How can I share code between Node.js and the browser?

If you want to write a module that can be used both client side and server side, I have a short blog post on a quick and easy method: Writing for Node.js and the browser, essentially the following (where this is the same as window):

(function(exports){

    // Your code goes here

   exports.test = function(){
        return 'hello world'
    };

})(typeof exports === 'undefined'? this['mymodule']={}: exports);

Alternatively there are some projects aiming to implement the Node.js API on the client side, such as Marak's gemini.

You might also be interested in DNode, which lets you expose a JavaScript function so that it can be called from another machine using a simple JSON-based network protocol.

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Epeli has a nice solution here http://epeli.github.com/piler/ that even works without the library, just put this in a file called share.js

(function(exports){

  exports.test = function(){
       return 'This is a function from shared module';
  };

}(typeof exports === 'undefined' ? this.share = {} : exports));

On the server side just use:

var share = require('./share.js');

share.test();

And on the client side just load the js file and then use

share.test();

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Checkout the jQuery source code that makes this work in the Node.js module pattern, AMD module pattern, and global in the browser:

(function(window){
    var jQuery = 'blah';

    if (typeof module === "object" && module && typeof module.exports === "object") {

        // Expose jQuery as module.exports in loaders that implement the Node
        // module pattern (including browserify). Do not create the global, since
        // the user will be storing it themselves locally, and globals are frowned
        // upon in the Node module world.
        module.exports = jQuery;
    }
    else {
        // Otherwise expose jQuery to the global object as usual
        window.jQuery = window.$ = jQuery;

        // Register as a named AMD module, since jQuery can be concatenated with other
        // files that may use define, but not via a proper concatenation script that
        // understands anonymous AMD modules. A named AMD is safest and most robust
        // way to register. Lowercase jquery is used because AMD module names are
        // derived from file names, and jQuery is normally delivered in a lowercase
        // file name. Do this after creating the global so that if an AMD module wants
        // to call noConflict to hide this version of jQuery, it will work.
        if (typeof define === "function" && define.amd) {
            define("jquery", [], function () { return jQuery; });
        }
    }
})(this)

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Don't forget that the string representation of a JavaScript function represents the source code for that function. You could simply write your functions and constructors in an encapsulated way so they can be toString()'d and sent to the client.

Another way to do it is use a build system, put the common code in separate files, and then include them in both the server and client scripts. I'm using that approach for a simple client/server game via WebSockets where the server and client both run essentially the same game loop and the client synchronises up with the server every tick to make sure nobody's cheating.

My build system for the game is a simple Bash script that runs the files through the C preprocessor and then through sed to clean up some junk cpp leaves behind, so I can use all the normal preprocessor stuff like #include, #define, #ifdef, etc.

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I would recommend looking into the RequireJS adapter for Node.js. The problem is that the CommonJS module pattern Node.js uses by default isn't asynchronous, which blocks loading in the web browser. RequireJS uses the AMD pattern, which is both asynchronous and compatible with both server and client, as long as you use the r.js adapter.

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Maybe this is not entirely in line with the question, but I thought I'd share this.

I wanted to make a couple of simple string utility functions, declared on String.prototype, available to both node and the browser. I simply keep these functions in a file called utilities.js (in a subfolder) and can easily reference it both from a script-tag in my browser code, and by using require (omitting the .js extension) in my Node.js script:

my_node_script.js

var utilities = require('./static/js/utilities')

my_browser_code.html

<script src="/static/js/utilities.js"></script>

I hope this is useful information to someone other than me.

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If you use use module bundlers such as webpack to bundle JavaScript files for usage in a browser, you can simply reuse your Node.js module for the frontend running in a browser. In other words, your Node.js module can be shared between Node.js and the browser.

For example, you have the following code sum.js:

Normal Node.js module: sum.js

const sum = (a, b) => {
    return a + b
}

module.exports = sum

Use the module in Node.js

const sum = require('path-to-sum.js')
console.log('Sum of 2 and 5: ', sum(2, 5)) // Sum of 2 and 5:  7

Reuse it in the frontend

import sum from 'path-to-sum.js'
console.log('Sum of 2 and 5: ', sum(2, 5)) // Sum of 2 and 5:  7

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The server can simply send JavaScript source files to the client (browser) but the trick is that the client will have to provide a mini "exports" environment before it can exec the code and store it as a module.

