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Utility type: Object entries tuple #32771

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nhardy opened this issue Aug 9, 2019 · 1 comment

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@nhardy
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commented Aug 9, 2019

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.
@thejohnfreeman

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commented Aug 14, 2019

I was about to report this as a bug. I'll leave the demonstration that I coded up, distilled from my personal use case:

type O = { [key: string]: unknown };
type B<A extends O> = { [K in keyof A]: A[K] };

class Test<A extends O> {
  public constructor(private a: A) {}

  public test() {
    const b = {} as B<A>;
    for (const [key, value] of Object.entries(this.a)) {
      b[key] = value; // error: Type 'string' cannot be used to index type 'B<A>'.
      b[key as keyof A] = value as A[keyof A];
    }
    return b;
  }
}
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