //fallthrough
                  case "lessThanOrEqual": {
                    return (expected: number) => {
                      if (isPromised) {
                        return (value as Promise<unknown>).then((v) =>
                          runMatcher(Reflect.get(target, p), v, [expected])
                        );
                      }
                      return runMatcher(Reflect.get(target, p), value, [
                        expected,
                      ]);
                    };
                  }
                  case "iterable": //fallthrough
                  case "asyncIterable": {
                    return () => {
                      if (isPromised) {
                        return (value as Promise<unknown>).then((v) =>
                          runMatcher(Reflect.get(target, p), v, [])
                        );
                      }
                      return runMatcher(Reflect.get(target, p), value, []);
                    };
                  }
                  case "a": //fallthrough
                  case "an": {
                    return new Proxy(target.a, {
                      apply(target, _thisArg, args) {
                        if (args.length !== 1) {
                          throw new TypeError(
                            `${p} expects exactly one argument, ${args.length} given`,
                          );
                        }
                        if (isPromised) {
                          return (value as Promise<unknown>).then((v) =>
                            runMatcher(target, v, args)
                          );
                        }
                        return runMatcher(target, value, args);
                      },
                      get(target, p) {
                        if (typeof p === "string") {
                          switch (p) {
                            case "function": //fallthrough
                            case "asyncFunction": //fallthrough
                            case "generatorFunction": //fallthrough
                            case "generator": //fallthrough
                            case "asyncGeneratorFunction": //fallthrough
                            case "asyncGenerator": //fallthrough
                            case "array": //fallthrough
                            case "object": //fallthrough
                            case "string": //fallthrough
                            case "number": //fallthrough
                            case "bigint": //fallthrough
                            case "boolean": //fallthrough
                            case "symbol": //fallthrough
                            case "null": //fallthrough
                            case "undefined": //fallthrough
                            case "instanceOf": //fallthrough
                            case "instanceof": //fallthrough
                            case "NaN": {
                              return (...args: unknown[]) => {
                                if (args.length !== 0) {
                                  throw new TypeError(
                                    `${p} expects zero arguments, ${args.length} given`,
                                  );
                                }
                                if (isPromised) {
                                  return (value as Promise<unknown>).then(
                                    (v) =>
                                      runMatcher(
                                        Reflect.get(target, p),
                                        v,
                                        args,
                                      ),
                                  );
                                }
                                return runMatcher(
                                  Reflect.get(target, p),
                                  value,
                                  args,
                                );
                              };
                            }
                          }
                        }
                        return Reflect.get(target, p);
                      },
                    });
                  }
                }
              }
            },
          });
        }

        return (...args: unknown[]) =>
          isPromised
            ? (value as Promise<unknown>).then((v) => applyMatcher(v, args))
            : applyMatcher(value, args);
      },
      ownKeys() {
        return Reflect.ownKeys(matchers);
      },
      getOwnPropertyDescriptor(_, p) {
        return Reflect.getOwnPropertyDescriptor(matchers, p);
      },
      getPrototypeOf: () =>
        Object.create(matchers, {
          [Symbol.toStringTag]: { value: "Expected" },
        }),
      setPrototypeOf: () => false, // dummy
      isExtensible: () => true, // dummy
      preventExtensions: () => true, // dummy
    },
  );
  return self;
}

matchers.ts

// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
// deno-lint-ignore-file no-explicit-any

import {
  type AnyConstructor,
  assertEquals,
  assertInstanceOf,
  AssertionError,
  assertIsError,
  assertMatch,
  assertNotEquals,
  assertNotInstanceOf,
  assertNotMatch,
  assertNotStrictEquals,
  assertObjectMatch,
  assertStrictEquals,
  AsyncFunction,
  AsyncGenerator,
  AsyncGeneratorFunction,
  equal,
  filterUndefined,
  format,
  Generator,
  GeneratorFunction,
  inspectArg,
  inspectArgs,
} from "./utils.ts";
import { getMockCalls } from "./mock.ts";
import { assertArrayIncludes } from "./utils.ts";

export interface UntypedMatcherContext {
  value: unknown;
  isNot: boolean;
  customMessage?: string | undefined;
}

export interface MatcherContext<T = unknown, N extends boolean = boolean>
  extends UntypedMatcherContext {
  value: T;
  isNot: N;
  customMessage?: string | undefined;
}

export interface Matcher<
  T = unknown,
  U = unknown,
  N extends boolean = boolean,
  A extends readonly unknown[] = readonly unknown[],
> {
  (
    this: MatcherContext<U, N>,
    context: MatcherContext<U, N>,
    ...args: A
  ): asserts context is RetypeContext<T, U, N>;
  (
    this: MatcherContext<T | U, N> | void,
    context: MatcherContext<T | U, N>,
    ...args: A
  ): asserts context is RetypeContext<T, U, N>;
  (
    this: MatcherContext<U, N>,
    context: MatcherContext<U, N>,
    ...args: A
  ): void | Promise<void>;
  (context: MatcherContext<T | U, N>): context is RetypeContext<T, U, N>;
  (context: MatcherContext<T, N>, ...args: A): void | Promise<void>;
}

export interface Matchers {
  [key: string]: Matcher;
}

export type MatchResult<T = void> = T | Promise<T> | boolean;

type RetypeContext<T, U, N extends boolean = boolean> = `${N}` extends
  infer S extends string ? "true" extends S ? MatcherContext<Exclude<U, T>, N>
  : MatcherContext<Extract<U, T>, N>
  : MatcherContext<T & U, N>;

