Can We Call the Function Written in One JavaScript in Another Js File

Calling a JavaScript function in another js file

A function cannot be called unless it was defined in the same file or one loaded before the attempt to call it.

A function cannot be called unless it is in the same or greater scope then the one trying to call it.

You declare function fn1 in first.js, and then in second you can just have fn1();

1.js:

function fn1 () {
    alert();
}

2.js:

fn1();

index.html :

<script type="text/javascript" src="1.js"></script>
<script type="text/javascript" src="2.js"></script>

Can we call the function written in one JavaScript in another JS file?

The function could be called as if it was in the same JS File as long as the file containing the definition of the function has been loaded before the first use of the function.

I.e.

File1.js

function alertNumber(number) {
    alert(number);
}

File2.js

function alertOne() {
     alertNumber("one");
}

HTML

<head>
....
    <script src="File1.js" type="text/javascript"></script> 
    <script src="File2.js" type="text/javascript"></script> 
....
</head>
<body>
....
    <script type="text/javascript">
       alertOne();
    </script>
....
</body>

~~The other way won't work.~~
As correctly pointed out by Stuart Wakefield. The other way will also work.

HTML

<head>
....
    <script src="File2.js" type="text/javascript"></script> 
    <script src="File1.js" type="text/javascript"></script> 
....
</head>
<body>
....
    <script type="text/javascript">
       alertOne();
    </script>
....
</body>

What will not work would be:

HTML

<head>
....
    <script src="File2.js" type="text/javascript"></script> 
    <script type="text/javascript">
       alertOne();
    </script>
    <script src="File1.js" type="text/javascript"></script> 
....
</head>
<body>
....
</body>

Although alertOne is defined when calling it, internally it uses a function that is still not defined (alertNumber).

Call JavaScript function from another JavaScript file

tl;dr: Load your dependencies before you depend on them.

You can't call a function that hasn't been loaded.

The functions defined in your second JS file won't be loaded until the first file has finished running all the top-level statements.

Reverse the order of your script elements.

How to call function from one js file to another

Modules have their own scope and top level var statements and function declarations do not create globals. (This is good, globals are problematic).

To deal with this:

export the variables you want to use outside the module
Change the <script> that loads viewer.js so it loads it as a module
Remove the <script> that loads player.js
Import the variables from player.js that you want to use with an import statement inside viewer.js

How do I call a JS Function in another file?

In the js File A I had to create a class to be able to export the function.
The class had to be exported with "module.exports"

module.exports = class A {
    getFiles(input) {
        ...
    }
}

And now in File B I had to add a require line to this class:

const A = require('./A.js');

And now I can call my function "getFiles" from File A:

// vscode extension function
function activate(context) {
    const a = new A();
    a.getFiles('');
}

https://nodejs.org/api/modules.html#modules_modules

Calling a function with same name in another JS file

Update: Nowadays you should prefer to use ES6 import/export in a <script> tag with type="module" or via a module bundler like webpack.

When both script files are included in the same page, they run in the same global JavaScript context, so the two names will overwrite each other. So no, you can not have two functions in different .js files with the same name and access both of them as you've written it.

The simplest solution would be to just rename one of the functions.

A better solution would be for you to write your JavaScript modularly with namespaces, so that each script file adds the minimum possible (preferably 1) objects to the global scope to avoid naming conflicts between separate scripts.

There are a number of ways to do this in JavaScript. The simplest way is to just define a single object in each file:

// In your first script file
var ModuleName = {
    myMain: function () {
        var count=0;
        count++;               
        myHelper(count);
        alert(count);
    },

    myHelper: function (count) {
        alert(count);
        count++;
    }
}

In a later script file, call the function
ModuleName.myMain();

A more popular method is to use a self-evaluating function, similar to the following:

(function (window, undefined) {

    // Your code, defining various functions, etc.
    function myMain() { ... }
    function myHelper(count) { ... }

    // More code...

    // List functions you want other scripts to access
    window.ModuleName = {
        myHelper: myHelper,
        myMain: myMain  
    };
})(window)

How do I include a JavaScript file in another JavaScript file?

The old versions of JavaScript had no import, include, or require, so many different approaches to this problem have been developed.

But since 2015 (ES6), JavaScript has had the ES6 modules standard to import modules in Node.js, which is also supported by most modern browsers.

For compatibility with older browsers, build tools like Webpack and Rollup and/or transpilation tools like Babel can be used.

ES6 Modules

ECMAScript (ES6) modules have been supported in Node.js since v8.5, with the --experimental-modules flag, and since at least Node.js v13.8.0 without the flag. To enable "ESM" (vs. Node.js's previous CommonJS-style module system ["CJS"]) you either use "type": "module" in package.json or give the files the extension .mjs. (Similarly, modules written with Node.js's previous CJS module can be named .cjs if your default is ESM.)

Using package.json:

{
    "type": "module"
}

Then module.js:

export function hello() {
  return "Hello";
}

Then main.js:

import { hello } from './module.js';
let val = hello();  // val is "Hello";

Using .mjs, you'd have module.mjs:

export function hello() {
  return "Hello";
}

Then main.mjs:

import { hello } from './module.mjs';
let val = hello();  // val is "Hello";

ECMAScript modules in browsers

Browsers have had support for loading ECMAScript modules directly (no tools like Webpack required) since Safari 10.1, Chrome 61, Firefox 60, and Edge 16. Check the current support at caniuse. There is no need to use Node.js' .mjs extension; browsers completely ignore file extensions on modules/scripts.

