What is Function Declaration

Also called function defination or function statement.

                
                    function square(num){
                        return num*num;
                    }
                
            

What is Function Expression

When a function is stored in a variable.

                
                    const square = function(num){
                        return num*num;
                    }
                
            

What is First Class Function.

Where a function can be treated like a variable, those functions are called first class function.
These functions can be passed into another functions can be manipulated and returned from those function. Basically a variable can do.
So functions are first class function in JS.

                
                    function square(num){
                        return num*num;
                    }

                    function displaySquare(fun){
                        console.log("square", fun(5));
                    }

                    displaySquare(square);
                
            

What is IIFE

Immediately invoked function expression

                
                    (function square(num){
                         console.log(num*num);
                    })(5);

                    (function (x){
                        return (function (y){
                            console.log(x); // 1
                        })(2);
                    })(1);
                
            

Function Scope

                
                    for(let i =0 ; i<5 ; i++){
                        setTimeout(function(){
                            console.log(i); // 01234
                        }, i*1000);
                    }
                    for(var i =0 ; i<5 ; i++){
                        setTimeout(function(){
                            console.log(i); // 44444
                        }, i*1000);
                    }
                
            

Function Hoisting

Functions are hoisted completely to the top of their scope.

                



    We have global scope of x.

    In local scope x is present.

    Hoisting is 2 step process:

    1. First is initialize the complete code. (init global scope.)

    2. Then it init local scope (first host this variable in top of this scope) So in this scope. x is present and is undefined right now.

    If we have variable in local scope we dont go to global scope.

                
            

Params vs argument

                
                    function square(num){ // num is params
                        return num*num;
                    }
                    square(5); // 5 is argument

                    

                    
                
            

Callback Function

A function passed into another function as an argument. Which is then invoked inside the outer function.
map, filter, settimeout, reduce

What is Function Expression