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copied!<p>You should read this question:</p> <p><a href="https://stackoverflow.com/questions/145053/javascript-as-a-functional-language">Javascript as a functional language</a></p> <p>There are lots of useful links, including:</p> <ul> <li><a href="http://www.ibm.com/developerworks/library/wa-javascript.html" rel="nofollow noreferrer">Use functional programming techniques to write elegant JavaScript</a></li> <li><a href="http://javascript.crockford.com/little.html" rel="nofollow noreferrer">The Little JavaScripter</a></li> <li><a href="http://interglacial.com/hoj/hoj.html" rel="nofollow noreferrer">Higher-Order JavaScript</a></li> <li><a href="http://eloquentjavascript.net/chapter6.html" rel="nofollow noreferrer">Eloquent JavaScript, Chapter 6: Functional Programming</a></li> </ul> <p>Now, for my opinion. A lot of people <a href="http://javascript.crockford.com/javascript.html" rel="nofollow noreferrer">misunderstand JavaScript</a>, possibly because its syntax looks like most other programming languages (where Lisp/Haskell/OCaml look completely different). JavaScript is <em>not</em> object-oriented, it is actually a <a href="http://en.wikipedia.org/wiki/Prototype-based_programming" rel="nofollow noreferrer">prototype-based language</a>. It doesn't have classes or classical inheritance so shouldn't really be compared to Java or C++.</p> <p>JavaScript can be better compared to a Lisp; it has closures and first-class functions. Using them you can create other functional programming techniques, such as <a href="http://ejohn.org/blog/partial-functions-in-javascript/" rel="nofollow noreferrer">partial application</a> (currying).</p> <p>Let's take an example (using <code>sys.puts</code> from node.js):</p> <pre><code>var external; function foo() { external = Math.random() * 1000; } foo(); sys.puts(external); </code></pre> <p>To get rid of global side effects, we can wrap it in a closure:</p> <pre><code>(function() { var external; function foo() { external = Math.random() * 1000; } foo(); sys.puts(external); })(); </code></pre> <p>Notice that we can't actually do anything with <code>external</code> or <code>foo</code> outside of the scope. They're completely wrapped up in their own closure, untouchable.</p> <p>Now, to get rid of the <code>external</code> side-effect:</p> <pre><code>(function() { function foo() { return Math.random() * 1000; } sys.puts(foo()); })(); </code></pre> <p>In the end, the example is not purely-functional because it <strong>can't</strong> be. Using a random number reads from the global state (to get a seed) and printing to the console is a side-effect.</p> <p>I also want to point out that mixing functional programming with objects is perfectly fine. Take this for example:</p> <pre><code>var Square = function(x, y, w, h) { this.x = x; this.y = y; this.w = w; this.h = h; }; function getArea(square) { return square.w * square.h; } function sum(values) { var total = 0; values.forEach(function(value) { total += value; }); return total; } sys.puts(sum([new Square(0, 0, 10, 10), new Square(5, 2, 30, 50), new Square(100, 40, 20, 19)].map(function(square) { return getArea(square); }))); </code></pre> <p>As you can see, using objects in a functional language can be just fine. Some Lisps even have things called property lists which can be thought of as objects.</p> <p>The real trick to using objects in a functional style is to make sure that you don't rely on their side effects but instead treat them as immutable. An easy way is whenever you want to change a property, just create a <em>new</em> object with the new details and pass that one along, instead (this is the approach often used in Clojure and Haskell).</p> <p>I strongly believe that functional aspects can be very useful in JavaScript but ultimately, you should use whatever makes the code more readable and what works for you.</p>

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