<script type="module">
  import { hello } from './hello.mjs'; // Or the extension could be just `.js`
  hello('world');
</script>

// hello.mjs -- or the extension could be just `.js`
export function hello(text) {
  const div = document.createElement('div');
  div.textContent = `Hello ${text}`;
  document.body.appendChild(div);
}

Read more at https://jakearchibald.com/2017/es-modules-in-browsers/

Dynamic imports in browsers

Dynamic imports let the script load other scripts as needed:

<script type="module">
  import('hello.mjs').then(module => {
      module.hello('world');
    });
</script>

Read more at https://developers.google.com/web/updates/2017/11/dynamic-import

Node.js require

The older CJS module style, still widely used in Node.js, is the module.exports/require system.

// mymodule.js
module.exports = {
   hello: function() {
      return "Hello";
   }
}

// server.js
const myModule = require('./mymodule');
let val = myModule.hello(); // val is "Hello"

There are other ways for JavaScript to include external JavaScript contents in browsers that do not require preprocessing.

AJAX Loading

You could load an additional script with an AJAX call and then use eval to run it. This is the most straightforward way, but it is limited to your domain because of the JavaScript sandbox security model. Using eval also opens the door to bugs, hacks and security issues.

Fetch Loading

Like Dynamic Imports you can load one or many scripts with a fetch call using promises to control order of execution for script dependencies using the Fetch Inject library:

fetchInject([
  'https://cdn.jsdelivr.net/momentjs/2.17.1/moment.min.js'
]).then(() => {
  console.log(`Finish in less than ${moment().endOf('year').fromNow(true)}`)
})

jQuery Loading

The jQuery library provides loading functionality in one line:

$.getScript("my_lovely_script.js", function() {
   alert("Script loaded but not necessarily executed.");
});

Dynamic Script Loading

You could add a script tag with the script URL into the HTML. To avoid the overhead of jQuery, this is an ideal solution.

The script can even reside on a different server. Furthermore, the browser evaluates the code. The <script> tag can be injected into either the web page <head>, or inserted just before the closing </body> tag.

Here is an example of how this could work:

function dynamicallyLoadScript(url) {
    var script = document.createElement("script");  // create a script DOM node
    script.src = url;  // set its src to the provided URL
   
    document.head.appendChild(script);  // add it to the end of the head section of the page (could change 'head' to 'body' to add it to the end of the body section instead)
}

This function will add a new <script> tag to the end of the head section of the page, where the src attribute is set to the URL which is given to the function as the first parameter.

Both of these solutions are discussed and illustrated in JavaScript Madness: Dynamic Script Loading.

Detecting when the script has been executed

Now, there is a big issue you must know about. Doing that implies that you remotely load the code. Modern web browsers will load the file and keep executing your current script because they load everything asynchronously to improve performance. (This applies to both the jQuery method and the manual dynamic script loading method.)

It means that if you use these tricks directly, you won't be able to use your newly loaded code the next line after you asked it to be loaded, because it will be still loading.

For example: my_lovely_script.js contains MySuperObject:

var js = document.createElement("script");

js.type = "text/javascript";
js.src = jsFilePath;

document.body.appendChild(js);

var s = new MySuperObject();

Error : MySuperObject is undefined

Then you reload the page hitting F5. And it works! Confusing...

So what to do about it ?

Well, you can use the hack the author suggests in the link I gave you. In summary, for people in a hurry, he uses an event to run a callback function when the script is loaded. So you can put all the code using the remote library in the callback function. For example:

function loadScript(url, callback)
{
    // Adding the script tag to the head as suggested before
    var head = document.head;
    var script = document.createElement('script');
    script.type = 'text/javascript';
    script.src = url;

    // Then bind the event to the callback function.
    // There are several events for cross browser compatibility.
    script.onreadystatechange = callback;
    script.onload = callback;

    // Fire the loading
    head.appendChild(script);
}

Then you write the code you want to use AFTER the script is loaded in a lambda function:

var myPrettyCode = function() {
   // Here, do whatever you want
};

Then you run all that:

loadScript("my_lovely_script.js", myPrettyCode);

Note that the script may execute after the DOM has loaded, or before, depending on the browser and whether you included the line script.async = false;. There's a great article on Javascript loading in general which discusses this.

Source Code Merge/Preprocessing

As mentioned at the top of this answer, many developers use build/transpilation tool(s) like Parcel, Webpack, or Babel in their projects, allowing them to use upcoming JavaScript syntax, provide backward compatibility for older browsers, combine files, minify, perform code splitting etc.

How can I use a function which is into another .js file?

As you can see, I've used $.getScript() to require sccript1.js file,
but still myfunc() is not defined there. How can I handle that?

myfunc is defined inside a document.ready event and is not exposed globally, hence you are not able to use the same in another function which is not inside this document ready event.

You need to either export this function globally or define it outside document.ready event.

Export function globally

$(document).ready(function(){
    .
    .
    function myfunc(){ ... }
    window.myfunc = myfunc; //this line added here
    .
    .
});

Or define it outside

$(document).ready(function(){
    .
    .

    .
    .
});
function myfunc(){ ... }

Can We Call the Function Written in One JavaScript in Another Js File

Calling a JavaScript function in another js file

Can we call the function written in one JavaScript in another JS file?

Call JavaScript function from another JavaScript file

How to call function from one js file to another

How do I call a JS Function in another file?

Calling a function with same name in another JS file

How do I include a JavaScript file in another JavaScript file?

ES6 Modules

ECMAScript modules in browsers

Dynamic imports in browsers

Node.js require

AJAX Loading

Fetch Loading

jQuery Loading

Dynamic Script Loading

Detecting when the script has been executed

Source Code Merge/Preprocessing

How can I use a function which is into another .js file?

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