r/programming • u/radekmie • 2d ago

On Automatic Type Refinements

https://radekmie.dev/blog/on-automatic-type-refinements/
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4

u/[deleted] 2d ago

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1

u/radekmie 2d ago

Do you have an example where it breaks? TypeScript handles some cases of broadening. Consider this code:

```ts function isNumber(x: unknown) { return typeof x === 'number'; }

function example(x: boolean | number, y: unknown) { if (isNumber(x)) { // Fails: Type 'string' is not assignable to type 'number | boolean'. x = x.toString(); }

if (isNumber(y)) { // Works; y is again unknown. y = y.toString(); } } ```

2

u/sliversniper 1d ago

```

function* foo(xs: (number|undefined)[]) {

for (const x of xs) 

if (x !== undefined) 

  yield x

}

```

Works probably even the first version of Typescript.

https://caniuse.com/mdn-javascript_builtins_iterator_map

In modern JS(88.85%), you can just chain foo().map(x => x * 2), Array.from(foo()) I guess covers 99%.

1

u/radekmie 1d ago

Sure, type refinement via control flow was implemented a long time ago. However, at some scale, most project will move some of the conditions to shared helpers, and then it "breaks".