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9 Lessons From A Review Of JavaScript Code

Before we start, I’d like to pose a question: when was the last time you asked someone to review your code? Reviewing code is possibly the single best technique to improve the overall quality of your solutions, and if you’re not actively taking advantage of it, then you’re missing out on identifying bugs and hearing suggestions that could make your code better.

None of us write 100% bug-free code all of the time, so don’t feel there’s a stigma attached to seeking help. Some of the most experienced developers in our industry, from framework authors to browser developers, regularly request reviews of their code from others; asking whether something could be tweaked should in no way be considered embarrassing. Reviews are a technique like any other and should be used where possible.

Further Reading on SmashingMag: Link

Today we’ll look at where to get your code reviewed, how to structure your requests, and what reviewers look for. I was recently asked to review some code for a new JavaScript application, and thought I’d like to share some of my feedback, because it covers some JavaScript fundamentals that are always useful to bear in mind.

Introduction Link

Reviewing code goes hand in hand with maintaining strong coding standards. That said, standards don’t usually prevent logical errors or misunderstandings about the quirks of a programming language, whether it’s JavaScript, Ruby, Objective-C or something else. Even the most experienced developers can make these kinds of mistakes, and reviewing code can greatly assist with catching them.

The first reaction most of us have to criticism is to defend ourselves (or our code), and perhaps lash back. While criticism can be slightly demoralizing, think of it as a learning experience that spurs us to do better and to improve ourselves; because in many cases, once we’ve calmed down, it actually does.

Also remember that no one is obliged to provide feedback on your work, and if the comments are indeed constructive, then be grateful for the time spent offering the input.

Reviews enable us to build on the experience of others and to benefit from a second pair of eyes. And at the end of the day, they are an opportunity for us to write better code. Whether we take advantage of them is entirely our choice.

Where Can I Get My Code Reviewed? Link

Often the most challenging part is actually finding an experienced developer who you trust to do the review. Below are some places where you can request others to review your code (sometimes in other languages, too).

  • JSMentors
    JSMentors is a mailing list that discusses everything to do with JavaScript (including Harmony), and a number of experienced developers are on its review panel (including JD Dalton, Angus Croll and Nicholas Zakas). These mentors might not always be readily available, but they do their best to provide useful, constructive feedback on code that’s been submitted. If you’re looking for assistance with a specific JavaScript framework beyond vanilla JavaScript, the majority of frameworks and libraries have mailing lists or forums that you can post to and that might provide a similar level of assistance.
  • freenode IRC5
    Many chat rooms here are dedicated both to discussing the JavaScript language and to requests for help or review. The most popular rooms are obviously named, and #javascript is particularly useful for generic JavaScript requests, while channels such as #jquery and #dojo are better for questions and requests related to particular libraries and frameworks.
  • Code Review6 (beta)
    You would be forgiven for confusing Code Review with StackOverflow, but it’s actually a very useful, broad-spectrum, subjective tool for getting peer review of code. While on StackOverflow you might ask the question “Why isn’t my code working?,” Code Review is more suited to questions like “Why is my code so ugly?” If you still have any doubt about what it offers, I strongly recommend checking out the FAQs7.
  • Twitter8
    This might sound odd, but at least half of the code that I submit for review is through social networks. Social networks work best, of course, if your code is open source, but trying them never hurts. The only thing I suggest is to ensure that the developers who you follow and interact with are experienced; a review by a developer with insufficient experience can sometimes be worse than no review at all, so be careful!
  • GitHub + reviewth.is9
    We all know that GitHub provides an excellent architecture for reviewing code. It comes with commits, file and line comments, update notifications, an easy way to track forks of gits and repositories, and more. All that’s missing is a way to actually initiate reviews. A tool called attempts to rectify that by giving you a post-commit hook that helps to automate this process, so changes that get posted in the wild have a clear #reviewthis hash tag, and you can tag any users who you wish to review your updates. If many of your colleagues happen to develop in the same language as you do, this set-up can work well for code reviews sourced closer to home. One workflow that works well with this10 (if you’re working on a team or on a collaborative project) is to perform your own work in a topic branch in a repository and then send through pull requests on that branch. Reviewers would examine the changes and commits and could then make line-by-line and file-by-file comments. You (the developer) would then take this feedback and do a destructive rebase on that topic branch, re-push it, and allow the review cycle to repeat until merging them would be acceptable.

