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132 lines
6.9 KiB
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132 lines
6.9 KiB
HTML
<section>
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<p><strong>So finally you're <a href="http://fakehost/code/2013/testing-frontend-javascript-code-using-mocha-chai-and-sinon/">testing your frontend JavaScript code</a>? Great! The more you
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write tests, the more confident you are with your code… but how much precisely?
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That's where <a href="http://en.wikipedia.org/wiki/Code_coverage">code coverage</a> might
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help.</strong>
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</p>
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<p>The idea behind code coverage is to record which parts of your code (functions,
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statements, conditionals and so on) have been executed by your test suite,
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to compute metrics out of these data and usually to provide tools for navigating
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and inspecting them.</p>
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<p>Not a lot of frontend developers I know actually test their frontend code,
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and I can barely imagine how many of them have ever setup code coverage…
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Mostly because there are not many frontend-oriented tools in this area
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I guess.</p>
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<p>Actually I've only found one which provides an adapter for <a href="http://visionmedia.github.io/mocha/">Mocha</a> and
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actually works…</p>
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<blockquote>
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<p>Drinking game for web devs:
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<br>(1) Think of a noun
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<br>(2) Google "<noun>.js"
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<br>(3) If a library with that name exists - drink</p>— Shay Friedman (@ironshay)
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<a href="https://twitter.com/ironshay/statuses/370525864523743232">August 22, 2013</a>
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</blockquote>
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<p><strong><a href="http://blanketjs.org/">Blanket.js</a></strong> is an <em>easy to install, easy to configure,
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and easy to use JavaScript code coverage library that works both in-browser and
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with nodejs.</em>
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</p>
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<p>Its use is dead easy, adding Blanket support to your Mocha test suite
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is just matter of adding this simple line to your HTML test file:</p>
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<pre><code><script src="vendor/blanket.js"
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data-cover-adapter="vendor/mocha-blanket.js"></script>
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</code></pre>
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<p>Source files: <a href="https://raw.github.com/alex-seville/blanket/master/dist/qunit/blanket.min.js">blanket.js</a>,
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<a href="https://raw.github.com/alex-seville/blanket/master/src/adapters/mocha-blanket.js">mocha-blanket.js</a>
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</p>
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<p>As an example, let's reuse the silly <code>Cow</code> example we used
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<a href="http://fakehost/code/2013/testing-frontend-javascript-code-using-mocha-chai-and-sinon/">in a previous episode</a>:</p>
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<pre><code>// cow.js
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(function(exports) {
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"use strict";
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function Cow(name) {
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this.name = name || "Anon cow";
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}
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exports.Cow = Cow;
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Cow.prototype = {
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greets: function(target) {
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if (!target)
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throw new Error("missing target");
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return this.name + " greets " + target;
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}
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};
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})(this);
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</code></pre>
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<p>And its test suite, powered by Mocha and <a href="http://chaijs.com/">Chai</a>:</p>
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<pre><code>var expect = chai.expect;
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describe("Cow", function() {
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describe("constructor", function() {
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it("should have a default name", function() {
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var cow = new Cow();
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expect(cow.name).to.equal("Anon cow");
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});
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it("should set cow's name if provided", function() {
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var cow = new Cow("Kate");
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expect(cow.name).to.equal("Kate");
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});
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});
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describe("#greets", function() {
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it("should greet passed target", function() {
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var greetings = (new Cow("Kate")).greets("Baby");
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expect(greetings).to.equal("Kate greets Baby");
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});
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});
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});
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</code></pre>
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<p>Let's create the HTML test file for it, featuring Blanket and its adapter
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for Mocha:</p>
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<pre><code><!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>Test</title>
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<link rel="stylesheet" media="all" href="vendor/mocha.css">
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</head>
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<body>
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<div id="mocha"></div>
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<div id="messages"></div>
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<div id="fixtures"></div>
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<script src="vendor/mocha.js"></script>
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<script src="vendor/chai.js"></script>
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<script src="vendor/blanket.js"
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data-cover-adapter="vendor/mocha-blanket.js"></script>
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<script>mocha.setup('bdd');</script>
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<script src="cow.js" data-cover></script>
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<script src="cow_test.js"></script>
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<script>mocha.run();</script>
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</body>
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</html>
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</code></pre>
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<p><strong>Notes</strong>:</p>
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<ul>
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<li>Notice the <code>data-cover</code> attribute we added to the script tag
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loading the source of our library;</li>
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<li>The HTML test file <em>must</em> be served over HTTP for the adapter to
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be loaded.</li>
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</ul>
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<p>Running the tests now gives us something like this:</p>
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<p>
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<img alt="screenshot" src="http://fakehost/static/code/2013/blanket-coverage.png">
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</p>
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<p>As you can see, the report at the bottom highlights that we haven't actually
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tested the case where an error is raised in case a target name is missing.
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We've been informed of that, nothing more, nothing less. We simply know
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we're missing a test here. Isn't this cool? I think so!</p>
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<p>Just remember that code coverage will only <a href="http://codebetter.com/karlseguin/2008/12/09/code-coverage-use-it-wisely/">bring you numbers</a> and
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raw information, not actual proofs that the whole of your <em>code logic</em> has
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been actually covered. If you ask me, the best inputs you can get about
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your code logic and implementation ever are the ones issued out of <a href="http://www.extremeprogramming.org/rules/pair.html">pair programming</a>
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sessions
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and <a href="http://alexgaynor.net/2013/sep/26/effective-code-review/">code reviews</a> —
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but that's another story.</p>
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<p><strong>So is code coverage silver bullet? No. Is it useful? Definitely. Happy testing!</strong>
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</p>
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</section> |