JavaScript - What is closure?

When a function is created using a variable in its outer function scope, the variable is stored as closure.

function functionHoldingVar() {
  var closureCandidate = 'Hello Geeks';
  return function() {
    console.log(closureCandidate);
  };
}
var closureFunc = functionHoldingVar();
//closureCandidate no more available in scope
//It is stored in Closure scope
closureFunc();//prints 'Hello Geeks'

Please see below a snapshot of variables when a breakpoint is placed.

The value of variable in closure does not depend on when the function is created rather when the function is executed. Below example would explain this.

var globalObj = {};
function returnFunctionWithClosure() {
  //store globalObj in local var
  var localRef = globalObj;
  localRef.test = 'Function Scope';
  return function() {
    //localRef is stored in closure
    console.log(localRef.test);
  };
}
var closureWithGlobalRef 
        = returnFunctionWithClosure();
//Changing the value in global scope
globalObj.test = 'Global Scope';
//As globalObj and localRef points to same object
closureWithGlobalRef(); //prints 'Global Scope'

That is all about closure.