Preserving tuple length

tuple-operations.ts

// #region Necessary types
type NonEmptyArray<T> = [head: T, ...T[]];
type Tuple<T> = [T, T];
declare function head<T>(collection: [T, ...T[]]): T;
// #endregion

const cloudflareDNS: NonEmptyArray<string> = ["1.1.1.1"];
const googleDNS: Tuple<string> = ["8.8.8.8", "8.8.4.4"]

// ✅ These Just Work™️
head(cloudflareDNS); // ✅
head(googleDNS); // ✅

// ⛔️ But you may want to massage your data first, and these won't work
head(cloudflareDNS.map(address => ({ target: address })));
head(googleDNS.slice().sort());
head(cloudflareDNS.concat());
head(cloudflareDNS.concat([]));
head(cloudflareDNS.concat("1.0.0.1"));
head(cloudflareDNS.concat(["1.0.0.1"]));
head(cloudflareDNS.concat(googleDNS));

const tuple: Tuple<string> = googleDNS.map(address => "Your address:" + " " + address)

// declare global {
//   interface Array<T> {
//     /**
//      * If two arrays are concatenated, and at least one of them has elements, then the array created by concatenation must also have elements.
//      */
//     concat(this: NonEmptyArray<T>, ...items: ConcatArray<T>[]): NonEmptyArray<T>;
//     concat(...items: NonEmptyArray<ConcatArray<T>>): NonEmptyArray<T>;
//     concat(...items: NonEmptyArray<T | ConcatArray<T>>): NonEmptyArray<T>;

//     /**
//      * When called on a tuple, the output has the same length as the input.
//      * Make sure to allow readonly arrays too.
//      */
//     map<U>(this: T[] | [T], callbackfn: (value: T, index: number, array: this) => U, thisArg?: any): { [index in keyof this]: U; };
//     map<U>(this: readonly T[] | readonly [T], callbackfn: (value: T, index: number, array: this) => U, thisArg?: any): { [index in keyof this]: U; };

//     /**
//      * When called on a tuple, the output has the same length as the input.
//      */
//     slice(): this
//   }
// }

// ⛔️ These also don't work out of the box
declare const salaries: Map<string, number>;

const name = "Bob";

if (salaries.has("Bob")) {
  const salary: number = salaries.get(name);
}

declare global {
  interface Map<K, V> {
    has<T extends K>(key: T): this is { get(key: T): V } & Map<K, V>;
  }
}

// #region Misc
export { }
export const hasElements = <T,>(collection: T[] | readonly T[]): collection is NonEmptyArray<T> => collection.length > 0;

// Takeaways
//
// • Leverage mapped tuple types https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-1.html
// • This can be risky (also the reason it doesn't work for maps)
// • Remember about including overloads for readonly arrays
// • https://gist.github.com/karol-majewski/bd00cbe38d8c31b7af87f66c896796f9
// #endregion

TypeScript Playground

TypeScript 3.1 and mapped tuple types