How and why to avoid global variables in JavaScript

global-variables-are-bad.js

// It is important to declare your variables.

(function() {
    var foo = 'Hello, world!';
    print(foo);  //=> Hello, world!
})();

// Because if you don't, the become global variables.

(function() {
    foo = 'Hello, world!';
    print(foo)  //=> Hello, world!
})();

print(foo)  //=> Hello, world!


// When global variables sneak into your code the can cause problems.
// Especially in applications with concurrency.

var count = function() {
    for (i = 0; i < 10; i += 1) {
        print(i);
    }
};

count();  //=> 0 1 2 3 4 5 6 7 8 9

var countSilently = function() {
    for (i = 0; i < 10; i += 1) {
        // don't print anything;
    }
};

// Both loops increment i at the same time, which causes strange behavior.
window.setTimeout(countSilently, 10);
window.setTimeout(count,         10);  //=> 2 3 7 8 9


// You can use 'this' in method definitions to refer to attributes of the
// method's object.

var obj = {
    name: 'foo',
    introduce: function() {
        print(this.name);
    }
};

obj.introduce();  //=> foo

// But 'this' does not follow the normal rules of scope in JavaScript. One
// might expect 'this' to be available with the same value via closure in the
// callback defined inside the method here.

var obj = {
    name: 'foo',
    introduce: function() {
        window.setTimeout(function() {
            print(this.name);
        }, 3000);
    }
};

obj.introduce(); //=> *pause* undefined

// In fact, this got bound to the global object in the callback. To get around
// this, assign the object reference to a regular variable that will have the
// same value inside the callback definition.

var obj = {
    name: 'foo',
    introduce: function() {
        var that = this;
        window.setTimeout(function() {
            print(that.name);
        }, 3000);
    }
};

obj.introduce();  //=> *pause* foo


// The keyword 'this' is actually dynamically assigned whenever a function is
// invoked. When a function is invoked as a method, i.e. obj.method(), 'this'
// is bound to 'obj'. But when a function is invoked by itself 'this' is bound
// to the global object.

var phrase = 'Hello, world!';
var printPhrase() {
    print(this.phrase);
}

printPhrase();  //=> Hello, world!

// This is true even of functions that were defined as a method.

var obj = {
    name: 'foo',
    introduce: function() {
        print(this.name);
    }
};

// When the function is invoked without 'obj.' in front of it, 'this' becomes
// the global namespace.

var introduce = obj.introduce;
introduce();  //=> undefined


// Method invocation and function invocation are two of the invocation patterns
// in JavaScript. A third is apply invocation, which gives us control over what
// 'this' will be assigned to during function execution.

introduce.apply(obj, null);  //=> foo

// 'apply' is a method on Function. The first argument is the value that 'this'
// will be bound to. Successive arguments to apply are passed as arguments to
// the function that is being invoked.

var chatty = function(repeatTimes) {
    var i;
    for (i = 0; i < repeatTimes; i += 1) {
        print(this.name + ' ');
    }
}
chatty.apply(obj, 3)  //=> foo foo foo


// The fourth and final invocation pattern in JavaScript is constructor
// invocation. This pattern was designed to provide a way to create new objects
// that would appear familiar to programmers who are used to programming with
// classes.

var Cat = function(name) {
    this.name = name;
};
Cat.prototype = {
    query: function() {
        print(this.name + ' says, "meow"');
    }
};

// When a function is called with the 'new' keyword in front of it, a new
// object is created and is bound to 'this' when the function runs. Special
// constructor functions use this feature to customize new objects as they are
// created.

var whiskers = new Cat('whiskers');
whiskers.query();  //=> whiskers says "meow"

// When a new object is created with 'new', the prototype of the new object is
// set to the prototype of the constructor function. So the new object inherits
// all of the attributes of the constructor's prototype value. In this case,
// new cat objects inherit the 'query' method from Cat.prototype.

var nibbler = new Cat('nibbler');
nibbler.query();  //=> nibbler says "meow"


// If a constructor function is called without the 'new' keyword, it is invoked
// with the ordinary function invocation pattern.

var gotcha = Cat('gotcha!');
gotcha.query();  //=> typein:165: TypeError: gotcha has no properties

// So 'this' is assigned to the global object instead of to a newly created object. That means that any attributes assigned to the new object by the constructor function become global variables!

print(name);  //=> gotcha!


