TypeScript ErrorHandling with Result Type Example

README.md

TypeScript ErrorHandling with Result Type Example

(todo)

deno.json

{
  "tasks": {
    "dev": "deno run --watch main.ts"
  },
  "imports": {
    "@std/assert": "jsr:@std/assert@1"
  }
}

deno.lock

{
  "version": "4",
  "specifiers": {
    "jsr:@std/assert@1": "1.0.8",
    "jsr:@std/internal@^1.0.5": "1.0.5"
  },
  "jsr": {
    "@std/[email protected]": {
      "integrity": "ebe0bd7eb488ee39686f77003992f389a06c3da1bbd8022184804852b2fa641b",
      "dependencies": [
        "jsr:@std/internal"
      ]
    },
    "@std/[email protected]": {
      "integrity": "54a546004f769c1ac9e025abd15a76b6671ddc9687e2313b67376125650dc7ba"
    }
  },
  "workspace": {
    "dependencies": [
      "jsr:@std/assert@1"
    ]
  }
}

lib.ts

export type SuccessResult<T> = {
  readonly ok: true;
  readonly value: T;
};

export type ErrorResult<E> = {
  readonly ok: false;
  readonly error: E;
};

export type Result<T, E> = SuccessResult<T> | ErrorResult<E>;

export function ok<T>(value: T): SuccessResult<T> {
  return {
    ok: true,
    value,
  };
}

export function err<E>(error: E): ErrorResult<E> {
  return {
    ok: false,
    error,
  };
}

export type ResultPromise<T, E> = Promise<Result<T, E>>;

lib_test.ts

import { assert, assertEquals } from "@std/assert";
import { err, ok, type Result } from "./lib.ts";

Deno.test("ok function creates an ErrorResult", () => {
  const value = 12;
  const result = ok(value);
  assertEquals(result, { ok: true, value: 12 });

  const complexValue = {
    a: -10.5,
    b: undefined,
    c: "Hello, World",
    d: false,
    e: { x: 100, y: {}, z: null },
  };
  const complexResult = ok(complexValue);
  assertEquals(complexResult, {
    ok: true,
    value: complexValue,
  });
});

Deno.test("err function creates an ErrorResult", () => {
  const result = err("error message");
  assertEquals(result, { ok: false, error: "error message" });

  const includeException = err(new Error("exception"));
  assertEquals(includeException.ok, false);
  assert(includeException.error instanceof Error);
});

Deno.test("Result type allows only SuccessResult or ErrorResult", () => {
  const success: Result<number, string> = ok(42);
  assertEquals(success.ok, true);
  assertEquals(success.value, 42);

  const failure: Result<number, string> = err("error");
  assertEquals(failure.ok, false);
  assertEquals(failure.error, "error");
});

main.ts

export function add(a: number, b: number): number {
  return a + b;
}

// Learn more at https://docs.deno.com/runtime/manual/examples/module_metadata#concepts
if (import.meta.main) {
  console.log("Add 2 + 3 =", add(2, 3));
}