Type predicates in filter

⬅️ Execute Program Advanced TypeScript

Type predicates in filter

• if we filter an Array<string | number> and just want to get the numbers out, the return type will still be (string | number)[]
• you can fix this with a type predicate e.g. isNumber

function isNumber(maybeNumber: number | string): maybeNumber is number {
  return typeof maybeNumber === 'number';
}
 
const numbersAndStrings: (number | string)[] = [1, 'a', 2, 'b'];
/* This assignment works because `filter` respects our `isNumber` type
 * predicate. This `filter` call narrows the array from (number |
 * string)[] to just number[]. */
const numbers: number[] = numbersAndStrings.filter(isNumber);
numbers;

• it does that by using generic constraints and function overloads

// ts filter definition
interface Array<T> {
  filter<S extends T>(
	// used when given a type predicate function as the callback
    predicate: (value: T) => value is S
  ): Array<S>
  filter(
    predicate: (value: T) => unknown
  ): Array<T>
}