CODEWITHSUNDEEP
typescript
kimsagro
kimsagro

Reputation: 17425

How to apply a function type to a function declaration

Given the following type:

type Add = (x:number, y:number) => number

I can declare a const of that type and define the rest of the function without having to specify the types explicitly. Typescript knows that x is a number and y is a number.

const add: Add = (x, y) => x + y;

I prefer to define functions using function rather than const however there doesn't appear to be a way to type the complete function declaration. It appears that you are forced to define the parameters and the return type separately. Is there any way to use the Add type on the declaration below so that I don't have to specify that x is a number and y is a number

function add(x, y) {
    return x + y;
}

without being forced to do something like

function add(x: Parameters<Add>[0], y: Parameters<Add>[1]): ReturnType<Add> {
    return x + y;
}

Upvotes: 39

Views: 8533

Answers (3)

mb21
mb21

Reputation: 39508

For function expressions:

export type Add = (x: number, y: number) => number

export const add: Add = function add (x, y) {
  return x + y;
}

For function declarations:

export function add (x: number, y: number): number

export function add (x: any, y: any) {
  return x + y;
}

Upvotes: -1

Doug Coburn
Doug Coburn

Reputation: 2575

You can specify types for function expressions but not function declarations.

// index.d.ts
export function add(a: string, b: string): string;
export function add(a: number, b: number): number;

// index.js
/**
 * @typedef {import('./index.d.ts').add} AddFunction
 */

/**
 * @type AddFunction
 * @param {any} a
 * @param {any} b
 */
export const add = function(a, b) {
    return a + b;
}

add(1, 2);
add('a', 'b');
add(1, '2')  

❯ ./node_modules/.bin/tsc
index.js:16:1 - error TS2769: No overload matches this call.
  Overload 1 of 2, '(a: string, b: string): string', gave the following error.
    Argument of type 'number' is not assignable to parameter of type 'string'.
  Overload 2 of 2, '(a: number, b: number): number', gave the following error.
    Argument of type 'string' is not assignable to parameter of type 'number'.

16 add(1, '2');
   ~~~~~~~~~~~

Found 1 error in index.js:16

Playground Link

Upvotes: 0

Titian Cernicova-Dragomir
Titian Cernicova-Dragomir

Reputation: 250246

You can't specify types for function declarations. You can only use Parameters and ReturnType. The only minor improvement would be to use destructuring for the parameters:

type Add = (x:number, y:number) => number
function add(...[x, y]: Parameters<Add>): ReturnType<Add> {
    return x + y;
}

Playground Link

Upvotes: 24

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