Truthy or Falsy Value

In Javascript, Truthy means true while it is using as a boolean purpose. And Falsy means false as a boolean purpose. Except few values all values are truthy. The falsy values are 0, "", null, false, NaN, Undefined Few confusion show in below:

var a = 0;
if(a){
 console.log("This is truthy");
}else{
console.log("This is falsy");
}

The above code executes This is falsy because a=0 is falsy value.

var a = "";
if(a){
 console.log("This is truthy");
}else{
console.log("This is falsy");
}

The above code also executes This is falsy because a="" is falsy value.

var a = " ";
if(a){
 console.log("This is truthy");
}else{
console.log("This is falsy");
}

The above code also executes This is truthy notice in a declaration and assigned a string White space. " " this is a truthy value.

Null Vs Undefined

Undefined is a datatype. It means a variable declares but the value is not assigned and Null is a value that can assign in a variable. The example is shown below

var hello; //variable declare but value not assigned
console.log(hello); // print here undefined

another way to get undefined

var hello = "hi"; //value assigned hi
console.log(hello);
hello = undefined; //undifined assign to hello
console.log(hello); 

hello = null;
console.log(hello); // print here null

Double equal (==) Vs Triple equal (===)

Double equal and triple equal use to compare two values.

Difference here

Triple equal (===) not only compare value but also variable datatype also. While both are the same then it is true.

var a = 77;
var b = '77';
console.log(a===b); // its false, values are same but datatype

var a =77;
var b=77;
console.log(a===b); //its true

var a = 'hello';
var b = 'hello';
console.log(a===b) //its true,

Double Equal only compare value don't datatype.

var a = 77;
var b = '77';
console.log(a==b); // its true, values are same 

Scope

Scope means a variable accessible range.

Global Scope

If a variable declares in global scope it can access anywhere. The variable is accessible to function scope and block scope.

var name= 'khan'
console.log(name);        //khan

function getName(){
    console.log(name);    //name is accessible here
}

Block Scope

It means a variable is accessible to a block likes if, switch, for, while, etc. Block means an area of code that start and end Carly brace {blcok scope area}

function getName(){
    let name1= 'khan';        //exist in function and block
    if(true){
        let name2= 'momin';     //exist in block scope

    }
    console.log(name1); //it is in the block
    console.log(name2);  //it is out of its block
}

Clouser

Closure definition is given by Douglas Crockford :

Closure means that an inner function always has access to the vars and parameters of its outer function, even after the outer function has returned.

function outerFunction(){
 var a = 5;
 function innerFunction(){
  console.log(a);
 }
return innerFunction; //here return the reference of the innerFunction
}

var innerF = outerFunction(); //innerF assigned reference of innerFunction.
innerF(); //call here innerFunction and print 5

The above code outerFunction() return the reference of innerFuncion(). Though innerFunction() is in the outerFunction() block scope. But it can access out of the outerFunction() block. This is clouser.