How Should I Structure My Review Requests? Link

The following are some guidelines (based on experience) on how to structure your requests for code reviews, to increase the chances of them being accepted. You can be more liberal with them if the reviewer is on your team; but if the reviewer is external, then these might save you some time:

  • Isolate what you would like to be reviewed; ensure that it can be easily run, forked and commented; be clear about where you think improvements could be made; and, above all, be patient.
  • Make it as easy as possible for the reviewer to look at, demo and change your code.
  • Don’t submit a ZIP file of your entire website or project; very few people have the time to go through all of this. The only situation in which this would be acceptable is if your code absolutely required local testing.
  • Instead, isolate and reduce what you would like to be reviewed on jsFiddle11, on jsbin12 or in a GitHub13 gist. This will allow the reviewer to easily fork what you’ve provided and to show changes and comments on what can be improved. If you would prefer a “diff” between your work and any changes they’ve recommended, you might also be interested in PasteBin14, which supports this.
  • Similarly, don’t just submit a link to a page and ask them to “View source” in order to see what can be improved. On websites with a lot of scripts, this task would be challenging and lowers the chances of a reviewer agreeing to help. No one wants to work to find what you want reviewed.
  • Clearly indicate where you personally feel the implementation could be improved. This will help the reviewer quickly home in on what you’re most interested in having reviewed and will save them time. Many reviewers will still look at other parts of the code you’ve submitted regardless, but at least help them prioritize.
  • Indicate what (if any) research you’ve done into techniques for improving the code. The reviewer might very well suggest the same resources, but if they’re aware that you already know of them, then they might offer alternative suggestions (which is what you want).
  • If English isn’t your first language, there’s no harm in saying so. When other developers inform me of this, I know whether to keep the language in my review technical or simple.
  • Be patient. Some reviews take several days to get back to me, and nothing’s wrong with that. Other developers are usually busy with other projects, and someone who agrees to schedule a look at your work is being kind. Be patient, don’t spam them with reminders, and be understanding if they get delayed. Doing this sometimes pay off, because the reviewer can provide even more detailed feedback when they have more time.

What Should Code Reviews Provide? Link

Jonathan Betz, a former developer at Google, once said that a code review should ideally address six things:

  1. Correctness
    Does the code do everything it claims?
  2. Complexity
    Does it accomplish its goals in a straightforward way?
  3. Consistency
    Does it achieve its goals consistently?
  4. Maintainability
    Could the code be easily extended by another member of the team with a reasonable level of effort?
  5. Scalability
    Is the code written in such a way that it would work for both 100 users and 10,000? Is it optimized?
  6. Style
    Does the code adhere to a particular style guide (preferably one agreed upon by the team if the project is collaborative)?

While I agree with this list, expanding it into an action guide of what reviewers should practically aim to give developers would be useful. So, reviewers should do the following:

  • Provide clear comments, demonstrate knowledge, and communicate well.
  • Point out the shortfalls in an implementation (without being overly critical).
  • State why a particular approach isn’t recommended, and, if possible, refer to blog posts, gists, specifications, MDN15 pages and jsPerf3216 tests to back up the statement.
  • Suggest alternative solutions, either in a separate runnable form or integrated in the code via a fork, so that the developer can clearly see what they did wrong.
  • Focus on solutions first, and style second. Suggestions on style can come later in the review, but address the fundamental problem as thoroughly as possible before paying attention to this.
  • Review beyond the scope of what was requested. This is entirely at the reviewer’s discretion, but if I notice issues with other aspects of a developer’s implementation, then I generally try to advise them on how those, too, might be improved. I’ve yet to receive a complaint about this, so I assume it’s not a bad thing.