export function toBe<T, N extends boolean, const A>(
  context: MatcherContext<unknown, N>,
  expect: T,
): asserts context is RetypeContext<T, unknown, N> {
  if (context.isNot) {
    assertNotStrictEquals(context.value, expect, context.customMessage);
  } else {
    assertStrictEquals(context.value, expect, context.customMessage);
  }
}

toBe.truthy = toBeTruthy;
toBe.falsy = toBeFalsy;
toBe.null = toBeNull;
toBe.undefined = toBeUndefined;
toBe.defined = toBeDefined;
toBe.NaN = toBeNaN;
toBe.instanceOf = toBe.instanceof = toBeInstanceOf;
toBe.a = toBe.an = Object.assign(
  ((...args: Parameters<typeof toBeType>) =>
    toBeType(...args)) as typeof toBeType,
  {
    function: toBeFunction,
    asyncFunction: toBeAsyncFunction,
    object: toBeObject,
    string: toBeString,
    number: toBeNumber,
    bigint: toBeBigInt,
    boolean: toBeBoolean,
    symbol: toBeSymbol,
    generator: toBeGeneratorObject,
    generatorFunction: toBeGeneratorFunction,
    asyncGenerator: toBeAsyncGeneratorObject,
    asyncGeneratorFunction: toBeAsyncGeneratorFunction,
    iterable: toBeIterable,
    asyncIterable: toBeAsyncIterable,
    array: toBeArray,
  },
);
toBe.type = toBeType;
toBe.closeTo = toBeCloseTo;
toBe.greaterThanOrEqual = toBeGreaterThanOrEqual;
toBe.greaterThan = toBeGreaterThan;
toBe.lessThanOrEqual = toBeLessThanOrEqual;
toBe.lessThan = toBeLessThan;
toBe.iterable = toBeIterable;
toBe.asyncIterable = toBeAsyncIterable;
toBe.array = toBeArray;

export function toBeGeneratorObject(
  context: MatcherContext,
): MatchResult {
  assertStrictEquals(
    Object.prototype.isPrototypeOf.call(
      Generator.prototype,
      Object(context.value),
    ),
    !context.isNot,
    context.customMessage ??
      `Expected value ${context.isNot ? "not " : ""}to be a generator object`,
  );
}

export function toBeGeneratorFunction(
  context: MatcherContext,
): MatchResult {
  if (context.isNot) {
    assertNotInstanceOf(
      context.value,
      GeneratorFunction,
      context.customMessage,
    );
  } else {
    assertInstanceOf(context.value, GeneratorFunction, context.customMessage);
  }
}

export function toBeAsyncGeneratorObject(
  context: MatcherContext,
): MatchResult {
  assertStrictEquals(
    Object.prototype.isPrototypeOf.call(
      AsyncGenerator.prototype,
      Object(context.value),
    ),
    !context.isNot,
    context.customMessage ??
      `Expected value ${
        context.isNot ? "not " : ""
      }to be an async generator object`,
  );
}

export function toBeAsyncGeneratorFunction(
  context: MatcherContext,
): MatchResult {
  if (context.isNot) {
    assertNotInstanceOf(
      context.value,
      AsyncGeneratorFunction,
      context.customMessage,
    );
  } else {
    assertInstanceOf(
      context.value,
      AsyncGeneratorFunction,
      context.customMessage,
    );
  }
}

export function toBeIterable<T>(
  context: MatcherContext,
): MatchResult<Iterable<T>> {
  if (context.isNot) {
    assertNotStrictEquals(
      typeof (context.value as Iterable<unknown>)?.[Symbol.iterator],
      "function",
      context.customMessage ?? "Expected value to not be iterable",
    );
  } else {
    assertStrictEquals(
      typeof (context.value as Iterable<unknown>)?.[Symbol.iterator],
      "function",
      context.customMessage ?? "Expected value to be iterable",
    );
  }
  return undefined!;
}

export function toBeIterator<T>(
  context: MatcherContext,
): MatchResult<Iterator<T>> {
  if (context.isNot) {
    assertNotStrictEquals(
      typeof (context.value as Iterable<unknown>)?.[Symbol.iterator],
      "function",
      context.customMessage ?? "Expected value to not be iterable",
    );
  } else {
    assertStrictEquals(
      typeof (context.value as Iterable<unknown>)?.[Symbol.iterator],
      "function",
      context.customMessage ?? "Expected value to be iterable",
    );
  }

  return undefined!;
}

export function toBeAsyncIterable<
  T = unknown,
  U = unknown,
  N extends boolean = boolean,
>(
  context: MatcherContext<U, N>,
): asserts context is MatcherContext<
  [`${N}`] extends ["true"] ? Exclude<U, AsyncIterable<T>>
    : Extract<U, AsyncIterable<T>>,
  N
> {
  if (context.isNot) {
    assertNotStrictEquals(
      typeof (context.value as AsyncIterable<unknown>)?.[Symbol.asyncIterator],
      "function",
      context.customMessage ??
        "Expected value to not be asynchronously iterable",
    );
  } else {
    assertStrictEquals(
      typeof (context.value as AsyncIterable<unknown>)?.[Symbol.asyncIterator],
      "function",
      context.customMessage ?? "Expected value to be asynchronously iterable",
    );
  }
}

