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Type Alias: TypePredicate()<T, U>

TypePredicate<T, U> = (value) => value is U

Defined in: predicate/types.ts:47

A function that takes a value and returns a type predicate.

Type predicates enable TypeScript to narrow types in control flow.

Type Parameters​

T​

T

The input type

U​

U extends T

The narrowed type (must extend T)

Parameters​

value​

T

Returns​

value is U

Example​

const isString: TypePredicate<unknown, string> = (value): value is string =>
  typeof value === 'string'

const value: unknown = 'hello'
if (isString(value)) {
  // TypeScript knows value is string here
  console.log(value.toUpperCase())
}