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Posts tagged: "javascript" - page 3

Javascript Object Promotion

So it’s not something you should have to worry about (often), but I just spent an inordinate amount of time figuring out exactly why a particularly innocuous-looking piece of code was failing. It’s also yet another surprising part of the javascript language, and if you often deal with javascript then it’s in your best interests to keep track of such oddities (there are a lot of them!).

To illustrate, consider the following array intersection function:

    var list1 = ["a", "b", "c"];
    var list2 = ["a", "b", "d"];

    function intersect(a,b) {
      // find the intersection of two arrays
      var intersection = [];
      jQuery.each(a, function() {
        if(b.indexOf(this) != -1) {
      return intersection;

You might expect calling intersect(list1, list2) to return ["a", "b"], since those are the common elements in the two sets. However, what you really get is [].

The problem is that I was a bit lazy, and used jQuery.each() to iterate over my collection (I hate the for x in y construct, and for loops are just so C). The each method calls your provided function on each member of the provided array, setting this in the context of each call to the current array element.

But it turns out that when you call a method on a primitive string object, the javascript (well, ECMAScript really) language specifies that this call takes place not on the string primitive type, but on the String object type (it’s kinda like a boxed value in Java). That is,"some string") actually ends up as if you had written String("some string")).

The String and string objects will compare equal when using the “==” operator - but since they are of different types, they will not compare as equal using the “===” operator. Evidently that is the type of equality that indexOf() uses, therefore none of the String objects will ever appear in the array of string primitives.

Normally, you will neither notice nor care. However, when extending the String class or calling a function with a primitive type as the subject, you should keep this conversion in mind. Note that the conversion does not apply to function arguments, only to the subject of a function application (i.e this).

I’m continually amazed by the high quality answers found on - I posted this question asking why the object promotion occurs (and if that is indeed what’s going on), and was rewarded with an explanation and a link to the precise part in the ECMAScript spec where the behaviour is detailed.

Making sense of async.js

async.js is a fascinating library for taming asynchronous javascript. It’s highly experimental, and as far as I know, it only works in firefox. But the idea is an important (and useful) one, so I think it’s definitely worth knowing about.

There are a few approaches to dealing with asynchronous javascript:

  1. Compile [some language] to javascript This is the approach taken by GWT, pyjamas, and many others. It’s usually extremely heavyweight, so it mainly makes sense for big apps.

  2. Compile [almost-javascript] to javascript Most notably this includes Narrative Javascript and its successor, Strands. This is a reasonable approach, but the lack of maturity / tool support makes debugging extremely hard. Also, I ran into a number of bugs in both these libraries. The thought of finding and fixing more of those bugs is not at all fun.

  3. Write a javascript library This is doable, but typically looks hideous, convoluted, and is usually quite burdensome to try and use.

Async.js is the most plausible attempt at #3 that I’ve seen. It uses a reasonably clever (but not unknown) trick to to turn Javascript 1.7’s generator functionality into an event-based coroutine system. Specifically, this allows for a program to “wait” for a callback, while not actually blocking the javascript interpreter (as a synchronous AJAX call would). Go read the async.js page to learn more, because the following isn’t going to make much sense if you don’t know roughly how to use async.js.

Javascript's fragile "this" statement

Javascript’s this statement must be the most fragile and confusing statement I’ve come across. Many languages have the concept of “this”, but none mess with it to hard as javascript does. As I have recently discovered, there are two fundamental issues with javascript’s this:

  1. this does not get captured when storing an object member into a function object
  2. you can never actually be certain what this will be

For object-oriented programming, these two facts are entirely terrifying. Let me illustrate each one:

1. this does not get captured when storing an object member into a function object

function Obj() {
	this.method = function() {
		return "method() called - I am " + this;

	this.toString = function() {
		return "[Obj instance]";

Now, consider the following scenarios:

var obj = new Obj();
// returns "method() called - I am [Obj instance]"

var obj_method = obj.method;
// returns "method() called - I am [object DOMWindow]"

What happened to this? I can find no explanation anywhere as to why it has been lost (and the window object used in its place), but it is consistent. This may not seem like a big deal, but even aside from the awkwardness, it’s completely non-obvious - and therefore a great candidate for sneaky bugs.

2. You can never actually be certain what this will be

Some see this as a feature, and it is in some cases. But the fact remains that the caller of any function can set this to be any object they choose is cause for great suspicion on the part of any callback code.