Collaborative Code Reviews Link

Although a review by one developer can work well, an alternative approach is to bring more people into the process. This has a few distinct advantages, including reducing the load on individual reviewers and exposing more people to your implementation, which could potentially lead to more suggestions for improvements. It also allows a reviewer’s comments to be screened and corrected if they happen to make a mistake.

To assist the group, you may wish to employ a collaborative tool to allow all reviewers to simultaneously inspect and comment on your code. Luckily, a few decent ones out there are worth checking out:

  • Review Board17
    This Web-based tool is available for free under the MIT license. It integrates with Git, CVS, Mercurial, Subversion and a number of other source-control systems. Review Board can be installed on any server running Apache or lighttpd and is free for personal and commercial use.
  • Crucible18
    This tool by Australian software company Atlassian is also Web-based. It’s aimed at the enterprise and works best with distributed teams. Crucible facilitates both live review and live commenting and, like Review Board, integrates with a number of source-control tools, including Git and Subversion.
  • Rietveld19
    Like the other two, Rietveld also supports collaborative review, but it was actually written by the creator of Python, Guido van Rossum. It is designed to run on Google’s cloud service and benefits from Guido’s experience writing Mondrian, the proprietary app that Google uses internally to review its code.
  • Others
    A number of other options for collaborative code review weren’t created for that purpose. These include CollabEdit20 (free and Web-based) and, my personal favorite, EtherPad21 (also free and Web-based).

(Image Source: joelogon23)

Lessons From A JavaScript Code Review Link

On to the review.

A developer recently wrote in, asking me to review their code and provide some useful suggestions on how they might improve it. While I’m certainly not an expert on reviewing code (don’t let the above fool you), here are the problems and solutions that I proposed.

Problem 1 Link

Problem: Functions and objects are passed as arguments to other functions without any type validation.

Feedback: Type validation is an essential step in ensuring that you’re working only with input of a desired type. Without sanitization checks in place, you run the risk of users passing in just about anything (a string, a date, an array, etc.), which could easily break your application if you haven’t developed it defensively. For functions, you should do the following at a minimum:

  1. Test to ensure that arguments being passed actually exist,
  2. Do a typeof check to prevent the app from executing input that is not a valid function at all.
if (callback && typeof callback === "function"){
    /* rest of your logic */
    /* not a valid function */

Unfortunately, a simple typeof check isn’t enough on its own. As Angus Croll points out in his post “Fixing the typeof operator24,” you need to be aware of a number of issues with typeof checking if you’re using them for anything other than functions.

For example, typeof null returns object, which is technically incorrect. In fact, when typeof is applied to any object type that isn’t a function, it returns object, not distinguishing between Array, Date, RegEx or whatever else.

The solution is to use Object.prototype.toString to call the underlying internal property of JavaScript objects known as [[Class]], the class property of the object. Unfortunately, specialized built-in objects generally overwrite Object.prototype.toString, but you can force the generic toString function on them:[1,2,3]); //"[object Array]"

You might also find Angus’s function below useful as a more reliable alternative to typeof. Try calling betterTypeOf() against objects, arrays and other types to see what happens.

function betterTypeOf( input ){

Here, parseInt() is being blindly used to parse an integer value of user input, but no base is specified. This can cause issues.

In JavaScript: The Good Parts25, Douglas Crockford refers to parseInt() as being dangerous. Although you probably know that passing it a string argument returns an integer, you should also ideally specify a base or radix as the second argument, otherwise it might return unexpected output. Take the following example:

parseInt('20');       // returns what you expect, however…
parseInt('020');      // returns 16
parseInt('000020');   // returns 16
parseInt('020', 10);  // returns 20 as we've specified the base to use

You’d be surprised by how many developers omit the second argument, but it happens quite regularly. Remember that your users (if permitted to freely enter numeric input) won’t necessarily follow standard number conventions (because they’re crazy!). I’ve seen 020, ,20, ;'20 and many other variations used, so do your best to parse as broad a range of inputs as possible. The following tricks to using parseInt() are occasionally better:

Math.floor("020");   // returns 20
Math.floor("0020");  //returns 20
Number("020");  //returns 20
Number("0020"); //returns 20
+"020"; //returns 20

Problem 2 Link

Problem: Checks for browser-specific conditions being met are repeated throughout the code base (for example, feature detection, checks for supported ES5 features, etc.).

