arguments: A JavaScript Oddity

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by Andrew Tetlaw

arguments is the name of a local, array-likeobject available inside every function. It’s quirky, often ignored, butthe source of much programming wizardry; all the major JavaScriptlibraries tap into the power of the arguments object. It’s something every JavaScript programmer should become familiar with.

Inside any function you can access it through the variable: arguments,and it contains an array of all the arguments that were supplied to thefunction when it was called. It’s not actually a JavaScript array; typeof arguments will return the value: "object". You can access the individual argument values through an array index, and it has a length property like other arrays, but it doesn’t have the standard Array methods like push and pop.

Create Flexible Functions

Even though it may appear limited, arguments is a very useful object. For example, you can make functions that accept a variable number of arguments. The format function, found in the base2 library by Dean Edwards, demonstrates this flexibility:

function format(string) {
  var args = arguments;
  var pattern = new RegExp("%([1-" + arguments.length + "])", "g");
  return String(string).replace(pattern, function(match, index) {
    return args[index];
  });
};

You supply a template string, in which you add place-holders for values using %1 to %9, and then supply up to 9 other arguments which represent the strings to insert. For example:

format("And the %1 want to know whose %2 you %3", "papers", "shirt", "wear");

The above code will return the string "And the papers want to know whose shirt you wear".

One thing you may have noticed is that, in the function definition for format, we only specified one argument: string. JavaScript allows us to pass any number of arguments to a function, regardless of the function definition, and the arguments object has access to all of them.

Convert it to a Real Array

Even though arguments is not an actual JavaScript array we can easily convert it to one by using the standard Array method, slice, like this:

var args = Array.prototype.slice.call(arguments);

The variable args will now contain a proper JavaScript Array object containing all the values from the arguments object.

Create Functions with Preset Arguments用已有参数创建函数

The arguments object allows us to perform all sorts of JavaScript tricks. Here is the definition for the makeFuncfunction. This function allows you to supply a function reference andany number of arguments for that function. It will return an anonymousfunction that calls the function you specified, and supplies the presetarguments together with any new arguments supplied when the anonymousfunction is called:

arguments 对象允许我们实现很多JavaScript 技巧。下面是一个makeFunc 函数的定义，这个函数允许你提供一个函数引用及其参数，它将返回一个调用你指定函数的匿名函数，当这个匿名函数被调用的时候，它还允许你提供已有参数和任何新的参数。

function makeFunc() {
  var args = Array.prototype.slice.call(arguments);
  var func = args.shift();
  return function() {
    return func.apply(null, args.concat(Array.prototype.slice.call(arguments)));
  };
}

The first argument supplied to makeFunc is consideredto be a reference to the function you wish to call (yes, there’s noerror checking in this simple example) and it’s removed from thearguments array. makeFunc then returns an anonymous function that uses the apply method of the Function object to call the function specified.

The first argument for apply refers to the scope the function will be called in; basically what the keyword this will refer to inside the function being called. That’s a little advanced for now, so we just keep it null. The second argument is an array of values that will be converted into the arguments object for the function. makeFuncconcatenates the original array of values onto the array of argumentssupplied to the anonymous function and supplies this to the calledfunction.

Lets say there was a message you needed to output where the templatewas always the same. To save you from always having to quote thetemplate every time you called the format function you could use the makeFunc utility function to return a function that will call format for you and fill in the template argument automatically:

var majorTom = makeFunc(format, "This is Major Tom to ground control. I'm %1.");

You can call the majorTom function repeatedly like this:

majorTom("stepping through the door");
majorTom("floating in a most peculiar way");

Each time you call the majorTom function it calls the format function with the first argument, the template, already filled in. The above calls return:

"This is Major Tom to ground control. I'm stepping through the door."
"This is Major Tom to ground control. I'm floating in a most peculiar way."

Create Self-referencing Functions

You may think that’s pretty cool, but wait, arguments has one more surprise; it has another useful property: callee. arguments.callee contains a reference to the function that created the arguments object. How can we use such a thing? arguments.callee is a handy way an anonymous function can refer to itself.

repeat is a function that takes a function reference,and 2 numbers. The first number is how many times to call the functionand the second represents the delay, in milliseconds, between eachcall. Here’s the definition for repeat:

function repeat(fn, times, delay) {
  return function() {
    if(times-- > 0) {
      fn.apply(null, arguments);
      var args = Array.prototype.slice.call(arguments);
      var self = arguments.callee;
      setTimeout(function(){self.apply(null,args)}, delay);
    }
  };
}

repeat uses arguments.callee to get a reference, in the variable self,to the anonymous function that runs the originally supplied function.This way the anonymous function can call itself again after a delayusing the standard setTimeout function.

So, I have this, admittedly simplistic, function in my applicationthat takes a string and pops-up an alert box containing that string:

function comms(s) {
  alert(s);
}

However, I want to create a special version of that function thatrepeats 3 times with a delay of 2 seconds between each time. With my repeat function, I can do this:

var somethingWrong = repeat(comms, 3, 2000);

somethingWrong("Can you hear me, major tom?");

The result of calling the somethingWrong function is an alert box repeated 3 times with a 2 second delay between each alert.

arguments is not often used, a little quirky, but full of surprises and well worth getting to know!

Share and Enjoy:

This entry was posted on Tuesday, November 11th, 2008 at 4:06 pm, contains 994 words, 6 images, and is filed under JavaScript & CSS. You can follow any responses to this entry through the RSS 2.0feed. You can skip to the end and leave a response. Pinging iscurrently not allowed. The views and opinions in this blog post arethose of its author.