export function toBeAsyncIterator<
  T = unknown,
  U = unknown,
  N extends boolean = boolean,
>(
  context: MatcherContext<U, N>,
): asserts context is MatcherContext<
  [`${N}`] extends ["true"] ? Exclude<U, AsyncIterator<T>>
    : Extract<U, AsyncIterator<T>>,
  N
> {
  if (context.isNot) {
    assertNotStrictEquals(
      typeof (context.value as AsyncIterable<unknown>)?.[Symbol.asyncIterator],
      "function",
      context.customMessage ??
        "Expected value to not be asynchronously iterable",
    );
  } else {
    assertStrictEquals(
      typeof (context.value as AsyncIterable<unknown>)?.[Symbol.asyncIterator],
      "function",
      context.customMessage ?? "Expected value to be asynchronously iterable",
    );
  }
}

export function toBeArray(context: MatcherContext): MatchResult {
  if (context.isNot) {
    assertNotInstanceOf(context.value, Array, context.customMessage);
    assertNotStrictEquals(
      Array.isArray(context.value),
      true,
      context.customMessage,
    );
  } else {
    assertInstanceOf(context.value, Array, context.customMessage);
    assertStrictEquals(
      Array.isArray(context.value),
      true,
      context.customMessage,
    );
  }
}

export function toEqual<const T>(
  context: MatcherContext,
  expected: T,
): MatchResult {
  const v = filterUndefined(context.value);
  const e = filterUndefined(expected);
  if (context.isNot) {
    assertNotEquals(v, e, context.customMessage);
  } else {
    assertEquals(v, e, context.customMessage);
  }
}

export function toStrictEqual(
  context: MatcherContext,
  expected: unknown,
): MatchResult {
  if (context.isNot) {
    assertNotEquals(context.value, expected, context.customMessage);
  } else {
    assertEquals(context.value, expected, context.customMessage);
  }
}

export function toBeCloseTo(
  context: MatcherContext,
  expected: number,
  numDigits = 2,
): MatchResult {
  if (numDigits < 0) {
    throw new Error(
      "toBeCloseTo second argument must be a non-negative integer. Got " +
        numDigits,
    );
  }
  const tolerance = 0.5 * Math.pow(10, -numDigits);
  const value = Number(context.value);
  const pass = Math.abs(expected - value) < tolerance;

  if (context.isNot) {
    if (pass) {
      throw new AssertionError(
        `Expected the value not to be close to ${expected} (using ${numDigits} digits), but it is`,
      );
    }
  } else {
    if (!pass) {
      throw new AssertionError(
        `Expected the value (${value} to be close to ${expected} (using ${numDigits} digits), but it is not`,
      );
    }
  }
}

export function toBeDefined(context: MatcherContext): MatchResult {
  if (context.isNot) {
    assertStrictEquals(context.value, undefined, context.customMessage);
  } else {
    assertNotStrictEquals(context.value, undefined, context.customMessage);
  }
}

export function toBeUndefined(context: MatcherContext): MatchResult {
  if (context.isNot) {
    assertNotStrictEquals(
      context.value,
      undefined,
      context.customMessage,
    );
  } else {
    assertStrictEquals(context.value, undefined, context.customMessage);
  }
}

export function toBeFunction(context: MatcherContext): MatchResult {
  if (context.isNot) {
    assertNotStrictEquals(
      typeof context.value,
      "function",
      context.customMessage,
    );
  } else {
    assertStrictEquals(typeof context.value, "function", context.customMessage);
  }
}

export function toBeAsyncFunction(context: MatcherContext): MatchResult {
  if (context.isNot) {
    assertNotInstanceOf(context.value, AsyncFunction, context.customMessage);
  } else {
    assertInstanceOf(context.value, AsyncFunction, context.customMessage);
  }
}

interface TypeMap {
  string: string;
  number: number;
  bigint: bigint;
  boolean: boolean;
  object: object;
  // deno-lint-ignore ban-types
  function: Function;
  symbol: symbol;
  null: null;
  undefined: undefined;
}

export type TypeNames = keyof TypeMap & string;

export function toBeType<const K extends readonly TypeNames[]>(
  context: MatcherContext,
  ...expectedTypes: [...K]
): MatchResult {
  const actual = context.value === null ? "null" : typeof context.value;
  const expected = expectedTypes.includes(actual as TypeNames);
  if (context.isNot) {
    if (expected) {
      throw new AssertionError(
        context.customMessage ??= `Expected value not to be of type ${
          expectedTypes.map((t, i, a) =>
            i === a.length - 1 ? `or '${t}'` : `'${t}'`
          ).join(", ")
        }. Received type: '${actual}'`,
      );
    }
  } else {
    assertArrayIncludes(expectedTypes, [actual], context.customMessage);
  }
}

export function toBeString(context: MatcherContext): MatchResult {
  return toBeType(context, "string");
}

export function toBeNumber(context: MatcherContext): MatchResult {
  return toBeType(context, "number");
}

export function toBeBigInt(context: MatcherContext): MatchResult {
  return toBeType(context, "bigint");
}

export function toBeBoolean(context: MatcherContext): MatchResult {
  return toBeType(context, "boolean");
}

export function toBeSymbol(context: MatcherContext): MatchResult {
  return toBeType(context, "symbol");
}

export function toBeObject(context: MatcherContext): MatchResult {
  return toBeType(context, "object");
}

export function toBeFalsy(
  context: MatcherContext,
): MatchResult {
  const isFalsy = !(context.value);
  if (context.isNot) {
    if (isFalsy) {
      throw new AssertionError(
        `Expected ${context.value} to NOT be falsy`,
      );
    }
  } else {
    if (!isFalsy) {
      throw new AssertionError(
        `Expected ${context.value} to be falsy`,
      );
    }
  }
}

