What is Functions in TypeScript? How many types you defined in TypeScript?

A function is a set of statements that perform a specific task and used to create readable, maintainable and re-usable code.

A function declaration tells to compiler about a function name, return type, and parameters.

TypeScript functions are almost similar to JavaScript functions but there are different ways of writing functions in TypeScript.

Stayed Informed – Learn Angular 2 with TypeScript

Different types of functions are available in the TypeScript i.e.

1. Normal function

2. Anonymous Function

3. Named Function

4. Lambda Function/Arrow Function

5. Class Function

6. Optional Parameters

7. Rest Parameters

8. Default Parameters

Anonymous Functions–

An anonymous function is a function that was declared without any named identifier to refer to it.

Example - Normal function

function printHello() {
    console.log('Hello Anil!');
}

printHello();

Examples - Anonymous function

JavaScript -

var hello = function () {
    console.log('Hello Anil!, I am Anonymous.');
};

hello();//Return - Hello Anil!, I am Anonymous.

OR 

setTimeout(function () {
    console.log('Hello Anil!, I am Anonymous.');
}, 2000); //Return - Hello Anil!, I am Anonymous.

TypeScript–

var anonymousFunc = function (num1: number, num2: number): number {
    return num1 + num2;
}

//RESULT
console.log(anonymousFunc(10, 20)); //Return is 30

//RESULT
console.log(anonymousFunc(10, "xyz")); 
// error: Argument of type 'number' is not assignable to parameter of type 'string'.
//because return type is number for anonymous function).

Named Function -

The named function is very similar to the JavaScript function and only one difference - we must declare the type on the passed parameters.

Example – JavaScript

function addTwoNumer(num1, num2) {
    return num1 + num2;
}

Example – TypeScript

function addTwoNumer(num1: number, num2: number): number {
    return num1 + num2;
}

Lambda Function/Arrow Function -

The arrow function is additional feature in typescript and it is also known as a lambda function.

A lambda function is a function without a name.

var addNum = (n1: number, n2: number) => n1 + n2;

In the above, the “=>” is a lambda operator and (n1 + n2) is the body of the function and (n1: number, n2: number) are inline parameters.

For example –

let addNum = (n1: number, n2: number): number => { return n1 + n2; }
let multiNum = (n1: number, n2: number): number => { return n1 * n2; }
let dividNum = (n1: number, n2: number): number => { return n1 / n2; }

addNum(10, 2);// Result - 12
multiNum(10, 2);// Result - 20
multiNum(10, 2);// Result - 5

Optional Parameters Function -

We can specify optional properties on interfaces and the property may be present or missing in the object.

In the below example, the address property is optional on the following “User” interface.

For Example as,

//USER INTERFACE 
interface User {
    name: string;
    age: number;
    address?: string //Optional
}

//FUNCTION USING USER INTERFACE 
let userInfo = function(user: User) {
   let info = "Hello, " + user.name + " Your Age is - " + user.age + " and Address is -" + user.address;

   return info;
}

//USER INFO JSON OBJECT
let info = {
    name: "Anil",
    age: 30
};

//RESULT
console.log(userInfo(info));

Rest Parameters –

The Rest parameters do not restrict the number of values that we can pass to a function and the passed values must be the same type otherwise throw the error.

For Example as,

//Rest Parameters
let addNumbers = function(...nums: number[]) {
    let p;
    let sum: number = 0;

    for (p = 0; p < nums.length; p++) {
        sum = sum + nums[p];
    }

    return sum;
}

//The Result
addNumbers(1, 2);
addNumbers(1, 2, 3);
addNumbers(1, 12, 10, 18, 17);

Default Parameters -

Function parameters can also be assigned values by default.

A parameter can't be declared as optional and default both at the same time.

For Example as,

let discount = function (price: number, rate: number = 0.40) {
    return price * rate;
}

//CALCULATE DISCOUNT
discount(500); // Result - 200

//CALCULATE DISCOUNT
discount(500, 0.45); // Result - 225