This post has 14 responses so far

Dougal Matthews Says:
November 11th, 2008 at 9:59 pm
Just a quick note to point out all Arrays show as ‘object’ when youuse the typeof operator. I just done the followng and got ‘object’;
alert(typeof []);
Nunks Says:
November 12th, 2008 at 1:13 am
Gotta love the Space Oddity examples ^^
jef2904 Says:
November 12th, 2008 at 1:40 am
Sounds interesting. Still a bit confused though. I will look into it though. Do you know any good documentations for?
Jesse Says:
November 12th, 2008 at 5:34 am
Excellent post on a lesser-understood topic, Andrew. One aspect ofthis sort of functional programming I struggle with, esp withjavascript’s peculiarities, is how to make clear either thru commentsor code just what is happening. Often this sort of code takes a whileto decipher, even if you understand the prototypal, first-classfunctional javascript way. Any thoughts?
Andrew Tetlaw Says:
November 12th, 2008 at 8:29 am
Dougal, you’re right! Sorry for making such a noob mistake. The correct way to check is this:
```
  var a = [];
  alert(a.constructor.toString());
```
In Firefox that will return:
```
function Array() { [native code] }
```
Inside a function, if you call:
```
  alert(arguments.constructor.toString());
```
You’ll get:
```
function Object() { [native code] }
```
Andrew Tetlaw Says:
November 12th, 2008 at 8:58 am
Jessie, yeah, it’s hard to write, hard to work out what the codedoes if you didn’t write it, and just as hard to explain what it’s dongin a blog post like this :)
There’s lots of invisible forces at work, especially regardingscope. But once, you are familiar with the patterns, you can identifyand write them easy enough.
It makes JavaScript so expressive though, and that makes it worth persisting with.
awasson Says:
November 12th, 2008 at 9:33 am
Really cool article!
A couple of years ago I stumbled upon the arguments variable whilewriting a quick-n-dirty slideshow script. I only learned enough to dothe job so it’s great to get some more insight & info from yourarticle.
Thanks,
Andrew
Jesse Says:
November 12th, 2008 at 9:56 am
Don’t get me wrong, I’m a big fan of Javascript and consider it oneof the most expressive languages I’ve worked with, but while it allowsfor some incredibly elegant solutions to problems, it also fosters atremendous amount in-elegance in readability and clarity of purpose. Iwas just wondering if there are any best-practices for code layout,naming, commenting, beyond the usual programming conventions.
Trep Says:
November 12th, 2008 at 10:05 am
Hmmm… that repeat() function is really one of those little thingy to sit down and meditate on! ;-)
-
I was wondering: the “apply” method should take an array as its second argument, yet you call it successfully as
–
fn.apply(null, arguments);
–
if made some experiments and it works just as well if you call on the “args” array created by splicing “arguments”.
-
On the other hand “self.apply(null,args)” doesn’t work if you call it on “arguments” instead of “args”.
-
Hm…ok, I got it. “self.apply(null,args)” is inside another anonymousfunction so “arguments” get stomped over by the new one. The power ofmeditation!
Trep Says:
November 12th, 2008 at 10:07 am
Errata: the third paragraph should read:
«I made some experiments and it works just as well if you call it on the “args” array created by splicing “arguments”».
Andrew Tetlaw Says:
November 12th, 2008 at 10:21 am
Trep, yeah I didn’t explain the closures that are used in the above code.
you’ll notice the variable times, is passed to the outer function, but used in the anonymous function. Which is fine, JavaScript let’s us do that.
However, every time a function is called the local variable arguments is created. So I can use it safely inside the inner anonymous function here:
```
fn.apply(null, arguments);
```
Because I know that when that anonymous function is eventually called, argumentswill contain the arguments that are supplied at the time it’s called,not at the time when we are defining the anonymous function.
However, to use setTimeout I have to create yet another anonymous function, the one that calls apply:
```
function(){self.apply(null,args)}
```
However, to the apply call I need to pass the original argumentsthat were supplied to the outer anonymous function… so I use yetanother closure:
```
var args = Array.prototype.slice.call(arguments);
```
The args variable is defined in the outer anonymous function, but used within the inner anonymous function.
Phew!
polvero Says:
November 13th, 2008 at 5:57 am
instead of…
var args = Array.prototype.slice.call(arguments);
do this…
var args = [].slice.call(arguments, 0);
brothercake Says:
November 13th, 2008 at 11:16 am
But don’t use arguments just for the sake of it - only if you really need to access them as a collection - because this:
function colorize() { ... }
is not as easy to understand at a glance, as this:
function colorize(foreground, background) { ... }
eric d. Says:
January 20th, 2009 at 12:01 am
Hi, thanks for that brilliant article. Made a lot of things a lot clearer!
Note: new RegExp("%([1-" + arguments.length + "])", "g"); will fail passed 9 arguments (the regexp would be "%([1-10])" so it will only match %0 and %1).
I think an easy fix would be something like:
function format(string) { var args = arguments; var pattern = new RegExp("%([0-9]+)", "g"); return String(string).replace(pattern, function(match, index) { if (index == 0 || index >= args.length) throw "Invalid index in format string"; return args[index]; }); };
(Sorry for nitpicking, I understand it was only an example and brevety is the main objective, but its a great function to have)