Understanding Closure In Javascript

Whenever we declare a variable, it is immediately given a scope. The scope which belongs to a variable depends to where it was declared on the code.

<?php
$foo = 'bar';
function printFooValue() {
    echo($foo);
}
printFooValue(); /* produce error 'Undefined variable' */

If we run the example, it will throw 'undeclared variable' error. Therefore we can conclude that whenever we make a function, it will make its own scope isolated from outside the function.

This behavior can be easily found in many other programming languages such as C, Java, or Ruby. However scope in Javascript behave differently.

var foo = 'bar';
function printFooValue() {
    console.log(foo);
}
printFooValue(); // produce output 'bar'

If we run example above, it will print out 'bar'. This shows us that even though we declare the variable outside a function, it is still recognized inside the function after we call out the function. Now look at the example below.

function printFooValue() {
    var bar = 'foo';
    console.log(bar);
}
printFooValue() // 'bar'
console.log(bar) // throw error 'Undefined variable'

Variable 'bar' which is declared inside a function is not recognized outside the function so it will throw an unrecognized variable error. Therefore we find that whenever we make a function in Javascript, it will create its own scope and a snapshot of enviroment outside. This particular scoping behavior is popularly known as lexical scoping.

Closure

function multiplyBy(x) {
    function multipleByX(y) {
        return x * y;
    }
    return multipleByX;
}
var multiplyBy2 = multiplyBy(2);
var multiplyBy10 = multiplyBy(10);
console.log(multiplyBy2(3)); // 6
console.log(multiplyBy10(5)); // 50

In programming languages which do not support lexical scoping, local variable will be erased after function's call. But it is not applied in Javascript. In example above, x can still be accessed by multipleByX function despite multipleBy has been invoked. This happens because multipleByX still save a scope outside of it. This concept is known as a closure.

Closures are functions that refer to independent (free) variables (variables that are used locally, but defined in an enclosing scope). In other words, these functions 'remember' the environment in which they were created. - Mozilla Developer Network

Private Method in Javascript

We already know that Javascript has no OOP concept. But we can still emulate a private method by utilize closure. Look at example below:

function Giraffe() {
    var eatingObject = 'leaves';

    function eat() {
        console.log('Giraffe is eating ' + place);
    }

    return {
        eating: function() {
            eat();
        },
        changeFood: function(food) {
            eatingObject = food;
        }
    };
}
var babyGiraffe = Giraffe();
babyGiraffe.eat(); // error 'Undefined property'
babyGiraffe.eatingObject; // error 'Undefined property'
babyGiraffe.eating(); // output 'Giraffe is eating leaves'
babyGiraffe.changeFood('french fries');
babyGiraffe.eating(); // output 'Giraffe is eating french fries'

Giraffe function return an object which contains two properties, eating and changeFood. Because of lexical scoping, these two properties save scope from Giraffe which are eatingObject and eat function. Therefore when babyGiraffe.eating and babyGiraffe.changeFood were called, they can still be accessed. The example above also shows that eatingObject and eat can only be accessed via properties which were returned by Giraffe function. This kind of model is often be used to avoid global scope pollution, and is popularly known as module pattern.

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Lexical scoping and closure are fundamental foundation of Javascript programming. The funny thing is, back in the days I actually often applied these kind of concept without knowing how it worked. Thus, I think I should write more about fundamental Javascript in the future.