export function toBeTruthy(
  context: MatcherContext,
): MatchResult {
  const isTruthy = !!(context.value);
  if (context.isNot) {
    if (isTruthy) {
      throw new AssertionError(
        `Expected ${context.value} to NOT be truthy`,
      );
    }
  } else {
    if (!isTruthy) {
      throw new AssertionError(
        `Expected ${context.value} to be truthy`,
      );
    }
  }
}

export function toBeGreaterThanOrEqual(
  context: MatcherContext,
  expected: number,
): MatchResult {
  const isGreaterOrEqual = Number(context.value) >= Number(expected);
  if (context.isNot) {
    if (isGreaterOrEqual) {
      throw new AssertionError(
        `Expected ${context.value} to NOT be greater than or equal ${expected}`,
      );
    }
  } else {
    if (!isGreaterOrEqual) {
      throw new AssertionError(
        `Expected ${context.value} to be greater than or equal ${expected}`,
      );
    }
  }
}

export function toBeGreaterThan(
  context: MatcherContext,
  expected: number,
): MatchResult {
  const isGreater = Number(context.value) > Number(expected);
  if (context.isNot) {
    if (isGreater) {
      throw new AssertionError(
        `Expected ${context.value} to NOT be greater than ${expected}`,
      );
    }
  } else {
    if (!isGreater) {
      throw new AssertionError(
        `Expected ${context.value} to be greater than ${expected}`,
      );
    }
  }
}

export function toBeLessThanOrEqual(
  context: MatcherContext,
  expected: number,
): MatchResult {
  const isLower = Number(context.value) <= Number(expected);
  if (context.isNot) {
    if (isLower) {
      throw new AssertionError(
        `Expected ${context.value} to NOT be lower than or equal ${expected}`,
      );
    }
  } else {
    if (!isLower) {
      throw new AssertionError(
        `Expected ${context.value} to be lower than or equal ${expected}`,
      );
    }
  }
}
export function toBeLessThan(
  context: MatcherContext,
  expected: number,
): MatchResult {
  const isLower = Number(context.value) < Number(expected);
  if (context.isNot) {
    if (isLower) {
      throw new AssertionError(
        `Expected ${context.value} to NOT be lower than ${expected}`,
      );
    }
  } else {
    if (!isLower) {
      throw new AssertionError(
        `Expected ${context.value} to be lower than ${expected}`,
      );
    }
  }
}

export function toBeNaN(context: MatcherContext): MatchResult {
  if (context.isNot) {
    assertNotEquals(
      isNaN(Number(context.value)),
      true,
      context.customMessage || `Expected ${context.value} to not be NaN`,
    );
  } else {
    assertEquals(
      isNaN(Number(context.value)),
      true,
      context.customMessage || `Expected ${context.value} to be NaN`,
    );
  }
}

export function toBeNull(context: MatcherContext): MatchResult {
  if (context.isNot) {
    assertNotStrictEquals(
      context.value as number,
      null,
      context.customMessage || `Expected ${context.value} to not be null`,
    );
  } else {
    assertStrictEquals(
      context.value as number,
      null,
      context.customMessage || `Expected ${context.value} to be null`,
    );
  }
}

export function toHaveLength(
  context: MatcherContext,
  expected: number,
): MatchResult {
  const { value } = context;
  // deno-lint-ignore no-explicit-any
  const maybeLength = (value as any)?.length;
  const hasLength = maybeLength === expected;

  if (context.isNot) {
    if (hasLength) {
      throw new AssertionError(
        `Expected value not to have length ${expected}, but it does`,
      );
    }
  } else {
    if (!hasLength) {
      throw new AssertionError(
        `Expected value to have length ${expected}, but it does not. (The value has length ${maybeLength})`,
      );
    }
  }
}

export function toBeInstanceOf<T extends AnyConstructor>(
  context: MatcherContext,
  expected: T,
): MatchResult {
  if (context.isNot) {
    assertNotInstanceOf(context.value, expected);
  } else {
    assertInstanceOf(context.value, expected);
  }
}

export function toHaveProperty(
  context: MatcherContext,
  propName: PropertyKey | PropertyKey[],
  v?: unknown,
): MatchResult {
  const { value } = context;

  let propPath = [] as PropertyKey[];
  if (Array.isArray(propName)) {
    propPath = propName;
  } else if (typeof propName === "string") {
    propPath = propName.split(".");
  } else {
    propPath = [propName];
  }

  propPath = propPath.flatMap<PropertyKey>((prop) => {
    if (typeof prop === "symbol") return prop;
    const p = String(prop);
    return p.split(/\.|\[(?=[^\]]*)\]/g).filter((p) => p);
  });

  // deno-lint-ignore no-explicit-any
  let current = value as any;
  while (true) {
    if (current === undefined || current === null) {
      break;
    }
    if (propPath.length === 0) {
      break;
    }
    const prop = propPath.shift()!;
    current = current[prop];
  }

  let hasProperty;
  if (v) {
    hasProperty = current !== undefined && propPath.length === 0 &&
      equal(current, v);
  } else {
    hasProperty = current !== undefined && propPath.length === 0;
  }

  let ofValue = "";
  if (v) {
    ofValue = ` of the value ${inspectArg(v)}`;
  }

  const joined = propPath.reduce((acc, p) => {
    if (!p) return acc.toString();
    if (typeof p === "symbol") {
      return `${acc.toString()}[${String(p)}]`;
    }
    if (!isNaN(Number(p))) {
      return `${acc.toString()}[${p}]`;
    }
    return `${acc ? acc.toString() + "." : ""}${p}`;
  }, "") as string;

