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Add Array.isArray improvement #23

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37 changes: 37 additions & 0 deletions readme.md
Original file line number Diff line number Diff line change
Expand Up @@ -256,6 +256,43 @@ const validate = (input: unknown) => {
};
```

### Make `Array.isArray` correctly narrow with `ReadonlyArray<Item>`

```ts
import "@total-typescript/ts-reset/array-includes";
```

This rule improves on TypeScript's default `Array.isArray` behaviour. With this rule enabled, TypeScript will now include `ReadonlyArray<Item>` when narrowing Array types.

```ts
// BEFORE
function makeArray(input: string | ReadonlyArray<string> | Array<string>) {
if (Array.isArray(input)) {
return input;
}
return [input];
}

type Result = ReturnType<typeof makeArray>; // (string | readonly string[])[]
```

This is almost definitely not what the user intended! While it works correctly at run-time (since JS has no concept of ReadonlyArray), the inferred types are very incorrect.

```ts
// AFTER
import "@total-typescript/ts-reset/array-includes";

function makeArray(input: string | ReadonlyArray<string> | Array<string>) {
// Array.isArray now
if (Array.isArray(input)) {
return input;
}
return [input];
}

type Result = ReturnType<typeof makeArray>; // string[]
```

## Rules we won't add

### `Object.keys`/`Object.entries`
Expand Down
2 changes: 1 addition & 1 deletion src/entrypoints/is-array.d.ts
Original file line number Diff line number Diff line change
@@ -1,3 +1,3 @@
interface ArrayConstructor {
isArray(arg: any): arg is unknown[];
isArray(arg: any): arg is ReadonlyArray<unknown> | Array<unknown>;
}
18 changes: 17 additions & 1 deletion src/tests/is-array.ts
Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,9 @@ doNotExecute(() => {
const maybeArr = [1, 2, 3] as unknown;

if (Array.isArray(maybeArr)) {
type tests = [Expect<Equal<typeof maybeArr, unknown[]>>];
type tests = [
Expect<Equal<typeof maybeArr, unknown[] | readonly unknown[]>>
];
}
});

Expand All @@ -15,3 +17,17 @@ doNotExecute(() => {
type tests = [Expect<Equal<typeof arrOrString, string[]>>];
}
});

doNotExecute(async () => {
function makeArray<Item>(input: Item | ReadonlyArray<Item> | Array<Item>) {
if (Array.isArray(input)) {
return input;
}
return [input];
}

const [first] = makeArray([{ a: "1" }, { a: "2" }, { a: "3" }] as const);

// No error!
first.a;
});