Feedback: Ideally, your code base should be as DRY as possible, and there are some elegant solutions to this problem. For example, you might benefit from the load-time configuration pattern here (also called load-time and init-time branching). The basic idea is that you test a condition only once (when the application loads) and then access the result of that test for all subsequent checks. This pattern is commonly found in JavaScript libraries that configure themselves at load time to be optimized for a particular browser.

This pattern could be implemented as follows:

var tools = {
    addMethod: null,
    removeMethod: null

if(/* condition for native support */){
    tools.addMethod = function(/* params */){
        /* method logic */
    /* fallback - eg. for IE */
    tools.addMethod = function(/* */){
        /* method logic */

The example below demonstrates how this can be used to normalize getting an XMLHttpRequest object.

var utils = {
    getXHR: null

    utils.getXHR = function(){
        return new XMLHttpRequest;
}else if(window.ActiveXObject){
    utils.getXHR = function(){
        /* this has been simplified for example sakes */
        return new ActiveXObject(’Microsoft.XMLHTTP’);

For a great example, Stoyan Stefanov applies this to attaching and removing event listeners cross-browser, in his book JavaScript Patterns26:

var utils = {
    addListener: null,
    removeListener: null
// the implementation
if (typeof window.addEventListener === ’function’) {
    utils.addListener = function ( el, type, fn ) {
        el.addEventListener(type, fn, false);
    utils.removeListener = function ( el, type, fn ) {
        el.removeEventListener(type, fn, false);
} else if (typeof document.attachEvent === ’function’) { // IE
    utils.addListener = function ( el, type, fn ) {
        el.attachEvent(’on’ + type, fn);
    utils.removeListener = function ( el, type, fn ) {
        el.detachEvent(’on’ + type, fn);
} else { // older browsers
    utils.addListener = function ( el, type, fn ) {
        el[’on’ + type] = fn;
    utils.removeListener = function ( el, type, fn ) {
        el[’on’ + type] = null;

Problem 3 Link

Problem: The native Object.prototype is being extended regularly.

Feedback: Extending native types is generally frowned upon, and few (if any) popular code bases should dare to extend Object.prototype. The reality is that there is not likely a situation in which you absolutely need to extend it in this way. In addition to breaking the object-as-hash tables in JavaScript and increasing the chance of naming collisions, it’s generally considered bad practice, and modifying it should only be a last resort (this is quite different from extending your own custom object properties).

If for some reason you do end up extending the object prototype, ensure that the method doesn’t already exist, and document it so that the rest of the team is aware why it’s necessary. You can use the following code sample as a guide:

if(typeof Object.prototype.myMethod != ’function’){
    Object.prototype.myMethod = function(){

Juriy Zaytsev3727 has a great post on extending native and host objects, which may be of interest.

Problem 4 Link

Problem: Some of the code is heavily blocking the page because it’s either waiting on processes to complete or data to load before executing anything further.

Feedback: Page-blocking makes for a poor user experience, and there are a number of ways to work around it without impairing the application.

One solution is to use “deferred execution” (via promises and futures). The basic idea with promises is that, rather than issuing blocking calls for resources, you immediately return a promise for a future value that will eventually be fulfilled. This rather easily allows you to write non-blocking logic that can be run asynchronously. It is common to introduce callbacks into this equation that execute once the request completes.

I’ve written a relatively comprehensive post28 on this with Julian Aubourg, if you’re interested in doing this through jQuery, but it can of course be implemented with vanilla JavaScript as well.

