FirstParameter type

Reads the type of the first parameter of a function, such as the event of a handler.

import type { FirstParameter } from 'utils';

Signature

export type FirstParameter<F extends (...arguments_: readonly never[]) => unknown> = Parameters<F>[0];

Parameters

Name	Description
F	The function type.

Example

type Event = FirstParameter<typeof onResize>; // ResizeObserverEntry

Tests

Line coverage: 0 %.

• ✅ reads the first parameter

Source

src/types/first-parameter.ts (on GitHub)

Source code

/**
 * Reads the type of the first parameter of a function, such as the event of a handler.
 *
 * @template F - The function type.
 * @example
 * type Event = FirstParameter<typeof onResize>; // ResizeObserverEntry
 */
export type FirstParameter<F extends (...arguments_: readonly never[]) => unknown> = Parameters<F>[0];

Test file

import type { FirstParameter } from './first-parameter';

describe('FirstParameter', () => {
  it('reads the first parameter', () => {
    expectTypeOf<FirstParameter<(event: MouseEvent, index: number) => void>>().toEqualTypeOf<MouseEvent>();
    expectTypeOf<FirstParameter<() => void>>().toEqualTypeOf<undefined>();
  });
});

Keywords: first, parameter, types, type