A function is a block of code which can be re-used multiple times. Beauty of functions is that they allow the code to call the functions multiple times and hence avoiding repetition of code.

How to declare a function

Syntax

function function-name(parameters){ // here parameters are optional
    //code
}

How to call a function

Syntax

function-name(parameters);

Examples

1. Function with out parameters

function greetings(){
    console.log('Happy learning!!');
}
greetings();
greetings(); // You can call the function as many times you want

check the result here

2. Function with parameters

function greetings(name){
    console.log('Hi ' + name + '! Happy learning!!');
}
greetings('foo');
greetings('bar'); 

check the result here

3. Function with parameters having default values

If parameters are not provided while calling the function, then it's value becomes undefined. If you want to default the parameter's values to some default text, then you can do this way.

Case : 1

If you are not providing any parameter then it's value becomes undefined.

function setName(name){ 
	return name;
};
console.log(setName()); // undefined
console.log(setName("foo")); // foo

Case : 2

If you want to default a value to a parameter, here is how you can do it.

function setName(name = "NA"){ // defaulting values to parameters
	return name;
};
console.log(setName()); // NA
console.log(setName("foo")); // foo

Case : 3

Below example helps you understand the difference between parameter value defaults and object literal defaults.

func = function({x = 10,y = 11,z = 12} = {x:1,y:2,z:3}) {
    console.log(x,y,z);
};

func(); //1 2 3  (hits the object literal default)
func({}); //10 11 12   (hits the value defaults)

Check the result here

4. Function Expressions

You can assign function to a variable explictly. In Javascript, function is a value and hence you can assign it to a variable like any other values.

Examples

let message = function () {
    console.log('Happy Learning!');
}

You can copy function to an another variable.

function greetings() {
    console.log('Happy Learning!');
}
let message = greetings; // if there are parameters to the function then you should specify greetings();

message(); // prints Happy Learning!
greetings(); // prints Happy Learning!

Quick take away:

• A function can be assigned to a variable.
• You can default values to the parameters of a function
• A function is created predominently for re-usability.