Boolean as a type guard #31164

Search Terms

boolean type guard

Suggestion

A common idiom in JavaScript is to filter an array using Boolean as a filter function to remove falsy values. To make this useful in TypeScript, Boolean should act as a type guard:

+type Falsy = false | 0 | '' | null | undefined;

interface BooleanConstructor {
  new (value?: any): Boolean;
- (value?: any): boolean;
+ <A>(value: A | Falsy): x is A;
  readonly prototype: Boolean;
}

Examples

// The type of `numbers` should be `number[]` but currently it is `(number | undefined)[]`.
const numbers = [1, 2, undefined, 3].filter(Boolean);

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code (I think this is true?)
• 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.