A simple map function for plain JavaScript arrays

12Aug/097

A simple map function for plain JavaScript arrays

So this isn't anything new, but sometimes you have to work with plain JavaScript and can't lean on a library. I thought I'd put this out there. This is my map function. There are many like it, but this one is mine.

PLAIN TEXT

CODE:

Array.prototype.map = function(fn) {
var r = [];
var l = this.length;
for(i=0;i<l;i++)
{
r.push(fn(this[i]));
}
return r;
};

note bene: Why do I declare the "l" variable at all instead of just checking i against this.length directly? The JavaScript for loop is much faster if you don't perform the length lookup and instead check against a static value.

Tagged as: Javascript Leave a comment

http://codebetter.com/blogs/gregyoung Greg Young

Its called “manual hoisting”

Cheers,

Greg
http://blog.dotsmart.net Duncan Smart

What about sparse arrays? (ok, maybe they’re rare on the wild) How about for … in to loop through?
http://kohari.org/ Nate Kohari

Very useful. (I have a similar implementation myself.) By the way, I’ve used this sort of syntax for loops:

for (int idx = 0, len = arr.length; idx++; idx < len) {
…
}
http://www.lazycoder.com Scott

Duncan: with a for-in loop and a JavaScript array, you end up looping over all members of the Array type. If you have modified the prototype of the native Array, you’ll end up iterating over them as well. See this post for a better explanation as well as a way to avoid that.

http://www.lazycoder.com/weblog/2008/05/23/modern-javascript-development-reflection-in-javascrpt/

edit: Ok, that post I linked to helps if you are dealing with objects that are not native JavaScript objects. Native JavaScript objects are a little bit different. The members of native JavaScript objects are marked as non-enumerable, so for-in works on them provided that their prototype object has not been modified. If it has, you’ll have to use a combination of the technique described in the post aboce as well as special casing out the members you know have been added to the native JavaScript object.
Pingback: Daily Links for Thursday, August 13th, 2009
Pingback: Dew Drop – August 13, 2009 | Alvin Ashcraft's Morning Dew
Pingback: JavaScript: Not for the faint at heart? | Lazycoder
Pingback: A simple map function for plain JavaScript arrays | Lazycoder
argimenes

Nice article… with respect to concise syntax, you could try:

Array.prototype.map = function(fn) {
for (i=0, r=[], l = this.length; i < l; r.push(fn(this[i++])));
return r;
};
jesus

it would be best if you did r = new Array(l)
thomd

what about:

Array.prototype.map = function(fn) {
for (i = this.length; this[i] = fn(this[i--]); );
return this;
};

JavaScript: Not for the faint at heart? » « Herding Code 56: Markus Völter on Model-Driven Development, DSLs and Product Line Engineering

Subscribe to Blog via Email

.NET HTTP Abstractions group

Subscribe to .NET HTTP Abstractions

Email:

Visit this group

Lazycoder