  if (context.isNot) {
    if (hasProperty) {
      throw new AssertionError(
        `Expected the value not to have the property ${joined}${ofValue}, but it does.`,
      );
    }
  } else {
    if (!hasProperty) {
      throw new AssertionError(
        `Expected the value to have the property ${joined}${ofValue}, but it does not.`,
      );
    }
  }
}

export function toContain(
  context: MatcherContext,
  expected: unknown,
): MatchResult {
  // deno-lint-ignore no-explicit-any
  const doesContain = (context.value as any)?.includes?.(expected);

  if (context.isNot) {
    if (doesContain) {
      throw new AssertionError("The value contains the expected item");
    }
  } else {
    if (!doesContain) {
      throw new AssertionError("The value doesn't contain the expected item");
    }
  }
}

export function toContainEqual(
  context: MatcherContext,
  expected: unknown,
): MatchResult {
  const { value } = context;
  assertIsIterable(value);
  let doesContain = false;
  for (const item of value) {
    if (equal(item, expected)) {
      doesContain = true;
      break;
    }
  }

  if (context.isNot) {
    if (doesContain) {
      throw new AssertionError("The value contains the expected item");
    }
  } else {
    if (!doesContain) {
      throw new AssertionError("The value doesn't contain the expected item");
    }
  }
}

// deno-lint-ignore no-explicit-any
function assertIsIterable(value: any): asserts value is Iterable<unknown> {
  if (value == null) {
    throw new AssertionError("The value is null or undefined");
  }
  if (typeof value[Symbol.iterator] !== "function") {
    throw new AssertionError("The value is not iterable");
  }
}

export function toMatch(
  context: MatcherContext,
  expected: RegExp,
): MatchResult {
  if (context.isNot) {
    assertNotMatch(
      String(context.value),
      expected,
      context.customMessage,
    );
  } else {
    assertMatch(String(context.value), expected, context.customMessage);
  }
}

export function toMatchObject(
  context: MatcherContext,
  expected: Record<PropertyKey, unknown>,
): MatchResult {
  if (context.isNot) {
    let objectMatch = false;
    try {
      assertObjectMatch(
        // deno-lint-ignore no-explicit-any
        context.value as Record<PropertyKey, any>,
        expected,
        context.customMessage,
      );
      objectMatch = true;
      const actualString = format(context.value);
      const expectedString = format(expected);
      throw new AssertionError(
        `Expected ${actualString} to NOT match ${expectedString}`,
      );
    } catch (e) {
      if (objectMatch) {
        throw e;
      }
      return;
    }
  } else {
    assertObjectMatch(
      // deno-lint-ignore no-explicit-any
      context.value as Record<PropertyKey, any>,
      expected,
      context.customMessage,
    );
  }
}

export function toHaveBeenCalled(context: MatcherContext): MatchResult {
  const calls = getMockCalls(context.value);
  const hasBeenCalled = calls.length > 0;

  if (context.isNot) {
    if (hasBeenCalled) {
      throw new AssertionError(
        `Expected mock function not to be called, but it was called ${calls.length} time(s)`,
      );
    }
  } else {
    if (!hasBeenCalled) {
      throw new AssertionError(
        `Expected mock function to be called, but it was not called`,
      );
    }
  }
}

export function toHaveBeenCalledTimes(
  context: MatcherContext,
  expected: number,
): MatchResult {
  const calls = getMockCalls(context.value);

  if (context.isNot) {
    if (calls.length === expected) {
      throw new AssertionError(
        `Expected mock function not to be called ${expected} time(s), but it was`,
      );
    }
  } else {
    if (calls.length !== expected) {
      throw new AssertionError(
        `Expected mock function to be called ${expected} time(s), but it was called ${calls.length} time(s)`,
      );
    }
  }
}

export function toHaveBeenCalledWith(
  context: MatcherContext,
  ...expected: unknown[]
): MatchResult {
  const calls = getMockCalls(context.value);
  const hasBeenCalled = calls.some((call) => equal(call.args, expected));

  if (context.isNot) {
    if (hasBeenCalled) {
      throw new AssertionError(
        `Expected mock function not to be called with ${
          inspectArgs(expected)
        }, but it was`,
      );
    }
  } else {
    if (!hasBeenCalled) {
      let otherCalls = "";
      if (calls.length > 0) {
        otherCalls = `\n  Other calls:\n     ${
          calls.map((call) => inspectArgs(call.args)).join("\n    ")
        }`;
      }
      throw new AssertionError(
        `Expected mock function to be called with ${
          inspectArgs(expected)
        }, but it was not.${otherCalls}`,
      );
    }
  }
}
export function toHaveBeenLastCalledWith(
  context: MatcherContext,
  ...expected: unknown[]
): MatchResult {
  const calls = getMockCalls(context.value);
  const hasBeenCalled = calls.length > 0 &&
    equal(calls.at(-1)?.args, expected);

  if (context.isNot) {
    if (hasBeenCalled) {
      throw new AssertionError(
        `Expected mock function not to be last called with ${
          inspectArgs(expected)
        }, but it was`,
      );
    }
  } else {
    if (!hasBeenCalled) {
      const lastCall = calls.at(-1);
      if (!lastCall) {
        throw new AssertionError(
          `Expected mock function to be last called with ${
            inspectArgs(expected)
          }, but it was not.`,
        );
      } else {
        throw new AssertionError(
          `Expected mock function to be last called with ${
            inspectArgs(expected)
          }, but it was last called with ${inspectArgs(lastCall.args)}.`,
        );
      }
    }
  }
}
export function toHaveBeenNthCalledWith(
  context: MatcherContext,
  nth: number,
  ...expected: unknown[]
): MatchResult {
  if (nth < 1) {
    new Error(`nth must be greater than 0. ${nth} was given.`);
  }

