Define a global variable in a JavaScript function
JavascriptFunctionVariablesScopeDeclarationJavascript Problem Overview
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>
Javascript Solutions
Solution 1 - Javascript
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>
Solution 2 - Javascript
Just declare
var trialImage;
outside. Then
function makeObj(address) {
trialImage = [address, 50, 50];
...
...
}
Solution 3 - Javascript
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 :)
Solution 4 - Javascript
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);
Solution 5 - Javascript
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];
Solution 6 - Javascript
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.
Solution 7 - Javascript
There are three types of scope in JavaScript:
- Global Scoop: where the variable is available through the code.
- Block Scoop: where the variable is available inside a certain area like a function.
- Local Scoop: where the variable is available in more certain areas, like an if-statement
If you add Var before the variable name, then its scoop 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
}
}
Solution 8 - Javascript
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];
...
}
Solution 9 - Javascript
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....
Solution 10 - Javascript
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/
Solution 11 - Javascript
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);
Solution 12 - Javascript
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.
Solution 13 - Javascript
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.
Solution 14 - Javascript
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.
Solution 15 - Javascript
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?
Solution 16 - Javascript
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();