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Define a global variable in a JavaScript function

Is it possible to define a global variable in a JavaScript function?

I want use the trailimage variable (declared in the makeObj function) in other functions.

<html xmlns="http://www.w3.org/1999/xhtml">
    <head id="Head1" runat="server">
        <title></title>
        <script type="text/javascript">
            var offsetfrommouse = [10, -20];
            var displayduration = 0;
            var obj_selected = 0;
            function makeObj(address) {
                **var trailimage = [address, 50, 50];**
                document.write('<img id="trailimageid" src="' + trailimage[0] + '" border="0"  style=" position: absolute; visibility:visible; left: 0px; top: 0px; width: ' + trailimage[1] + 'px; height: ' + trailimage[2] + 'px">');
                obj_selected = 1;
            }

            function truebody() {
                return (!window.opera && document.compatMode && document.compatMode != "BackCompat") ? document.documentElement : document.body;
            }
            function hidetrail() {
                var x = document.getElementById("trailimageid").style;
                x.visibility = "hidden";
                document.onmousemove = "";
            }
            function followmouse(e) {
                var xcoord = offsetfrommouse[0];
                var ycoord = offsetfrommouse[1];
                var x = document.getElementById("trailimageid").style;
                if (typeof e != "undefined") {
                    xcoord += e.pageX;
                    ycoord += e.pageY;
                }
                else if (typeof window.event != "undefined") {
                    xcoord += truebody().scrollLeft + event.clientX;
                    ycoord += truebody().scrollTop + event.clientY;
                }
                var docwidth = 1395;
                var docheight = 676;
                if (xcoord + trailimage[1] + 3 > docwidth || ycoord + trailimage[2] > docheight) {
                    x.display = "none";
                    alert("inja");
                }
                else
                    x.display = "";
                x.left = xcoord + "px";
                x.top = ycoord + "px";
            }

            if (obj_selected = 1) {
                alert("obj_selected = true");
                document.onmousemove = followmouse;
                if (displayduration > 0)
                    setTimeout("hidetrail()", displayduration * 1000);
            }
        </script>
    </head>
    <body>
        <form id="form1" runat="server">
        <img alt="" id="house" src="Pictures/sides/right.gif" style="z-index: 1; left: 372px;
            top: 219px; position: absolute; height: 138px; width: 120px" onclick="javascript:makeObj('Pictures/sides/sides-not-clicked.gif');" />
        </form>
    </body>
</html>
to declare a global simply don't use the "var" keyword
@Ibrahim: "to declare a global simply don't use the "var" keyword" Gak! The Horror! ;-) Thankfully, strict mode does away with implicit globals.
@Ibrahim Diallo - not using var doesn't declare a global variable. A consequence of assigning a value to an undeclared variable is the creation of a property on the global object, which is quite different to declaring a variable.
useful info in this answer, too stackoverflow.com/a/4862268/1356098 :)

T
T.J. Crowder

As the others have said, you can use var at global scope (outside of all functions and modules) to declare a global variable:

<script>
var yourGlobalVariable;
function foo() {
    // ...
}
</script>

(Note that that's only true at global scope. If that code were in a module — <script type="module">...</script> — it wouldn't be at global scope, so that wouldn't create a global.)

Alternatively:

In modern environments, you can assign to a property on the object that globalThis refers to (globalThis was added in ES2020):

<script>
function foo() {
    globalThis.yourGlobalVariable = ...;
}
</script>

On browsers, you can do the same thing with the global called window:

<script>
function foo() {
    window.yourGlobalVariable = ...;
}
</script>

...because in browsers, all global variables global variables declared with var are properties of the window object. (In the latest specification, ECMAScript 2015, the new let, const, and class statements at global scope create globals that aren't properties of the global object; a new concept in ES2015.)

(There's also the horror of implicit globals, but don't do it on purpose and do your best to avoid doing it by accident, perhaps by using ES5's "use strict".)

All that said: I'd avoid global variables if you possibly can (and you almost certainly can). As I mentioned, they end up being properties of window, and window is already plenty crowded enough what with all elements with an id (and many with just a name) being dumped in it (and regardless that upcoming specification, IE dumps just about anything with a name on there).

Instead, in modern environments, use modules:

<script type="module">
let yourVariable = 42;
// ...
</script>

The top level code in a module is at module scope, not global scope, so that creates a variable that all of the code in that module can see, but that isn't global.

