Functions (1) - wiki.busyhe.com

Rest Parameters

https://www.typescriptlang.org/docs/handbook/functions.html#rest-parameters

this parameters

https://www.typescriptlang.org/docs/handbook/functions.html#this-parameters


function fn( this: void ) {
		// Property 'alert' does not exist on type 'void'.(2339)
    this.alert();
}

function fn2( this: Window ) {
    this.alert();
}

interface Person {
    age: number;
}

function fn3( this: Person ) {
    console.log( this.age );
}

// The 'this' context of type 'void' is not assignable to method's 'this' of type 'Person'.(2684)
fn3();

fn3.call( new class implements Person {
    age = 100;
} )


interface UIElement {
	addClickListener( onclick: ( this: void, e: Event ) => void ): void;
}

this: void means that addClickListener expects onclick to be a function that does not require a this type.


function add( this: { x : number }, y: number ): number {
    return this.x + 1;
}

// The 'this' context of type 'void' is not assignable to method's 'this' of type '{ x: number; }'.(2684)
add( 10 );

add.call( { x : 10 }, 10 );

add.bind( { x : 10 } )( 10 );

const obj = {
    x : 10,
    add
};

obj.add( 10 );

Overloads

https://www.typescriptlang.org/docs/handbook/functions.html#overloads

In order for the compiler to pick the correct type check, it follows a similar process to the underlying JavaScript. It looks at the overload list and, proceeding with the first overload, attempts to call the function with the provided parameters. If it finds a match, it picks this overload as the correct overload. For this reason, it's customary to order overloads from most specific to least specific.


function create( name: string ): boolean;
function create( name: boolean ): string;
function create( name: string | boolean ): boolean | string {
    if( typeof name === 'string' ) {
        return 'string';
    }
    return true;
}


ParameterType<create>;

// Type 'boolean' is not assignable to type 'string'.(2322)
const a: string = create( 'A' );

// Type 'string' is not assignable to type 'boolean'.(2322)
const b: boolean = create( true );

Note that the function create( name: string | boolean ): boolean | string piece is not part of the overload list, so it only has two overloads.

Literal Types


function create( name: 'A' ): boolean;
function create( name: 'B' ): string;
function create( name: string ): boolean | string {
		if( name === 'A' ) {
        return 'A';
    }
    return true;
}

// Type 'boolean' is not assignable to type 'string'.(2322)
const a: string = create( 'A' );

// Type 'string' is not assignable to type 'boolean'.(2322)
const b: boolean = create( 'B' );


// This overload signature is not compatible with its implementation signature.(2394)
function create( name: 'A' ): boolean;
function create( name: 'B' ): string;
function create( name: string ): number {
    return 1;
}