Micro-framework Q29 offers a CommonJS-compatible implementation of promises and futures that is relatively comprehensive and can be used as follows:

/* define a promise-only delay function that resolves when a timeout completes */
function delay(ms) {
    var deferred = Q.defer();
    setTimeout(deferred.resolve, ms);
    return deferred.promise;

/* usage of Q with the 'when' pattern to execute a callback once delay fulfils the promise */
Q.when(delay(500), function () {
        /* do stuff in the callback */

If you’re looking for something more basic that can be read through, then here is Douglas Crockford’s implementation of promises:

function make_promise() {
  var status = ’unresolved’,
      waiting = [],
      dreading = [];

  function vouch( deed, func ) {
    switch (status) {
    case ’unresolved’:
      (deed === ’fulfilled’ ? waiting : dreading).push(func);
    case deed:

  function resolve( deed, value ) {
    if (status !== ’unresolved’) {
      throw new Error(’The promise has already been resolved:’ + status);
    status = deed;
    outcome = value;
    (deed == ’fulfilled’ ? waiting : dreading).forEach(function (func) {
      try {
      } catch (ignore) {}
    waiting = null;
    dreading = null;

  return {
    when: function ( func ) {
      vouch(’fulfilled’, func);
    fail: function ( func ) {
      vouch(’smashed’, func);
    fulfill: function ( value ) {
      resolve(’fulfilled’, value);
    smash: function ( string ) {
      resolve(’smashed’, string);
    status: function () {
      return status;

Problem 5 Link

Problem: You’re testing for explicit numeric equality of a property using the == operator, but you should probably be using === instead

Feedback: As you may or may not know, the identity == operator in JavaScript is fairly liberal and considers values to be equal even if they’re of completely different types. This is due to the operator forcing a coercion of values into a single type (usually a number) prior to performing any comparison. The === operator will, however, not do this conversion, so if the two values being compared are not of the same type, then === will just return false.

The reason I recommend considering === for more specific type comparison (in this case) is that == is known to have a number of gotchas and is considered to be unreliable by many developers.

You might also be interested to know that in abstractions of the language, such as CoffeeScript, the == operator is completely dropped in favor of === beneath the hood due to the former’s unreliability.

Rather than take my word for it, see the examples below of boolean checks for equality using ==, some of which result in rather unexpected outputs.

3 == "3" // true
3 == "03" // true
3 == "0003" // true
3 == "+3" //true
3 == [3] //true
3 == (true+2) //true
’ trn ’ == 0 //true
"trn" == 0 //true
"t" == 0 // true
"tn" == 0 // true
"tr" == 0 // true
" " == 0 // true
" t" == 0 // true
"  " == 0 // true
" rn " == 0 //true

The reason that many of the (stranger) results in this list evaluate to true is because JavaScript is a weakly typed language: it applies type coercion wherever possible. If you’re interested in learning more about why some of the expressions above evaluate to true, look at the Annotated ES530 guide, whose explanations are rather fascinating.

Back to the review. If you’re 100% certain that the values being compared cannot be interfered with by the user, then proceed with using the == operator with caution. Just remember that === covers your bases better in the event of an unexpected input.

Problem 6 Link

Problem: An uncached array length is being used in all for loops. This is particularly bad because you’re using it when iterating through an HTMLCollection.

Here’s an example:

for( var i=0; i<myArray.length;i++ ){
    /* do stuff */

Feedback: The problem with this approach (which I still see a number of developers using) is that the array length is unnecessarily re-accessed on each loop’s iteration. This can be very slow, especially when working with HTMLCollections (in which case, caching the length can be anywhere up to 190 times faster than repeatedly accessing it, as Nicholas C. Zakas mentions in his book High-Performance JavaScript31). Below are some options for caching the array length.

/* cached outside loop */
var len = myArray.length;
for ( var i = 0; i < len; i++ ) {

/* cached inside loop */
for ( var i = 0, len = myArray.length; i < len; i++ ) {

/* cached outside loop using while */
var len = myArray.length;
while (len--) {

A jsPerf3216 test that compares the performance benefits of caching the array length inside and outside the loop, using prefix increments, counting down and more is also available, if you would like to study which performs the best.

Problem 7 Link

Problem: jQuery’s $.each() is being used to iterate over objects and arrays, in some cases while for is being used in others.

