What is Javascript closure?

I have been asked this question many times and have learned about it more times than i can think.

Lets see what I have learned from various sources about the concept of closures.

function outer() {
  var name = "World"
  function inner() {
    // access to the name variable is the facilitated
    // by closure.
    console.log(`Hello ${name}`)
  }
  inner()
}

outer() // Hello World

In the above code there is no name variable declared inside the inner function block. It can still access the variable. Why is that?

Accessing the name variable is done via lexical scoping. Dont sweat on the word lexical. Nested functions have access to variables declared in their outer scope. That is closure.

function userName() {
  --------------------Lexical scope-----------------
  |  var name = "World"                            |
  | function printName() {                         |
  |    console.log(`user name is ${name}`)         |
  |  }                                             |
  |  printName()                                   |
  --------------------Lexical scope-----------------
}

userName() // user name is World

Get your hands on the closure. Try it yourself

How can you use it? or What is the use for it?

Lets say you want to write a function that add 2 to a given integer argument. Simple enough!

function add2To(number) {
  console.log(2 + number)
}

Then we have requirement to create a function that can add 5 to a given integer argument. Now that is just same code over and over again. Lets see how closure can make it interesting.

function addXToY(x) {
  // x is the number you want to set 2,5 or any other
  function sum(y) {
    // y is the second number that needs to be added
    // to x.
    console.log(x + y)
  }
  return sum
}

var add2 = addXToY(2)
add2(10) // output on console = 12

var add5 = addXToY(5)
add5(10) // output on console = 15

I hope the accessing of the outer variables which are stored in some lexical scope for nested function to use is clear enough and that is closure.