In obsolete environments without module support, wrap your code in a scoping function and use variables local to that scoping function, and make your other functions closures within it:

<script>
(function() { // Begin scoping function
    var yourGlobalVariable; // Global to your code, invisible outside the scoping function
    function foo() {
        // ...
    }
})();         // End scoping function
</script>

Note that using window won't work in Node. The easiest trick here is to set: GLOBAL.window = GLOBAL; -- as explained in this related question. Of course, it's not pretty conceptually. I prefer to do things the other way around, so I can use GLOBAL instead of window.
@CasparKleijne, I don't understand it. Why would you assign it on window when you literally have no evidence that the window object actually exists. You know nothing about how your code will be used in the future. It might be even used in the MongoDB environment or rino instead of your node. And the window object is also not visible even in the browser if you use web workers for instance. This will completely kill reusability.
window.yourGlobalVariable = ...; works like a charm after reading 5 to 6 questions on stack site.
@JacquesKoekemoer: There's no reason whatsoever to use eval there. Instead: window[dynamic_variable_name] = dynamic_variable_value;
@AstritSpanca - That's the Horror of Implicit Globals I mention in a parenthetical in the middle of the answer. :-)
P
Peter Mortensen

Just declare

var trialImage;

outside. Then

function makeObj(address) {
    trialImage = [address, 50, 50];
    ...
    ...
}

P
Peter Mortensen

If you read the comments there's a nice discussion around this particular naming convention.

It seems that since my answer has been posted the naming convention has gotten more formal. People who teach, write books, etc. speak about var declaration, and function declaration.

Here is the additional Wikipedia post that supports my point: Declarations and definitions ...and to answer the main question. Declare variable before your function. This will work and it will comply to the good practice of declaring your variables at the top of the scope :)


If You want to define your variables elsewhere be sure to understand what hoisting is. Here is a very nice article about it adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting. Good luck!
This isn't what definition and declaration mean in C. Your first line could be either a declaration or a definition (depending on where it is); the second is just an assignment. A declaration just specifies the interpretation of the identifier (ie. myVar is an int); if the declaration also reserves storage, it is a definition. This is unrelated to typing; it's part of how compilation units understand references to other compilation units.
Even in JS, var myVar is called declaration (it doesn't need to be typed) and myVar = 10 an assignment. I've heard the term "defintion" for the compound (var myVar = 10;).
This doesn't help to answer the question. It should have been a comment, not an answer.
@Stuntddude you're probably right :( I started answering the question and then decided to diverge a bit, and well this is what we've got. Still some ppl still find it useful so I've left it here. Thanks for your feedback!
P
Peter Mortensen

Just declare it outside the functions, and assign values inside the functions. Something like:

<script type="text/javascript">
    var offsetfrommouse = [10, -20];
    var displayduration = 0;
    var obj_selected = 0;
    var trailimage = null ;  // Global variable
    function makeObj(address) {
        trailimage = [address, 50, 50];  // Assign value

Or simply removing "var" from your variable name inside function also makes it global, but it is better to declare it outside once for cleaner code. This will also work:

var offsetfrommouse = [10, -20];
var displayduration = 0;
var obj_selected = 0;

function makeObj(address) {
    trailimage = [address, 50, 50];  // Global variable, assign value

I hope this example explains more: http://jsfiddle.net/qCrGE/

var globalOne = 3;
testOne();

function testOne()
{
    globalOne += 2;
    alert("globalOne is :" + globalOne );
    globalOne += 1;
}

alert("outside globalOne is: " + globalOne);

testTwo();

function testTwo()
{
    globalTwo = 20;
    alert("globalTwo is " + globalTwo);
    globalTwo += 5;
}

alert("outside globalTwo is:" + globalTwo);

Pretty clean and working solution +1
G
Guffa

No, you can't. Just declare the variable outside the function. You don't have to declare it at the same time as you assign the value:

var trailimage;

function makeObj(address) {
  trailimage = [address, 50, 50];