  const calls = getMockCalls(context.value);
  const callIndex = nth - 1;
  const hasBeenCalled = calls.length > callIndex &&
    equal(calls[callIndex]?.args, expected);

  if (context.isNot) {
    if (hasBeenCalled) {
      throw new AssertionError(
        `Expected the n-th call (n=${nth}) of mock function is not with ${
          inspectArgs(expected)
        }, but it was`,
      );
    }
  } else {
    if (!hasBeenCalled) {
      const nthCall = calls[callIndex];
      if (!nthCall) {
        throw new AssertionError(
          `Expected the n-th call (n=${nth}) of mock function is with ${
            inspectArgs(expected)
          }, but the n-th call does not exist.`,
        );
      } else {
        throw new AssertionError(
          `Expected the n-th call (n=${nth}) of mock function is with ${
            inspectArgs(expected)
          }, but it was with ${inspectArgs(nthCall.args)}.`,
        );
      }
    }
  }
}

export function toHaveReturned(context: MatcherContext): MatchResult {
  const calls = getMockCalls(context.value);
  const returned = calls.filter((call) => call.returns);

  if (context.isNot) {
    if (returned.length > 0) {
      throw new AssertionError(
        `Expected the mock function to not have returned, but it returned ${returned.length} times`,
      );
    }
  } else {
    if (returned.length === 0) {
      throw new AssertionError(
        `Expected the mock function to have returned, but it did not return`,
      );
    }
  }
}

export function toHaveReturnedTimes(
  context: MatcherContext,
  expected: number,
): MatchResult {
  const calls = getMockCalls(context.value);
  const returned = calls.filter((call) => call.returns);

  if (context.isNot) {
    if (returned.length === expected) {
      throw new AssertionError(
        `Expected the mock function to not have returned ${expected} times, but it returned ${returned.length} times`,
      );
    }
  } else {
    if (returned.length !== expected) {
      throw new AssertionError(
        `Expected the mock function to have returned ${expected} times, but it returned ${returned.length} times`,
      );
    }
  }
}
export function toHaveReturnedWith(
  context: MatcherContext,
  expected: unknown,
): MatchResult {
  const calls = getMockCalls(context.value);
  const returned = calls.filter((call) => call.returns);
  const returnedWithExpected = returned.some((call) =>
    equal(call.returned, expected)
  );

  if (context.isNot) {
    if (returnedWithExpected) {
      throw new AssertionError(
        `Expected the mock function to not have returned with ${
          inspectArg(expected)
        }, but it did`,
      );
    }
  } else {
    if (!returnedWithExpected) {
      throw new AssertionError(
        `Expected the mock function to have returned with ${
          inspectArg(expected)
        }, but it did not`,
      );
    }
  }
}

export function toHaveLastReturnedWith(
  context: MatcherContext,
  expected: unknown,
): MatchResult {
  const calls = getMockCalls(context.value);
  const returned = calls.filter((call) => call.returns);
  const lastReturnedWithExpected = returned.length > 0 &&
    equal(returned.at(-1)?.returned, expected);

  if (context.isNot) {
    if (lastReturnedWithExpected) {
      throw new AssertionError(
        `Expected the mock function to not have last returned with ${
          inspectArg(expected)
        }, but it did`,
      );
    }
  } else {
    if (!lastReturnedWithExpected) {
      throw new AssertionError(
        `Expected the mock function to have last returned with ${
          inspectArg(expected)
        }, but it did not`,
      );
    }
  }
}

export function toHaveNthReturnedWith(
  context: MatcherContext,
  nth: number,
  expected: unknown,
): MatchResult {
  if (nth < 1) {
    throw new Error(`nth(${nth}) must be greater than 0`);
  }

  const calls = getMockCalls(context.value);
  const returned = calls.filter((call) => call.returns);
  const returnIndex = nth - 1;
  const maybeNthReturned = returned[returnIndex];
  const nthReturnedWithExpected = maybeNthReturned &&
    equal(maybeNthReturned.returned, expected);

  if (context.isNot) {
    if (nthReturnedWithExpected) {
      throw new AssertionError(
        `Expected the mock function to not have n-th (n=${nth}) returned with ${
          inspectArg(expected)
        }, but it did`,
      );
    }
  } else {
    if (!nthReturnedWithExpected) {
      throw new AssertionError(
        `Expected the mock function to have n-th (n=${nth}) returned with ${
          inspectArg(expected)
        }, but it did not`,
      );
    }
  }
}

export function toThrow<E extends Error = Error>(
  context: MatcherContext,
  // deno-lint-ignore no-explicit-any
  expected?: string | RegExp | E | (new (...args: any[]) => E),
): MatchResult {
  if (typeof context.value === "function") {
    try {
      context.value = context.value();
    } catch (err) {
      context.value = err;
    }
  }

