Closure 

A closure is an inner function that has access to the outer (enclosing) function’s variables—scope chain. The closure has three scope chains: it has access to its own scope (variables defined between its curly brackets), it has access to the outer function’s variables, and it has access to the global variables.

You create a closure by adding a function inside another function.
A Basic Example

function showName (firstName, lastName) {

​var nameIntro = "Your name is ";
    // this inner function has access to the outer
// function's variables, including the parameter​
​function makeFullName () {
        
​return nameIntro + firstName + " " + lastName;
    
}
​
​return makeFullName ();

}

​
showName ("Michael", "Jackson"); // Your name is Michael Jackson


Closures have access to the outer function’s variable even after the outer function returns:

function celebrityName (firstName) {
    var nameIntro = "This celebrity is ";
    // this inner function has access to the outer function's variables, including the parameter​
   function lastName (theLastName) {
        return nameIntro + firstName + " " + theLastName;
    }
    return lastName;
}
​
​var mjName = celebrityName ("Michael"); // At this juncture, the celebrityName outer function has returned.​
​
​// The closure (lastName) is called here after the outer function has returned above​
​// Yet, the closure still has access to the outer function's variables and parameter​
mjName ("Jackson"); // This celebrity is Michael Jackson


Closures store references to the outer function’s variables

function celebrityID () {
    var celebrityID = 999;
    // We are returning an object with some inner functions​
    // All the inner functions have access to the outer function's variables​
    return {
        getID: function ()  {
            // This inner function will return the UPDATED celebrityID variable​
            // It will return the current value of celebrityID, even after the changeTheID function changes it​
          return celebrityID;
        },
        setID: function (theNewID)  {
            // This inner function will change the outer function's variable anytime​
            celebrityID = theNewID;
        }
    }
​