Sorry! "Is it possible to define a global variable in a JavaScript function?" -- "No, you can't" isn't correct, as the first answer shows!
@mustafa.0x: You are mistaken. You can't define a global variable inside a function. You can implicitly create a global variable, or create a window property inside a function, but you can't define a global variable inside a function.
With regards to JavaScript in a browser environment, a global variable and a property of window are synonymous. Either way, the semantic distinction you're making is clear, so I don't mind un-downvoting. Edit: unable to change my vote, sorry!
if you're not using strict (but why aren't you using strict?), you actually can declare and define global vars inside a function by going against all good practices and just not using the var, which is what Guffa meant by implicitly create (such as @DhruvPathak pointed there, as well).
S
Shmack
    var Global = 'Global';

    function LocalToGlobalVariable() {

        // This creates a local variable.

        var Local = '5';

        // Doing this makes the variable available for one session
        // (a page refresh - it's the session not local)

        sessionStorage.LocalToGlobalVar = Local;

        // It can be named anything as long as the sessionStorage
        // references the local variable.
        // Otherwise it won't work.
        // This refreshes the page to make the variable take
        // effect instead of the last variable set.

        location.reload(false);
    };

    // This calls the variable outside of the function for whatever use you want.

    sessionStorage.LocalToGlobalVar;

I realize there is probably a lot of syntax errors in this but its the general idea... Thanks so much LayZee for pointing this out... You can find what a local and session Storage is at http://www.w3schools.com/html/html5_webstorage.asp. I have needed the same thing for my code and this was a really good idea.

//Edit

As of (I believe) 2015, a new "standard" for javascript (if you will) was introduced. This standard introduced many new ideas into javascript, one of them being the implementation of scope.

https://www.w3schools.com/js/js_scope.asp has all the details concerning this idea, but the cliff notes:

const defines a constant.

var has "global" scope.

let has "function" or "block" scope.


So far this is the only answer that works at all, but it requires reloading the page and location.reload(false); refreshes the page repeatedly.
In my case I replace sessionStorage with localStorage and this suppress the need to function LocalToGlobalVariable(). I am using localStorage.setItem("xxx", "yyy") and localStorage.getItem("xxx") for setting and getting the variables.
V
ViRuSTriNiTy

There are three types of scope in JavaScript:

Global Scope: where the variable is available through the code.

Block Scope: where the variable is available inside a certain area like a function.

Local Scope: where the variable is available in more certain areas, like an if-statement

If you add Var before the variable name, then its scope is determined where its location is

Example:

var num1 = 18; // Global scope
function fun() {
  var num2 = 20; // Local (Function) Scope
  if (true) {
    var num3 = 22; // Block Scope (within an if-statement)
  }
}

num1 = 18; // Global scope
function fun() {
  num2 = 20; // Global Scope
  if (true) {
    num3 = 22; // Global Scope
  }
}

P
Peter Mortensen

Classic example:

window.foo = 'bar';

A modern, safe example following best practice by using an IIFE:

;(function (root) {
    'use strict'

    root.foo = 'bar';
)(this));

Nowadays, there's also the option of using the WebStorage API:

localStorage.foo = 42;

or

sessionStorage.bar = 21;

Performance-wise, I'm not sure whether it is noticeably slower than storing values in variables.

Widespread browser support as stated on Can I use....


localStorage and sessionStorage only works for string values.
That is right, @HereToLearn_, but then you can use localStorage.foo = JSON.stringify({ arbitrary: 'object', meaning: 42, awesome: true }); and var foo = JSON.decode(localStorage.foo);.
What localStorage has to do with global variables?
OP is looking for at solution to this problem: "I want use the trailimage variable (declared in the makeObj function) in other functions." Who says that the solution has to involve global variables? Global variables are seldom a good solution to anything.
P
Peter Mortensen

It is very simple. Define the trailimage variable outside the function and set its value in the makeObj function. Now you can access its value from anywhere.

var offsetfrommouse = [10, -20];
var displayduration = 0;
var obj_selected = 0;
var trailimage;

function makeObj(address) {
    trailimage = [address, 50, 50];
    ...
}

G
Gawrion

Yes, You can. Just don't use var, don't use let. Just initialize variable and it will be automaticly assigned global:

    function firstFunction() {
        if (typeof(testVar) === "undefined") {testVar = 1;} //initializing variable if not initialized
        testVar += 1;
        console.log('Test variable inside 1st function: '+testVar);
    }
    function secondFunction() {
        testVar += 1;
        console.log('Test variable inside 2nd function: '+testVar);
    } 
    firstFunction();
    secondFunction();
    testVar += 1;
    console.log('Test variable outside: '+testVar);

S
Shrey Mittal

Global variables are declared outside of a function for accessibility throughout the program, while local variables are stored within a function using var for use only within that function’s scope. If you declare a variable without using var, even if it’s inside a function, it will still be seen as global. References = https://www.freecodecamp.org/news/global-variables-in-javascript-explained/


P
Peter Mortensen

If you want the variable inside the function available outside of the function, return the results of the variable inside the function.

var x = function returnX { var x = 0; return x; } is the idea...

