2011-08-28

How to write and unit-test universal JavaScript modules (browser, Node.js)

Update 2011-11-19. This post is now mostly superseded by “Bridging the module gap between Node.js and browsers”.

Node.js has a very nice module system that is easy to understand, yet distinguishes between the exports of a module and things that should be private to it. This post explains how the code of a Node.js module can be modified so that it works on both Node.js and web browsers. It also explains how to unit-test such code.

1. Writing a universal module

1.1. Node.js modules

A Node.js module looks as follows:
    var privateVariable = 123;
    function privateFunction() {
    }
    
    exports.publicVariable = 345;
    exports.publicFunction = function () {
        return privateVariable * exports.publicVariable;
    }
You add your public data to the object exports which Node.js manages for you. Additionally, Node.js ensures that all other data stays private to your module. Inside the module, you refer to private data directly, to public data via exports. Sect. 2 shows how such a module is used.

1.2. Making Node.js code universal

You can use Node.js module code in browsers, if you wrap it as follows:
    "use strict";

    (function(exports) {
        
        // Your Node.js code goes here

    }(typeof exports === "undefined"
        ? (this.moduleName = {})
        : exports));
Let’s examine the wrapper code:
  • Enabling strict mode: The first line enables strict mode [1] under ECMAScript 5 and does nothing under older versions of JavaScript.
  • An immediately invoked function definition (IIFE): We define a function and immediately invoke it [2]. This serves two purposes:
    • It keeps non-exported data private in browsers, where Node.js does not do this for us.
    • It conditionally creates the variable exports which exists on Node.js, but must be created in a browser. Creating a new scope with a new variable exports in it is the only way of doing this, you cannot add a variable to an existing scope if it isn’t there, yet. That is, you cannot do the following:
          if (typeof exports !== "undefined") {
              var exports = {}; // (*)
          }
      
      Reason: JavaScript is function-scoped – a variable always exists inside the complete (inner-most) enclosing function. Additionally, variable declarations are hoisted – moved to the beginning of the function. Thus, the var declaration (but not the assignment!) at (*) is moved before the if statement and exports will always be undefined when the check is performed. Note that the code would work without hoisting, but not with block-scoping, because then the exports declared at (*) would only exist inside the then-block.
  • typeof exports === "undefined": Does the variable exports exist?
  • this.moduleName = {}: At the global level, this refers to the global object. Therefore, the assignment creates the global variable moduleName (which holds the module in a browser).
Running example:
    "use strict";

    (function(exports) {

        exports.StringSet = function () {
            this.data = {};
            // arguments is not an array; borrow forEach
            Array.prototype.forEach.call(arguments, function(elem) {
                this.add(elem);
            }, this); // pass "this" on to the function
        }

        exports.StringSet.prototype.add = function(elem) {
            if (typeof elem !== "string") {
                throw new TypeError("Argument is not a string: "+elem);
            }
            this.data[elem] = true;
            return this; // allow method chaining
        }

        exports.StringSet.prototype.contains = function(elem) {
            // Comparison ensures boolean result
            return this.data[elem] === true;
        }

    }(typeof exports === "undefined"
        ? (this.strset = {})
        : exports));

2. Using a universal module

Node:
    var strset = require("./strset");
    var s = new strset.StringSet();
Browser:
    <script src="strset.js"></script>
    <script>
        var s = new strset.StringSet();
    </script>

3. Unit-testing a universal module

3.1. Node.js: unit-testing via the module assert

We use Node.js’ built-in means for unit-testing – the module assert.
    var assert = require('assert');
    var strset = require('./strset');

    // constructor
    (function() {
        var sset = new strset.StringSet("a"); 
        assert.ok(sset.contains("a"));
        assert.ok(!sset.contains("b"));
    }());

    // add - illegal arguments
    (function() {
        assert.throws(function() {
            new strset.StringSet().add(3);
        });
    }());
These tests are run by storing them in a file strset-node-test.js and executing it via the command line:
    > node strset-node-test.js
If one of the tests fails, there will be an exception. If all tests succeed, nothing happens.

3.2. Browser: unit-testing via jQuery’s QUnit

In a browser, we have (too) many options: Most frameworks come with their own unit-testing support. Let us use jQuery’s QUnit.
<!doctype html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
        <title>strset-test</title>
        <script src="http://code.jquery.com/jquery-latest.js"></script>
        <link rel="stylesheet" href="http://code.jquery.com/qunit/git/qunit.css" type="text/css" media="screen">
        <script type="text/javascript" src="http://code.jquery.com/qunit/git/qunit.js"></script>
        <script type="text/javascript" src="strset.js"></script>
        <script>
            $(document).ready(function(){
                test("constructor", function() {
                    var sset = new strset.StringSet("a"); 
                    ok(sset.contains("a"));
                    ok(!sset.contains("b"));
                });
                test("add - illegal arguments", function() {
                    raises(function() {
                        new strset.StringSet().add(3);
                    });
                });
            });
        </script>
    </head>
    <body>
        <h1 id="qunit-header">strset-test</h1>
        <h2 id="qunit-banner"></h2>
        <div id="qunit-testrunner-toolbar"></div>
        <h2 id="qunit-userAgent"></h2>
        <ol id="qunit-tests"></ol>
        <div id="qunit-fixture">test markup, will be hidden</div>
    </body>
</html>
You execute these tests by saving them in an .html file and opening it in a browser.

4. Related reading

Related reading
  1. JavaScript’s strict mode: a summary
  2. JavaScript variable scoping and its pitfalls
  3. Modules and namespaces in JavaScript
  4. pattern for module targeting browser and nodejs. Inspiration for the above pattern. Does the same thing, but in reverse – one uses the same variable to export public data on both Node.js and browsers. On Node.js that variable points to the exports object, on browsers, it points to an object in a global variable.

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