  // deno-lint-ignore no-explicit-any
  type ErrorClass = new (...args: any[]) => Error;
  let expectClass: undefined | ErrorClass = undefined;
  let expectMessage: undefined | string | RegExp = undefined;
  if (expected instanceof Error) {
    expectClass = expected.constructor as ErrorClass;
    expectMessage = expected.message;
  }
  if (typeof expected === "function") {
    expectClass = expected as ErrorClass;
  }
  if (typeof expected === "string" || expected instanceof RegExp) {
    expectMessage = expected;
  }

  if (context.isNot) {
    let isError = false;
    try {
      assertIsError(
        context.value,
        expectClass,
        expectMessage,
        context.customMessage,
      );
      isError = true;
      throw new AssertionError(`Expected to NOT throw ${expected}`);
    } catch (e) {
      if (isError) {
        throw e;
      }
      return;
    }
  }
  return assertIsError(
    context.value,
    expectClass,
    expectMessage,
    context.customMessage,
  );
}

export const matchers = {
  lastCalledWith: toHaveBeenLastCalledWith,
  lastReturnedWith: toHaveLastReturnedWith,
  nthCalledWith: toHaveBeenNthCalledWith,
  nthReturnedWith: toHaveNthReturnedWith,
  toBeCalled: toHaveBeenCalled,
  toBeCalledTimes: toHaveBeenCalledTimes,
  toBeCalledWith: toHaveBeenCalledWith,
  toBeCloseTo,
  toBeDefined,
  toBeFalsy,
  toBeGreaterThanOrEqual,
  toBeGreaterThan,
  toBeInstanceOf,
  toBeLessThanOrEqual,
  toBeLessThan,
  toBeNaN,
  toBeNull,
  toBeTruthy,
  toBeUndefined,
  toBe,
  toBeBigInt,
  toBeBoolean,
  toBeNumber,
  toBeObject,
  toBeString,
  toBeSymbol,
  toBeType,
  toBeAsyncFunction,
  toBeFunction,
  toBeGeneratorFunction,
  toBeGeneratorObject,
  toBeAsyncGeneratorFunction,
  toBeAsyncGeneratorObject,
  toBeIterable,
  toBeAsyncIterable,
  toContainEqual,
  toContain,
  toEqual,
  toHaveBeenCalledTimes,
  toHaveBeenCalledWith,
  toHaveBeenCalled,
  toHaveBeenLastCalledWith,
  toHaveBeenNthCalledWith,
  toHaveLength,
  toHaveLastReturnedWith,
  toHaveNthReturnedWith,
  toHaveProperty,
  toHaveReturnedTimes,
  toHaveReturnedWith,
  toHaveReturned,
  toMatchObject,
  toMatch,
  toReturn: toHaveReturned,
  toReturnTimes: toHaveReturnedTimes,
  toReturnWith: toHaveReturnedWith,
  toStrictEqual,
  toThrow,
} as const;
export type matchers = typeof matchers;

mock.ts

const MOCK_SYMBOL = Symbol.for("@MOCK");
type MOCK_SYMBOL = typeof MOCK_SYMBOL;

export interface MockCall<
  A extends readonly unknown[] = readonly unknown[],
  R = unknown,
  T = unknown,
> {
  args: A;
  returned?: R;
  thrown?: T;
  timestamp: number;
  returns: boolean;
  throws: boolean;
}

export function getMockCalls<
  const F extends {
    readonly [MOCK_SYMBOL]: {
      readonly calls: readonly MockCall<any, any, any>[];
    };
  },
>(
  f: F,
): F[MOCK_SYMBOL] extends
  { readonly calls: infer C extends readonly MockCall[] } ? C
  : readonly MockCall[];

export function getMockCalls(f: unknown): readonly MockCall[];

export function getMockCalls(f: any): readonly MockCall[] {
  if (f == null || !(typeof f === "function" || typeof f === "object")) {
    throw new TypeError("Received value must be a function or object");
  }
  if (!(MOCK_SYMBOL in f)) {
    throw new TypeError("Received value must be a mock or spy function");
  }
  const mockInfo = f[MOCK_SYMBOL];
  if (!mockInfo) {
    throw new Error("Received function must be a mock or spy function");
  }

  return [...mockInfo.calls] as any;
}

export function fn<A extends readonly unknown[], R = unknown>(
  ...stubs: readonly ((...args: A) => R)[]
): {
  <const Args extends A>(...args: Args): R | undefined;
  readonly [MOCK_SYMBOL]: {
    readonly calls: readonly MockCall[];
  };
} {
  const calls: MockCall[] = [];
  const f = (...args: A) => {
    const stub = stubs.length === 1
      // keep reusing the first
      ? stubs[0]
      // pick the exact mock for the current call
      : stubs[calls.length];

    try {
      const returned = stub ? stub(...args) : undefined;
      calls.push({
        args,
        returned,
        timestamp: Date.now(),
        returns: true,
        throws: false,
      });
      return returned;
    } catch (thrown) {
      calls.push({
        args,
        timestamp: Date.now(),
        returns: false,
        thrown,
        throws: true,
      });
      throw thrown;
    }
  };

  Reflect.defineProperty(f, MOCK_SYMBOL, {
    value: { calls },
    writable: false,
    configurable: true,
    enumerable: false,
  });

  return f as unknown as ReturnType<typeof fn>;
}

mod.ts

// runtime exports
import { expect } from "./expect.ts";
import { matchers } from "./matchers.ts";
import { fn, getMockCalls } from "./mock.ts";

// type-only exports
import type { Expected } from "./expect.ts";
import type { Matcher, MatcherContext, Matchers } from "./matchers.ts";
import type { MockCall } from "./mock.ts";

