JavaScript Closure

Before everything, we need to know JavaScript has 2 types of variables (based on Variable Scope to categorize), global & local variable !!

Concept:

function having access to parent scope, especially function can use variable which is defined outside of the function!!!

Example:

var value = 0; // variable declared outside of function outputValue
function outputValue() {
  function plus () { value = value + 1 }; // value can be used inside the function
  plus();
  return value; // will return 1 as result
};
outputValue();

Another Example for scope of this:

var person = {
  name: 'damon',
  printPersonAttribute() {
    console.log('this.name: ', this.name); // damon
    function unableToPrintPersonAttribute() {
      console.log('nested this.name is not working: ', this.name); // undefined
    };
    unableToPrintPersonAttribute();
  },
}
person.printPersonAttribute();

How JS closure works

javascript engine does not find the variable in local scope, it tries to check for the variable in the outer scope. If the variable does not exist in the outer scope, it tries to find the variable in the global scope.