Feedback: In jQuery, we have two ways to seamlessly iterate over objects and arrays. The generic $.each33 iterates over both of these types, whereas $.fn.each()34 iterates over a jQuery object specifically (where standard objects can be wrapped with $() should you wish to use them with the latter). While the lower-level $.each performs better than $.fn.each(), both standard JavaScript for and while loops perform much better than either, as proven by this jsPerf test. Below are some examples of loop alternatives that also perform better:

/* jQuery $.each */
$.each(a, function() {
 e = $(this);

/* classic for loop */
var len = a.length;
for ( var i = 0; i < len; i++ ) {
    //if this must be a jQuery object do..
    e = $(a[i]);
    //otherwise just e = a[i] should suffice

/* reverse for loop */
for ( var i = a.length; i-- ) {
    e = $(a[i]);

/* classic while loop */
var i = a.length;
while (i--) {
    e = $(a[i]);

/* alternative while loop */
var i = a.length - 1;

while ( e = a[i--] ) {

You might find Angus Croll’s post on “Rethinking JavaScript for Loops35” an interesting extension to these suggestions.

Given that this is a data-centric application with a potentially large quantity of data in each object or array, you should consider a refactor to use one of these. From a scalability perspective, you want to shave off as many milliseconds as possible from process-heavy routines, because these can build up when hundreds or thousands of elements are on the page.

Problem 8 Link

Problem: JSON strings are being built in-memory using string concatenation.

Feedback: This could be approached in more optimal ways. For example, why not use JSON.stringify(), a method that accepts a JavaScript object and returns its JSON equivalent. Objects can generally be as complex or as deeply nested as you wish, and this will almost certainly result in a simpler, shorter solution.

var myData = {};
myData.dataA = [’a’, ’b’, ’c’, ’d’];
myData.dataB = {
    ’animal’: ’cat’,
    ’color’: ’brown’
myData.dataC = {
    ’vehicles’: [{
        ’type’: ’ford’,
        ’tint’: ’silver’,
        ’year’: ’2015’
    }, {
        ’type’: ’honda’,
        ’tint’: ’black’,
        ’year’: ’2012’
myData.dataD = {
    ’buildings’: [{
        ’houses’: [{
            ’streetName’: ’sycamore close’,
            ’number’: ’252’
        }, {
            ’streetName’: ’slimdon close’,
            ’number’: ’101’
console.log(myData); //object
var jsonData = JSON.stringify(myData);

{"dataA":["a","b","c","d"],"dataB":{"animal":"cat","color":"brown"},"dataC":{"vehicles":[{"type":"ford","tint":"silver","year":"2015"},{"type":"honda","tint":"black","year":"2012"}]},"dataD":{"buildings":[{"houses":[{"streetName":"sycamore close","number":"252"},{"streetName":"slimdon close","number":"101"}]}]}}

As an extra debugging tip, if you would like to pretty-print JSON in your console for easier reading, then the following extra arguments to stringify() will achieve this:

JSON.stringify({ foo: "hello", bar: "world" }, null, 4);

Problem 9 Link

Problem: The namespacing pattern used is technically invalid.

Feedback: While namespacing is implemented correctly across the rest of the application, the initial check for namespace existence is invalid. Here’s what you currently have:

if ( !MyNamespace ) {
  MyNamespace = { };

The problem is that !MyNamespace will throw a ReferenceError, because the MyNamespace variable was never declared. A better pattern would take advantage of boolean conversion with an inner variable declaration, as follows:

if ( !MyNamespace ) {
  var MyNamespace = { };

var myNamespace = myNamespace || {};

// Although a more efficient way of doing this is:
// myNamespace || ( myNamespace = {} );
// jsPerf test:

if ( typeof MyNamespace == ’undefined’ ) {
  var MyNamespace = { };

This could, of course, be done in numerous other ways. If you’re interested in reading about more namespacing patterns (as well as some ideas on namespace extension), I recently wrote “Essential JavaScript Namespacing Patterns36.” Juriy Zaytsev3727 also has a pretty comprehensive post on namespacing patterns38.

Conclusion Link

That’s it. Reviewing code is a great way to enforce and maintain quality, correctness and consistency in coding standards at as high a level as possible. I strongly recommend that all developers give them a try in their daily projects, because they’re an excellent learning tool for both the developer and the reviewer. Until next time, try getting your code reviewed, and good luck with the rest of your project!