    <html xmlns="http://www.w3.org/1999/xhtml">
    <head id="Head1" runat="server">
    <title></title>
    <script type="text/javascript">

        var offsetfrommouse = [10, -20];
        var displayduration = 0;
        var obj_selected = 0;

        function makeObj(address) {
            var trailimage = [address, 50, 50];
            document.write('<img id="trailimageid" src="' + trailimage[0] + '" border="0"  style=" position: absolute; visibility:visible; left: 0px; top: 0px; width: ' + trailimage[1] + 'px; height: ' + trailimage[2] + 'px">');
            obj_selected = 1;
            return trailimage;
        }

        function truebody() {
            return (!window.opera && document.compatMode && document.compatMode != "BackCompat") ? document.documentElement : document.body;
        }

        function hidetrail() {
            var x = document.getElementById("trailimageid").style;
            x.visibility = "hidden";
            document.onmousemove = "";
        }

        function followmouse(e) {
            var xcoord = offsetfrommouse[0];
            var ycoord = offsetfrommouse[1];
            var x = document.getElementById("trailimageid").style;
            if (typeof e != "undefined") {
                xcoord += e.pageX;
                ycoord += e.pageY;
            }

            else if (typeof window.event != "undefined") {
                xcoord += truebody().scrollLeft + event.clientX;
                ycoord += truebody().scrollTop + event.clientY;
            }
            var docwidth = 1395;
            var docheight = 676;
            if (xcoord + trailimage[1] + 3 > docwidth || ycoord + trailimage[2] > docheight) {
                x.display = "none";
                alert("inja");
            }
            else
                x.display = "";
            x.left = xcoord + "px";
            x.top = ycoord + "px";
        }

        if (obj_selected = 1) {
            alert("obj_selected = true");
            document.onmousemove = followmouse;
            if (displayduration > 0)
                setTimeout("hidetrail()", displayduration * 1000);
        }

    </script>
</head>
<body>
    <form id="form1" runat="server">
    <img alt="" id="house" src="Pictures/sides/right.gif" style="z-index: 1; left: 372px; top: 219px; position: absolute; height: 138px; width: 120px" onclick="javascript:makeObj('Pictures/sides/sides-not-clicked.gif');" />
    </form>
</body>
</html>

I haven't tested this, but if your code worked prior to that small change, then it should work.


P
Peter Mortensen

To use the window object is not a good idea. As I see in comments,

'use strict';

function showMessage() {
    window.say_hello = 'hello!';
}

console.log(say_hello);

This will throw an error to use the say_hello variable we need to first call the showMessage function.


u
user3777368

All variables declared without "var" or "let" or anything else preceding it like "const" are global. So ....

function myFunction(){

     myVar = "value"; // myVar is a global variable and can be used anywhere inside <script> tags.

}

P
Peter Mortensen

If you are making a startup function, you can define global functions and variables this way:

function(globalScope)
{
    // Define something
    globalScope.something()
    {
        alert("It works");
    };
}(window)

Because the function is invoked globally with this argument, this is global scope here. So, the something should be a global thing.


this is undefined in strict mode outside a function and should not be used to refer to the global object.
P
Peter Mortensen

Here is sample code that might can be helpful.

var Human = function() {
    name = "Shohanur Rahaman";  // Global variable
    this.name = "Tuly"; // Constructor variable 
    var age = 21;
};

var shohan = new Human();

document.write(shohan.name + "<br>");
document.write(name);
document.write(age); // Undefined because it's a local variable 

Here I found a nice answer: How can one declare a global variable in JavaScript?


Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference.
a
am2124429

Here is another easy method to make the variable available in other functions without having to use global variables:

function makeObj() { // var trailimage = 'test'; makeObj.trailimage = 'test'; } function someOtherFunction() { document.write(makeObj.trailimage); } makeObj(); someOtherFunction();