Functions

Optional and Default Parameters

Optional Parameters

In TypeScript, every parameter is assumed to be required by the function. You can add a ? at the end of a parameter name to set it as optional.

For example, the lastName parameter of this function is optional:

function buildName(firstName: string, lastName?: string) {
    // ...
}

Optional parameters must come after all non-optional parameters:

function buildName(firstName?: string, lastName: string) // Invalid

Default Parameters

If the user passes undefined or doesn’t specify an argument, the default value will be assigned. These are called default-initialized parameters.

For example, “Smith” is the default value for the lastName parameter.

function buildName(firstName: string, lastName = "Smith") {
    // ...
}
buildName('foo', 'bar');      // firstName == 'foo', lastName == 'bar'
buildName('foo');             // firstName == 'foo', lastName == 'Smith'
buildName('foo', undefined);  // firstName == 'foo', lastName == 'Smith'

Types of Functions

Named functions

function multiply(a, b) {
    return a * b;
}

Anonymous functions

let multiply = function(a, b) { return a * b; };

Lambda / arrow functions

let multiply = (a, b) => { return a * b; };

Function as a parameter

Suppose we want to receive a function as a parameter, we can do it like this:

function foo(otherFunc: Function): void {
    ...
}

If we want to receive a constructor as a parameter:

function foo(constructorFunc: { new() }) {
    new constructorFunc();
}

function foo(constructorWithParamsFunc: { new(num: number) }) {
    new constructorWithParamsFunc(1);
}

Or to make it easier to read we can define an interface describing the constructor:

interface IConstructor {
    new();
}

function foo(contructorFunc: IConstructor) {
    new constructorFunc();
}

Or with parameters:

interface INumberConstructor {
    new(num: number);
}

function foo(contructorFunc: INumberConstructor) {
    new contructorFunc(1);
}

Even with generics:

interface ITConstructor<T, U> {
    new(item: T): U;
}

function foo<T, U>(contructorFunc: ITConstructor<T, U>, item: T): U {
    return new contructorFunc(item);
}

If we want to receive a simple function and not a constructor it’s almost the same:

function foo(func: { (): void }) {
    func();
}

function foo(constructorWithParamsFunc: { (num: number): void }) {
    new constructorWithParamsFunc(1);
}

Or to make it easier to read we can define an interface describing the function:

interface IFunction {
    (): void;
}

function foo(func: IFunction ) {
    func();
}

Or with parameters:

interface INumberFunction {
    (num: number): string;
}

function foo(func: INumberFunction ) {
    func(1);
}

Even with generics:

interface ITFunc<T, U> {
    (item: T): U;
}

function foo<T, U>(contructorFunc: ITFunc<T, U>, item: T): U {
    return func(item);
}

Functions with Union Types

A TypeScript function can take in parameters of multiple, predefined types using union types.

function whatTime(hour:number|string, minute:number|string):string{
    return hour+':'+minute;
}

whatTime(1,30)         //'1:30'
whatTime('1',30)       //'1:30'
whatTime(1,'30')       //'1:30'
whatTime('1','30')     //'1:30'

Typescript treats these parameters as a single type that is a union of the other types, so your function must be able to handle parameters of any type that is in the union.

function addTen(start:number|string):number{
    if(typeof number === 'string'){
        return parseInt(number)+10;
    }else{
        else return number+10;
    }
}

Remarks

Typescript documentation link for Functions