narrow array type with `.filter(Boolean)`

[mmkal]

Search Terms

narrow, array, filter

Suggestion

add this overload to Array<T>.filter:

filter(fn: typeof Boolean): Array<Exclude<T, null | undefined | 0 | ''>>

This may be considered too specific - but the use case is very common so hopefully worth it.

Use Cases

This will allow using array.filter(Boolean) to get rid of null/undefined/falsy values, and have the type system track it correctly even with strictNullChecks.

Right now the options are to use lodash.compact or write a similar function (not ideal to add a dependency), or to use ! (myArray.filter(Boolean).map(val => val!.foo(...))) which is a little dangerous.

Examples

Example

const maybeStrings = ['foo', null, 'bar']
const definitelyStrings = maybeStrings.filter(Boolean)

definitelyStrings is of type (string | null)[] even though it's impossible for nulls to be in it.

current behaviour:

proposed behaviour:

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code (I don't think so - types would become more specific)
• 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.

[jack-williams]

See #29955

[mmkal]

@jack-williams interesting, it'd be great if this could be solved by modifying the type of Boolean rather than filter. But I just tried manually putting in @ForbesLindesay 's signature in typescript@next and it doesn't allow using .filter(Boolean) without explicitly supplying the generic i.e. .filter<string>(Boolean). That's fine-ish for strings but when it's a complex inferred type it wouldn't be feasible.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[ForbesLindesay]

@mmkal That can be fixed, but will require improvements to the inference engine.