What’s worse, as a library writer there are cases where it’s impossible to not mess with the value of this. Other languages have the concept of a unsplat operator. Google it if you don’t know this term, but basically it will turn a list of objects into an argument list. That is, func(1,2,3) is the same as func(*[1,2,3]) (where * is the un-splat operator). This is very important for higher-order / functional programming, where you might write a proxy function that wraps a normal function call with some useful behaviour.

Anyways, javascript does have an unsplat operator. Kind of… The following code will work:

function call_other() {
	var _arguments =;
	var func = _arguments[0];
	var func_args = _arguments.slice(1);
	// do whatever proxy stuff you need to do here
	func.appy(null, func_args);

Except for that first parameter to the apply function. Whatever you pass in there is what this will be set to in the context of the called function. With no discernible means of extracting what this would normally be for the given function, it becomes impossible not to clobber the otherwise extremely-useful this statement.

navim update

So I’ve kept working on navim, my jQuery plugin for easily adding vim-style keyboard navigation to web pages. New features include:

  • shift+enter to open links in a new window (and the ability to tell if shift is pressed from a custom action callback)
  • fixed a bug that interfered with pressing return to submit a form
  • using focus(), blur() and tab-key navigation to better effect

I’ve also now implemented navim in my “read later” webapp, pagefeed. It was trivial enough to add “d” as an additional keyboard shortcut to delete the currently active item, which serves as a good example for anyone wanting to add their own custom action keys. The code is simply:

$(window).keypress(function(e) {
	if(e.which == 100) { // 'd'
		return false;

The rise of vi keybindings on the web

Vi? On the internet? No, not that one. I just mean vi keybindings, not the rest of vi. And I’ve just written a jQuery plugin to help make this happen.

On the UNIX terminal, many commands use common keyboard shortcuts to navigate screenfuls of text. j/k for down/up, h/l for left/right, and so on. As far as I know, vi was the first program to use these shortcuts. vi is famous for its modal interface, where different keys mean different things depending on what mode you’re in. In text insertion mode, “hjkl” means exactly those letters. But in normal (or visual) mode, they are the keys you use for navigation. Using standard letters instead of the arrow keys could have come about for a number of reasons. Firstly, there’s the issue that different terminals send different key-codes for special keys like the arrow keys. That’s generally been solved these days, but the other reason still remains: By using the whole alphabet as control keys, you get a startling amount of “command bandwidth” - that is, commands in vi generally require much less finger-contortion than pretty much every other text editor.

vi keybindings are useful on the internet for a third (but related) reason - keybinding collisions. We already have meanings for what the arrow keys do (scroll), and trying to use control-key combinations for webpage-specific functionality is fraught with user frustration, confusion and technical issues.

I’ve noticed it already with a bunch of google products, especially since I started using gmail’s online version most of the time now (instead of Gmail and Google reader are the two big ones that I use, but I’m sure there are more. Both use vi-style keyboard navigation, and both are a delight to use with the keyboard. I’d be surprised if bespin lasts long before a vi-mode is added.

I guess many people think only data-heavy, webapps are worth learning keyboard shortcuts for. But the real benefit comes when everything (or at least most things you care about) use the same convenient shortcuts. I was absolutely delighted when I noticed that every issue of The Big Picture allows you to use j/k to jump to successive images - incredibly useful in this case, because page up/down rarely manages to line up to image boundaries. It’s an unobtrusive addition, and it won’t hurt regular users. But for those who do use it, it quickly becomes indispensable.

So what I’d love is for all websites with many conceptual “items” on a page to implement the j/k keybinding as a minimum (with horizontal and inter-page navigation coming later, hopefully). To this end, I have written a jQuery plugin that should make the process fairly trivial, and allow for “active item” decoration via CSS. Click that link for a demo you can try out yourself.

If there’s enough interest, maybe someone could turn it into a community-based firefox extension (a-la AutoPager) that allows users to define the navigation items for websites that haven’t supplied their own (google search would be a handy one).

It’s worth mentioning Vimperator, which brings vi keybindings to firefox’s UI. To clarify, I’m not talking about browser functionality. I’m happy to use the existing controls for a browser, and leave the alphabet keys for use by the web-page. That way there’s no overlap, and webpages can provide contextual navigation controls that are much more powerful than the basic “scroll down 30 pixels” that a browser provides.