Is It Bad Practice Declaring Global Variables In A.js File?

I have a .js file where I am initialising two parameters which are used in a seperate function : var submyvar1; var submyvar2; function init(myvar1 , myvar2){ submyvar1= myva

Solution 1:

At least it's not a good practice, you could use an immediated invoked function expression:

(function() {
    var x;
    var y;

    window.init = function(xValue, yValue) {
        x = xValue;
        y = yValue;
    }

    window.doWhateverYouWant = function() {
        console.log(x + y);
    }
}());

You could use this pattern unless you need access your global variable outside this .js file, when an cross-accessing between .js files or <script> elements are required, global variable is a simple solution (although you could use AMD or sth else instead).

Solution 2:

I decided to use a revealing module pattern as described at : Nice article here on a possible implementation using the 'Revealing Module Pattern' : http://enterprisejquery.com/2010/10/how-good-c-habits-can-encourage-bad-javascript-habits-part-1/

Its very similar to accepted answer.

Here is pattern explained pasted from above enterprisejquery :

We looked at the Self-Executing Anonymous Function earlier as a pattern you could use to keep all your code private. As it turns out, you can actually modify the pattern somewhat so that you can achieve the same benefits of the Revealing Module Pattern. Not only can we achieve public and private properties and methods, but we can also provide an easy way to extend the namespace while keeping the content protected from the global namespace. In addition, the following pattern can protect the $ from conflicting with other JavaScript libraries and also protect undefined from being redefined.

Take a look at the following example, and we will walk through the code explaining the key changes to the pattern.


//Self-Executing Anonymous Func: Part 2 (Public & Private)
(function( skillet, $, undefined ) {
    //Private Property
    var isHot = true;

    //Public Property
    skillet.ingredient = "Bacon Strips";

    //Public Method
    skillet.fry = function() {
        var oliveOil;

        addItem( "\t\n Butter \n\t" );
        addItem( oliveOil );
        console.log( "Frying " + skillet.ingredient );
    };

    //Private Method
    function addItem( item ) {
        if ( item !== undefined ) {
            console.log( "Adding " + $.trim(item) );
        }
    }    
}( window.skillet = window.skillet || {}, jQuery ));

//Public Properties
console.log( skillet.ingredient ); //Bacon Strips

//Public Methods
skillet.fry(); //Adding Butter & Fraying Bacon Strips

//Adding a Public Property
skillet.quantity = "12";
console.log( skillet.quantity ); //12

//Adding New Functionality to the Skillet
(function( skillet, $, undefined ) {
    //Private Property
    var amountOfGrease = "1 Cup";

    //Public Method
    skillet.toString = function() {
        console.log( skillet.quantity + " " + 
                     skillet.ingredient + " & " + 
                     amountOfGrease + " of Grease" );
        console.log( isHot ? "Hot" : "Cold" );
    };    
}( window.skillet = window.skillet || {}, jQuery ));

try {
    //12 Bacon Strips & 1 Cup of Grease
    skillet.toString(); //Throws Exception
} catch( e ) {
    console.log( e.message ); //isHot is not defined
}
You can execute and modify the above code from jsFiddle.

First, since we have a Self-Executing Anonymous Function, we can actually provide some parameters to pass to it when it executes. In this case we are passing 2 arguments to the anonymous function.

The first argument looks quite strange. What is window.skillet = window.skillet || {} doing? The code checks to see if skillet exists in the global namespace (window). If it does not exist, then window.skillet is assigned an empty object literal. Using this approach we can build a library across JavaScript files. If another script uses the same technique, then it will pass in the existing instance and append logic to it. Inside the Anonymous Function, if we want something to be public, then we append it to the skillet object. Any other properties or methods will be considered private.

The second argument passed in jQuery. The benefit of this is that the named parameter is referenced as $, which allows us to refer to jQuery as $ within the Anonymous Function without having to worry that it will conflict with the $ declared in other JavaScript libraries. This is a common practice that you will most likely run across when looking at well written jQuery code.

You might notice a 3rd parameter to the Anonymous Function called undefined. Why did we add that parameter and why didn’t we pass an argument to it? In JavaScript, you can unfortunately redefine what undefined means. Imagine that some code somewhere deep in one of your 3rd party libraries redefines undefined to something strange like true. If anywhere in your code you test against undefined, then you code will most likely not behave like you intended. In JavaScript, if you have a parameter that doesn’t have a matching argument, then it’s value is set as undefined. By using this trick, it can save us from the bad code someone wrote to redefine undefined.

Pros

Gives you the ability to have public and private properties and methods
The code inside doesn’t use the Object Literal notation
Keeps undefined’s value as undefined in case someone overrode the value
Allows you to use $ inside your code without worrying about clashing with other libraries
Allows your library to grow across files using the “window.namespace = window.namespace || {}” technique
A common pattern that you will see in many libraries, widgets, and plugins
Cons

Slightly more complicated pattern, but not overly difficult to understand
If you are interested in digging deeper into some of the patterns I mentioned above then I encourage you to check out Klaus Komenda’s post entitled JavaScript Programming Patterns. The article provides an insightful view of how JavaScript patterns have changed over the years. 

Solution 3:

You should avoid it wherever possible to prevent cluttering up the global namespace. There'll probably be bits of your code you want to expose though, so in that case it's fine. For example, jQuery obviously has to make the jQuery and $ variables global (it does that by making them properties of window.

To avoid it, you could include your code in a function with brackets following it:

var mySingleGlobalVar = (function () {
    var submyvar1;
    var submyvar2;

    function init(myvar1 , myvar2){
        submyvar1= myvar1;
        submyvar2= myvar2;
    }

    function (){
        //subvar1 & subvar 2 used here
    }

    return init;
}());

This is a function which executes immediately. In this example, I've allocated the return value (the init function) to a global var, and this is the only global var your script will create (as long as you declare them all properly with var).

Solution 4:

In JavaScript: The Good Parts (also summarized here) it's suggested that :

Global variables weaken the resiliency of programs and should be avoided. One way to minimize the use of global variables is to create a single global variable for your application:

var MYAPP = {};

That variable then becomes the container for your application:

MYAPP.stooge = {
    "first-name": "Joe",
    "last-name": "Howard" };

By reducing your global footprint to a single name, you significantly reduce the chance of bad interactions with other applications, widgets, or libraries. Your program also becomes easier to read because it is obvious that MYAPP.stooge refers to a top-level structure.