A simple way to make such an environment is to use a closure. For example, say your server provides source files via HTTP like http://example.com/js/foo.js. The browser can load the required files via an XMLHttpRequest and load the code like so:

ajaxRequest({
  method: 'GET',
  url: 'http://example.com/js/foo.js',
  onSuccess: function(xhr) {
    var pre = '(function(){var exports={};'
      , post = ';return exports;})()';
    window.fooModule = eval(pre + xhr.responseText + post);
  }
});

The key is that client can wrap the foreign code into an anonymous function to be run immediately (a closure) which creates the "exports" object and returns it so you can assign it where you'd like, rather than polluting the global namespace. In this example, it is assigned to the window attribute fooModule which will contain the code exported by the file foo.js.

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None of the previous solutions bring the CommonJS module system to the browser.

As mentioned in the other answers, there are asset manager/packager solutions like Browserify or Piler and there are RPC solutions like dnode or nowjs.

But I couldn't find an implementation of CommonJS for the browser (including a require() function and exports / module.exports objects, etc.). So I wrote my own, only to discover afterwards that someone else had written it better than I had: https://github.com/weepy/brequire. It's called Brequire (short for Browser require).

Judging by popularity, asset managers fit the needs of most developers. However, if you need a browser implementation of CommonJS, Brequire will probably fit the bill.

2015 Update: I no longer use Brequire (it hasn't been updated in a few years). If I'm just writing a small, open-source module and I want anyone to be able to easily use, then I'll follow a pattern similar to Caolan's answer (above) -- I wrote a blog post about it a couple years ago.

However, if I'm writing modules for private use or for a community that is standardized on CommonJS (like the Ampersand community) then I'll just write them in CommonJS format and use Browserify.

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Use case: share your app configuration between Node.js and the browser (this is just an illustration, probably not the best approach depending on your app).

Problem: you cannot use window (does not exist in Node.js) nor global (does not exist in the browser).

Edit: now we can thx to globalThis and Node.js >= 12.

Obsolete solution:

File config.js: var config = { foo: 'bar' }; if (typeof module === 'object') module.exports = config;

In the browser (index.html): You can now open the dev tools and access the global variable config

In Node.js (app.js): const config = require('./config'); console.log(config.foo); // Prints 'bar'

With Babel or TypeScript: import config from './config'; console.log(config.foo); // Prints 'bar'

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now.js is also worth a look. It allows you to call server-side from the client-side, and client-side functions from the server-side

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If you want to write your browser in Node.js-like style you can try dualify.

There is no browser code compilation, so you can write your application without limitations.

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Write your code as RequireJS modules and your tests as Jasmine tests.

This way code can be loaded everywhere with RequireJS and the tests be run in the browser with jasmine-html and with jasmine-node in Node.js without the need to modify the code or the tests.

Here is a working example for this.

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I wrote a simple module, that can be imported (either using require in Node, or script tags in the browser), that you can use to load modules both from the client and from the server.

Example usage

1. Defining the module

Place the following in a file log2.js, inside your static web files folder:

let exports = {};

exports.log2 = function(x) {
    if ( (typeof stdlib) !== 'undefined' )
        return stdlib.math.log(x) / stdlib.math.log(2);

    return Math.log(x) / Math.log(2);
};

return exports;

Simple as that!

2. Using the module

Since it is a bilateral module loader, we can load it from both sides (client and server). Therefore, you can do the following, but you don't need to do both at once (let alone in a particular order):

In Node

In Node, it's simple:

var loader = require('./mloader.js');
loader.setRoot('./web');

var logModule = loader.importModuleSync('log2.js');
console.log(logModule.log2(4));

This should return 2.

If your file isn't in Node's current directory, make sure to call loader.setRoot with the path to your static web files folder (or wherever your module is).

In the browser:

First, define the web page:

<html>
    <header>
        <meta charset="utf-8" />
        <title>Module Loader Availability Test</title>

        <script src="mloader.js"></script>
    </header>

    <body>
        <h1>Result</h1>
        <p id="result"><span style="color: #000088">Testing...</span></p>

        <script>
            let mod = loader.importModuleSync('./log2.js', 'log2');

            if ( mod.log2(8) === 3 && loader.importModuleSync('./log2.js', 'log2') === mod )
                document.getElementById('result').innerHTML = "Your browser supports bilateral modules!";

            else
                document.getElementById('result').innerHTML = "Your browser doesn't support bilateral modules.";
        </script>
    </body>
</html>

Make sure you don't open the file directly in your browser; since it uses AJAX, I suggest you take a look at Python 3's http.server module (or whatever your superfast, command line, folder web server deployment solution is) instead.

If everything goes well, this will appear:

https://i.stack.imgur.com/i9Wgi.png

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I wrote this, it is simple to use if you want to set all variables to the global scope:

(function(vars, global) {
    for (var i in vars) global[i] = vars[i];
})({
    abc: function() {
        ...
    },
    xyz: function() {
        ...
    }
}, typeof exports === "undefined" ? this : exports);

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