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how to avoid global variables in javascript


how to avoid global variables in javascript  using -'javascript,global-variables'

We all know that global variables are anything but best practice. But there are several instances when it is difficult to code without them. What techniques do you use to avoid the use of global variables?

For example, given the following scenario, how would you not use a global variable?

JavaScript code:

var uploadCount = 0;

window.onload = function() {
    var frm = document.forms[0];

    frm.target = "postMe";
    frm.onsubmit = function() {
        startUpload();
        return false;
    }
}

function startUpload() {
    var fil = document.getElementById("FileUpload" + uploadCount);

    if (!fil || fil.value.length == 0) {
        alert("Finished!");
        document.forms[0].reset();
        return;
    }

    disableAllFileInputs();
    fil.disabled = false;
    alert("Uploading file " + uploadCount);
    document.forms[0].submit();
}


Relevant markup:

<iframe src="test.htm" name="postHere" id="postHere"
  onload="uploadCount++; if(uploadCount > 1) startUpload();"></iframe>

<!-- MUST use inline JavaScript here for onload event
     to fire after each form submission. -->


This code comes from a web form with multiple <input type="file">. It uploads the files one at a time to prevent huge requests. It does this by POSTing to the iframe, waiting for the response which fires the iframe onload, and then triggers the next submission.

You don't have to answer this example specifically, I am just providing it for reference to a situation in which I am unable to think of a way to avoid global variables.
    
asked Sep 16, 2015 by android_master
0 votes
2 views



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answered Sep 16, 2015 by kinnari
0 votes

First off, it is impossible to avoid global JavaScript, something will always be dangling the global scope. Even if you create a namespace, which is still a good idea, that namespace will be global.

There are many approaches, however, to not abuse the global scope. Two of the simplest are to either use closure, or since you only have one variable you need to keep track of, just set it as a property of the function itself (which can then be treated as a static variable).

Closure

var startUpload = (function() {
  var uploadCount = 1;  // <----
  return function() {
    var fil = document.getElementById("FileUpload" + uploadCount++);  // <----

    if(!fil || fil.value.length == 0) {    
      alert("Finished!");
      document.forms[0].reset();
      uploadCount = 1; // <----
      return;
    }

    disableAllFileInputs();
    fil.disabled = false;
    alert("Uploading file " + uploadCount);
    document.forms[0].submit();
  };
})();

* Note that incrementing of uploadCount is happening internally here

Function Property

var startUpload = function() {
  startUpload.uploadCount = startUpload.count || 1; // <----
  var fil = document.getElementById("FileUpload" + startUpload.count++);

  if(!fil || fil.value.length == 0) {    
    alert("Finished!");
    document.forms[0].reset();
    startUpload.count = 1; // <----
    return;
  }

  disableAllFileInputs();
  fil.disabled = false;
  alert("Uploading file " + startUpload.count);
  document.forms[0].submit();
};

I'm not sure why uploadCount++; if(uploadCount > 1) ... is necessary, as it looks like the condition will always be true. But if you do need global access to the variable, then the function property method I described above will allow you to do so without the variable actually being global.


However, if that's the case, then you should probably use an object literal or instantiated object and go about this in the normal OO way (where you can use the module pattern if it strikes your fancy).

answered Sep 16, 2015 by dahiyabecomp
0 votes

I suggest the module pattern.

YAHOO.myProject.myModule = function () {

    //"private" variables:
    var myPrivateVar = "I can be accessed only from within YAHOO.myProject.myModule.";

    //"private" method:
    var myPrivateMethod = function () {
    	YAHOO.log("I can be accessed only from within YAHOO.myProject.myModule");
    }

    return  {
    	myPublicProperty: "I'm accessible as YAHOO.myProject.myModule.myPublicProperty."
    	myPublicMethod: function () {
    		YAHOO.log("I'm accessible as YAHOO.myProject.myModule.myPublicMethod.");

    		//Within myProject, I can access "private" vars and methods:
    		YAHOO.log(myPrivateVar);
    		YAHOO.log(myPrivateMethod());

    		//The native scope of myPublicMethod is myProject; we can
    		//access public members using "this":
    		YAHOO.log(this.myPublicProperty);
    	}
    };

