# functions define and calling a function

1) //defining a function
function <function-name>(){
// code to be executed
};

//calling a function
<function-name>();

Example: Define and Call a Function

//defining a function
function ShowMessage() {
alert("Hello World!");
}

ShowMessage(); //calling a function

2) A function can return another function in JavaScript.

Example: Function Returning a Function
function multiple(x) {

function fn(y)
{
return x * y;
}

return fn;
}

var triple = multiple(3);
triple(2); // returns 6
triple(3); // returns 9

3) Type of function:----
1. Regular function

// Function declaration: starts with "function"
function multiply(x, y){
return x * y;
}

// Function expression: starts with "var,let,const"

JavaScript allows us to assign a function to a variable and then use that variable as a function. It is called function expression.

var add = function sum(val1, val2) {
return val1 + val2;
};

var result1 = add(10,20);

An easy to remember rule: the function declaration in a statement always starts with the keyword function. Otherwise it’s a function expression

2. Anonymous Function

a function without any name.This unnamed function is called anonymous function. Anonymous function must be assigned to a variable.

example:--
var showMessage = function (){
alert("Hello World!");
};

showMessage();

3.Nested Functions

Inner function can access variables and parameters of outer function. However, outer function cannot access variables defined inside inner functions.
example:---
function ShowMessage(firstName)
{
function SayHello() {
alert("Hello " + firstName);
}

return SayHello();
}

ShowMessage("Steve");

4. Arrow Function OR Function Shorthand methods:
Example:--
a) var double = (value) => {return value * 2;} //multiple line then use return
b) var double = (value) => value * 2;
b) var double = (value) => value * 2; //for use single line

4. Generator Function:
Example:----
function * generatorFunction() {
yield '1';
yield '2';
yield '3';
yield '4';
yield '5';
}
var generatorObject = generatorFunction();
console.log(generatorObject.next().value);
console.log(generatorObject.next().value);
console.log(generatorObject.next().value);

5. Function with: new Function

var sum = new Function('a', 'b', 'return a + b');

var add = function(a, b){
return a + b;
}