Utility type: Object entries tuple

Search Terms

Utility type, object entries, tuple, mapped type

Suggestion

An Entries<T> utility type that constructs a union of all object entry tuples of type T in lib.es5.d.ts.

/**
 * Construct a union of all object entry tuples of type T
 */
type Entries<T> = {
    [P in keyof T]: [P, T[P]];
}[keyof T];

Use Cases

I understand the history behind why Object.entries provides string as the key type rather than the actual keys (because there may exist more actual keys that those defined in the type). I refer to discussion here.

Sometimes, however, we can be confident that only properties defined in the type will be present at runtime, and have found that a utility type like this can be helpful to more strictly define such types, particularly in tests.

Examples

const expected = {
   foo: "foo",
   bar: "Bar",
   baz: "BAZ",
} as const;

(Object.entries(expected) as Entries<typeof expected>).forEach(([input, output]) => {
  // where `myFunction`'s first parameter only accepts `"foo" | "bar" | "baz"` (or a superset)
  expect(myFunction(input)).to.equal(output);
});

function thatAcceptsJsxAttributeTuples(attributes: readonly Entries<JSX.IntrinsicElements["path"][]>): void {
  // ...
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Der Vorschlag zielt auf lib.es5.d.ts; beginne dort und vergleiche die vorhandenen Deklarationen von Utility Types mit der vorgeschlagenen Form Entries. Als erledigt gilt die Aufgabe, wenn der Utility Type konsistent mit der Standardbibliothek hinzugefügt wurde und die im Issue beschriebenen Beispiele für Tupel von Objekt-Einträgen unterstützt.