(al) (il)

Footnotes Link

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  35. 35
  36. 36
  37. 37
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Addy Osmani is a Developer Programs Engineer on the Chrome team at Google. A passionate JavaScript developer, he has written open-source books like 'Learning JavaScript Design Patterns' and 'Developing Backbone Applications', having also contributed to open-source projects like Modernizr and jQuery. He is currently working on 'Yeoman' - an opinionated workflow for building beautiful applications.

  1. 1

    JSLint can perform as a so so code reviewer.

    • 2

      JSHint do it better than JSLint, unless you are Douglas Crockford.

    • 4

      The debate regarding whether you should or shouldn’t Lint your code aside (I believe you should and prefer using jsHint), I’m curious as to why one might prefer JSLint over Hint (@Florian).

      Do you just prefer the level of strictness it enforces during the lint process? :)

  2. 5

    Nice and well layed out article. I am not much into web programmming but still liked it. Thanks for awakening.

  3. 6

    Hey Addy,

    That’s one of the best articles I read this year. You drew exactly what I try to tell everyday to new developers.

    Every JS developer should read this.

  4. 8

    cancel bubble

    October 27, 2011 6:00 am

    Asked for just this yesterday morning on JSMentors (provided link to full web page, isolated .js file and also put the JS up on jsfiddle. Haven’t received a response yet :~(

    It must because there’s no flaws to be found. That’s gotta be it. Definitely.

    • 9

      Good work posting to JSMentors!. As I mentioned in the post, it can sometimes take a while to actually get reviews back, but fingers crossed you’ll get a response. If not, do feel free to reach out on Twitter or on IRC and I’m sure someone in the community will be happy to have a look at your code.

  5. 10

    One thing : thank you very much… It’s incredible : I had just read your article and exactly at the same time I had a Js problem with parseInt (I hadn’t specified the second argument, shame on me) !
    This is the fastest solution bring in my developer’s life !

  6. 11

    I’m not sure I agree with the feedback in Problem 1. The typeof operator returns one of six possible types: undefined, boolean, number, string, function, and object. Types such as Array, Date, and, yes, null are objects. If you need to know more specifically what type of object a variable references, you are looking for the instanceof operator.

    Great Article and a nice read!

    • 12

      Hey Jason. If I recall correctly, the feedback I provided with respect to typeof was how to use it correctly if you *were* going to opt for that approach to type-checking in an application (really just to ensure the developer understood that it came with a number of gotchas they should be aware of). One could of course opt for instanceof, but I really wanted to hit home how to ‘fix’ that particular operator :)

  7. 13

    Thanks so much i like a lot this article :D

  8. 15

    About problem 1, is the “fail-fast” approach really bad ? Today, we have good JavaScript debuggers which allow us to identify easily where a script failed. If we really want to do “defensive programming”, should we really put type checking *everywhere* in our application code and keep these tests in production as suggested here ? Wouldn’t it be better to apply a programming by contract approach where type checking code could be removed in production ?

    • 16

      I think I may have mentioned this in another comment, but in the first problem I wanted to ensure the developer was aware first about the gotchas involved in using the typeof operator, but also how to ‘fix’ that particular behaviour if they were going to opt to use it.

      I would say that the argument for or against defensive programming might fall under a separate discussion, but my thoughts on this are, employ defensive programming only where:

      1) You believe there’s a strong chance the user is likely to pass input through which is either not caught during client-side validation (or for whatever reason cannot be).

      2) You believe data being passed through your application, regardless of whether this is being returned by a third party API, methods with variable output forms or some other process where validating the data in an earlier step is not possible, would benefit from a level of type-checking.

      Where issues like the above are unlikely to be run into, I would say defensive development probably doesn’t make the most sense.

      • 17

        I think check_ca raises an interesting point. There’s so much emphasis on performance in Javascript, how much of a performance hit should users be expected to bear just to ensure the programmer doesn’t make mistakes? I don’t know if typeof could be called “expensive” but it involves reflection so would at least be non-trivial. There’s benchmarks on things like using array length for loops. I wonder about benchmarks for having rigorous typechecking all through your code as opposed to none. Surely it’s necessary to know the costs and benefits?

