Typescript Function

Functions

Use type annotations for function parameters and return type to keep the calling code inline and ensure the type checking within the function body.

function add(a: number, b: number): number {
    return a + b;
}

Types

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

let add: (a: number, b: number) => number =
    function (x: number, y: number) {
        return x + y;
    };

Optional Parameters

Use the parameter? type syntax to make a parameter optional.

Use the expression typeof(parameter) !== 'undefined' to check if the parameter has been initialized.

function multiply(a: number, b: number, c?: number): number {

    if (typeof c !== 'undefined') {
        return a * b * c;
    }
    return a * b;
}

Default Parameters

1. Use default parameter syntax parameter:=defaultValue if you want to set the default initialized value for the parameter.

2. Default parameters are optional.

3. To use the default initialized value of a parameter, you omit the argument when calling the function or pass the undefined into the function.

function applyDiscount(price: number, discount: number = 0.05): number {
  // ...
}

function applyDiscount(price: number, discount?: number): number {
  // ...
}

Rest Parameters

in this tutorial, you will learn about the TypeScript rest parameters that allows you to represent an indefinite number of arguments as an array.

function getTotal(...numbers: number[]) : number {
    let total = 0;
    numbers.forEach((num) => total += num);
    return total;
}

Function Overloadings

TypeScript function overloadings allow you to describe the relationship between parameter types and results of a function.

function add(a: number, b: number): number;
function add(a: string, b: string): string;
function add(a: any, b: any): any {
   return a + b;
}