}(); // the parens here cause the anonymous function to execute and return
answered Sep 16, 2015 by girisha
0 votes
window.onload = function() {
  var frm = document.forms[0];
  frm.target = "postMe";
  frm.onsubmit = function() {
    frm.onsubmit = null;
    var uploader = new LazyFileUploader();
    uploader.startUpload();
    return false;
  }
}

function LazyFileUploader() {
    var uploadCount = 0;
    var total = 10;
    var prefix = "FileUpload";	
    var upload = function() {
    	var fil = document.getElementById(prefix + uploadCount);

    	if(!fil || fil.value.length == 0) {    
    		alert("Finished!");
    		document.forms[0].reset();
    		return;
    	 }

    	disableAllFileInputs();
    	fil.disabled = false;
    	alert("Uploading file " + uploadCount);
    	document.forms[0].submit();
    	uploadCount++;

    	if (uploadCount < total) {
    		setTimeout(function() {
    			upload();
    		}, 100); 
    	}
    }

    this.startUpload = function() {
    	setTimeout(function() {
    		upload();
    	}, 100);  
    } 		
}
answered Sep 16, 2015 by ajit.chavhan
0 votes

Sometimes it makes sense to have global variables in JavaScript. But don't leave them hanging directly off window like that.

Instead, create a single "namespace" object to contain your globals. For bonus points, put everything in there, including your methods.

answered Sep 16, 2015 by patilkiran.101
0 votes

Use closures. Something like this gives you a scope other than global.

(function() {
    // Your code here
    var var1;
    function f1() {
        if(var1){...}
    }

    window.var_name = something; //<- if you have to have global var
    window.glob_func = function(){...} //<- ...or global function
})();
answered Sep 16, 2015 by gauravg.gwl
0 votes

Some things are going to be in the global namespace -- namely, whatever function you're calling from your inline JavaScript code.

In general, the solution is to wrap everything in a closure:

(function() {
    var uploadCount = 0;
    function startupload() {  ...  }
    document.getElementById('postHere').onload = function() {
        uploadCount ++;
        if (uploadCount > 1) startUpload();
    };
})();

and avoid the inline handler.

answered Sep 16, 2015 by dhananjayksharma
0 votes

Using closures might be OK for small to medium projects. However, for big projects, you might want to split your code into modules and save them in different files.

Therefore I wrote jQuery Secret plugin to solve the problem.

In your case with this plugin the code would look something like the following.

JavaScript:

// Initialize uploadCount.
$.secret( 'in', 'uploadCount', 0 ).

// Store function disableAllFileInputs.
secret( 'in', 'disableAllFileInputs', function(){
  // Code for 'disable all file inputs' goes here.

// Store function startUpload
}).secret( 'in', 'startUpload', function(){
    // 'this' points to the private object in $.secret
    // where stores all the variables and functions
    // ex. uploadCount, disableAllFileInputs, startUpload.

    var fil = document.getElementById( 'FileUpload' + uploadCount);

    if(!fil || fil.value.length == 0) {
        alert( 'Finished!' );
        document.forms[0].reset();
        return;
    }

    // Use the stored disableAllFileInputs function
    // or you can use $.secret( 'call', 'disableAllFileInputs' );
    // it's the same thing.
    this.disableAllFileInputs();
    fil.disabled = false;

    // this.uploadCount is equal to $.secret( 'out', 'uploadCount' );
    alert( 'Uploading file ' + this.uploadCount );
    document.forms[0].submit();

// Store function iframeOnload
}).secret( 'in', 'iframeOnload', function(){
    this.uploadCount++;
    if( this.uploadCount > 1 ) this.startUpload();
});

window.onload = function() {
    var frm = document.forms[0];

    frm.target = "postMe";
    frm.onsubmit = function() {
        // Call out startUpload function onsubmit
        $.secret( 'call', 'startUpload' );
        return false;
    }
}

Relevant markup:


Open your Firebug, you will find no globals at all, not even the funciton :)

For full documentation, please see here.

For a demo page, please see this.

Source code on GitHub.

answered Sep 16, 2015 by akasati02

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