// Constructor invocation is pretty complicated and prone to disastrous global
// variable creation. Here is a cleaner way to create new objects that inherit
// from other objects.

// This defines Object.create, a method that simplifies the behavior of the
// 'new' keyword. This method was invented by Douglas Crockford.
// http://javascript.crockford.com/prototypal.html
if (typeof Object.create !== 'function') {
    Object.create = function(o) {
        var F = function() {};
        F.prototype = o;
        return new F();
    };
}

// Object.create(obj) returns a new object that inherits all of the attributes
// of obj. The 'cat' prototype object here defines a 'clone' method that wraps
// around Object.create to customize new 'cat' objects as they are created.

var cat = {
    query: function() {
        print(this.name + ' says "meow"');
    },
    clone: function(name) {
        var newCat = Object.create(this);
        newCat.name = name;
        return newCat;
    }
};

var fluffy = cat.clone('fluffy');
fluffy.query();  //=> fluffy says "meow"

// In addition to inheriting 'query', new cats also inherit 'clone'.

var fluffy2 = fluffy.clone('fluffy2');
fluffy2.query();  //=> fluffy2 says "meow"

// Methods and attributes are inherited, not copied. If you change the
// definition of 'clone' on 'cat' at this point, the change will be reflected
// in cat objects that have already been created.

fluffy2.hasOwnProperty('clone')  //=> false
fluffy.hasOwnProperty('clone')  //=> false
cat.hasOwnProperty('clone')  //=> true

It's one of the best explanations I found about the different invokation patterns :) ... I'll put a link on my recent blog entry about globals and JavaScript :)

great post, the examples make a big difference in learning!

great tutorial !

👍

Fantastic.

Thanks for post, great tutorial.

Yes, I agree with others: really great.

Loud and clear.

Why did you use "print" keyword?
It does not output text, it invokes the actual printer from the browser. I didn't test it on browsers from 2009 though.

wow, what a bullshit: https://gist.github.com/hallettj/64478#file-global-variables-are-bad-js-L18-L37

Thanks. Gr8 tutorial

Thanks. Great Summary.

👍

Still a great breakdown! Thanks

Simple to Understand ,Thanks

Clear and deep. Thanks.

Great Stuff

Great Summary.

Excellent summary of how globals can trip you up!

A minor correction: on line 128, when calling chatty.apply(obj, 3), the second argument should be an array with 3 as the only element. If the function is executed as written, you will get Uncaught TypeError: CreateListFromArrayLike called on non-object. However, if you used call() instead of apply(), then you could execute it as is, because call() takes the arguments as a list instead of an array.

// Both loops increment i at the same time, which causes strange behavior.
window.setTimeout(countSilently, 10);
window.setTimeout(count,         10);  //=> 2 3 7 8 9

No, they don't :) Javascript always works in one thread. countSilently is called the first, count is called as soon as countSilently is finished. You can try it in a console of your browser.

Great Examples

I agree with @mrThomasTeller.

When you build SPA/PWA you will not be able to avoid global variables because you need acccess to your objects which you cannot achieve from you isolated IIFE, or use classes as flags, for instance.

I think @mrThomasTeller is right, in browser's console, use console.log to prove it.

Hello there,
Another solution is a to use a dependency injection framework,
I coded di-ninja for this
It implements the Composition-Root design pattern and work very well for server and browser

// Both loops increment i at the same time, which causes strange behavior. window.setTimeout(countSilently, 10); window.setTimeout(count, 10); //=> 2 3 7 8 9
Have a look at https://www.youtube.com/watch?v=8aGhZQkoFbQ to understand how js works.

Excellent summary!