// vendored behavior-driven development (BDD) helpers
import {
  afterAll,
  afterEach,
  beforeAll,
  beforeEach,
  describe,
  it,
} from "https://deno.land/[email protected]/testing/bdd.ts";

export {
  afterAll,
  afterEach,
  beforeAll,
  beforeEach,
  describe,
  expect,
  fn,
  fn as mock,
  getMockCalls,
  it,
  matchers,
};

export type { Expected, Matcher, MatcherContext, Matchers, MockCall };

export default {
  afterAll,
  afterEach,
  beforeAll,
  beforeEach,
  describe,
  expect,
  fn,
  getMockCalls,
  it,
  matchers,
  mock: fn,
};

utils.ts

// deno-lint-ignore-file no-explicit-any
import { AssertionError } from "https://deno.land/[email protected]/assert/mod.ts";
export {
  assertArrayIncludes,
  assertEquals,
  assertInstanceOf,
  AssertionError,
  assertIsError,
  assertMatch,
  assertNotEquals,
  assertNotInstanceOf,
  assertNotMatch,
  assertNotStrictEquals,
  assertObjectMatch,
  assertStrictEquals,
  assertStringIncludes,
  equal,
} from "https://deno.land/[email protected]/assert/mod.ts";

export type Constructor<T, A extends readonly unknown[]> = {
  new (...args: A): T;
};

export type AnyConstructor = Constructor<any, any>;

export interface AsyncFunctionConstructor
  extends Constructor<AsyncFunction, string[]> {
  readonly prototype: AsyncFunction;

  (...args: string[]): AsyncFunction;
}

export interface AsyncFunction<TReturn = unknown> {
  <T extends TReturn>(...args: unknown[]): Promise<T>;

  readonly prototype: AsyncFunction;
}

export type Fn<T = any, A extends readonly unknown[] = any> = (...args: A) => T;

/**
 * Converts all the methods in an interface to be async functions,
 * including deeply nested methods, preserving the original types + structure.
 */
export type Async<T> = [T] extends [
  ((...a: infer A) => infer R) & (infer O),
] ?
    & (A extends readonly [] ? () => Promise<Awaited<R>>
      : <const Args extends A>(...args: Args) => Promise<Awaited<R>>)
    & ([keyof O] extends [never] ? unknown : { [P in keyof O]: Async<O[P]> })
  : [T] extends [(...args: infer A) => infer R]
    ? <const Args extends A>(...args: Args) => Promise<Awaited<R>>
  : { [K in keyof T]: T[K] extends Fn ? Async<T[K]> : T[K] };

export const AsyncFunction: AsyncFunctionConstructor =
  Object.getPrototypeOf(async () => {}).constructor;

export const AsyncGenerator: AsyncGeneratorFunction = Object.getPrototypeOf(
  async function* () {},
);

export const Generator: GeneratorFunction = Object.getPrototypeOf(
  function* () {},
);

export const AsyncGeneratorFunction: AsyncGeneratorFunctionConstructor =
  AsyncGenerator.constructor as AsyncGeneratorFunctionConstructor;

export const GeneratorFunction: GeneratorFunctionConstructor = Generator
  .constructor as GeneratorFunctionConstructor;

/**
 * Converts the input into a string. Objects, Sets and Maps are sorted so as to
 * make tests less flaky
 * @param v Value to be formatted
 */
export function format(v: unknown): string {
  const { Deno } = globalThis as any;
  return typeof Deno?.inspect === "function"
    ? Deno.inspect(v, {
      depth: Infinity,
      sorted: true,
      trailingComma: true,
      compact: false,
      iterableLimit: Infinity,
      // getters should be true in assertEquals.
      getters: true,
      strAbbreviateSize: Infinity,
    })
    : `"${String(v).replace(/(?=["\\])/g, "\\")}"`;
}

export function inspectArgs<const A extends readonly unknown[]>(
  args: A,
): string {
  return args.map(inspectArg).join(", ");
}

export function inspectArg(arg: unknown): string {
  const { Deno } = globalThis as any;
  return typeof Deno !== "undefined" && Deno.inspect
    ? Deno.inspect(arg)
    : String(arg);
}

export function isPromiseLike<T>(value: unknown): value is PromiseLike<T> {
  if (value == null) {
    return false;
  } else {
    return typeof ((value as Record<string, unknown>).then) === "function";
  }
}

export function assertIsIterable<T>(
  value: unknown,
): asserts value is Iterable<T> {
  if (value == null || typeof value !== "object") {
    throw new AssertionError("The value is null or undefined");
  }
  if (
    Symbol.iterator in value && typeof value[Symbol.iterator] !== "function"
  ) {
    throw new AssertionError("The value is not iterable");
  }
}

export type Filter<A extends readonly unknown[], T = never> = A extends
  readonly [infer B, ...infer C] ? B extends T ? Filter<C, T>
  : B extends readonly unknown[] // filter deeeep bro
    ? [Filter<B, T>, ...Filter<C, T>]
  : [B, ...Filter<C, T>]
  : [];

export function filterUndefined<const T extends readonly unknown[]>(
  array: T,
): Filter<T, undefined>;
export function filterUndefined<const T extends object>(obj: T): {
  [K in keyof T as [T[K]] extends [undefined] ? never : K]: T[K];
};
export function filterUndefined<T>(obj: T): Exclude<T, undefined>;
export function filterUndefined(obj: any) {
  if (
    typeof obj !== "object" || obj instanceof Date || obj instanceof RegExp ||
    ArrayBuffer.isView(obj) || obj === null || "size" in obj ||
    Symbol.iterator in obj
  ) {
    return obj;
  } else if (Array.isArray(obj)) {
    return obj.map(filterUndefined);
  }

  const result = structuredClone(obj);
  for (const key in result) {
    const val = result[key];
    if (val === undefined) {
      Reflect.deleteProperty(result, key);
      continue;
    }
    result[key] = filterUndefined(val);
  }
  return result;
}