  9. 18

    Igors Brezinskis

    October 27, 2011 12:51 pm

    Very usefull post! Thanks Addy! I am looking forward to reed more so good explained works as this one!

  10. 19

    Nice article mate.

  11. 20

    Veeeeery useful article ! I use jQuery a lot and must admit I don’t pay enough attention to writing “clean” code… shame on me! But this article has really given me the will to change that and pay attention to “details”. Thanks a lot!

  12. 21

    to solve the first problem I’ve created a library:

    and by the way, code +’020′ also may cause problem because ‘+’ is also used to concatenate strings. So instead of +’020′ I use 1*’020′

  13. 22

    Very nice article, thanks you!

  14. 23

    scot mcintire

    October 28, 2011 8:10 am

    Concise and well written. Thank you!

  15. 24

    Nices tips, as usual, Thanks you for sharing

    Also these days i come across the testing of performances between 4 functions for mapping object’s keys to an array. Here the result.

    Newly ES5 standards ( give JavaScript strength… var arr = Object.keys({a:1,b:2}); is nearly 2 times faster than iterating… of course…

  16. 25

    Hi Addy — The linked JSperf test for iterating over objects/arrays is incorrect because you’re querying the cached selector for all of the tests except jQuery.fn.each.

    Here’s an updated test:

    As you can see, there’s no substantive performance difference between the alternatives.

    • 26

      Hey Dave. Thanks for pointing that out! I’ve looked through your test case and I think that redeclaring the selection on each iteration for each test may have a negative overall impact on the figures being reported.

      Here’s the original test case which uses a cached selector for the final test on jQuery.fn.each (which hopefully means that all cases are being tested on an equal playing ground).

      Whilst in Chrome you’re absolutely right in there being a negligible difference, you’ll notice that in browsers like Firefox, IE and Opera $.fn.each() doesn’t perform as well as some of the other alternatives :)

      • 27

        Hi Addy — Your tests are still unequal because you’re retrieving the DOM elements in the preparation code using .get() for all of the tests *except* jQuery.fn.each. Obviously, the .get() call doesn’t come for free so it should be included in the tests, not as prep code.

        What you’ll find is that there isn’t much of a performance difference between using native `for` loops and `jQuery.each` or `jQuery.fn.each` (FYI: jQuery.fn.each is just a wrapper for jQuery.each which is a wrapper for a `for` loop).

        Readability and consistency of usage are far more important than ekeing out a 2% performance boost on an already fast piece of code.

        • 28

          Weird. I’m not sure if this is simply an issue with jsPerf or the way I’ve setup the test, but although iterating over collections seems to work absolutely fine offline/on jsFiddle/jsBin using jQuery.each, with jsPerf I’m encountering errors when iterating over ‘a’ with that method (unless the underlying DOM nodes are accessed) which is why I opted for the way it’s done now.

          If you know a better way of getting the test setup (based on revision 44) please feel free to share it.

          Btw, I know about jQuery.each vs jQuery.fn.each.. :) Had to explain the differences for a few years. Where I think it’s important to realize that there can be benefits (occasionally) to performance benchmarking these various approaches is where you’re not just working with small sets of elements, but significantly larger ones. That 2% speed boost won’t matter in 80% of cases where you just have a few items on a page, but if a page has thousands of elements on it, it may have some positive impact using the most optimal approach available.

          For this particular review, the developer in question was working on something that required quite extensive DOM manipulation or a large collection which is why I was suggesting looking at these other approaches :)

  17. 29

    Great information. Realy helpful.Thank you for sharing.

  18. 30

    thanks for share great articles. I learn a lot.

  19. 31

    Patricia Sousa

    November 8, 2011 6:17 pm

    Thanks for this blog really great keep up with it. specially the way of writing your blog is awesome and fabulous.

  20. 32

    Another great collaborative code editing tool is .

  21. 33

    Appears that jsMentors is no more.


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