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libphutil/src/docs/drafts/javascript_object_array.txt
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This document describes the behaviors of Object and Array in Javascript.
=Primitives=
Javascript has two native datatype primitives, Object and Array. Both are classes, so you can use <tt>new</tt> to instantiate new objects and arrays:
<code badcode>
var a = new Array(); // Not preferred.
var o = new Object();
</code>However, <b>you should prefer the shorthand notation</b> because it's more concise:
<code> var a = []; // Preferred.
var o = {};
</code>(A possible exception to this rule is if you want to use the allocation behavior of the Array constructor, but you almost certainly don't.)
The language relationship between Object and Array is somewhat tricky. Object and Array are both classes, but "object" is also a primitive type. Object is <i>also</i> the base class of all classes.
<code> typeof Object; // "function"
typeof Array; // "function"
typeof {}; // "object"
typeof []; // "object"
var a = [], o = {};
o instanceof Object; // true
o instanceof Array; // false
a instanceof Object; // true
a instanceof Array; // true
</code>If this is confusing (it is), refer to [[Articles/Object-Oriented Javascript]] for a run-down on how this stuff works.
=Objects are Maps, Arrays are Lists=
PHP has a single <tt>array</tt> datatype which behaves like as both map and a list, and a common mistake is to treat Javascript arrays (or objects) in the same way. <b>Don't do this.</b> It sort of works until it doesn't. Instead, learn how Javascript's native datatypes work and use them properly.
<b>In Javascript, you should think of Objects as maps ("dictionaries") and Arrays as lists ("vectors").</b>
You store keys-value pairs in a map, and store ordered values in a list. So, store key-value pairs in Objects.
<code> var o = { // Good, an object is a map.
name: 'Hubert',
species: 'zebra'
};
console.log(o.name);
</code>And store ordered values in Arrays.
<code> var a = [1, 2, 3]; // Good, an array is a list.
a.push(4);
</code>Don't store key-value pairs in Arrays and don't expect Objects to be ordered.
<code badcode>
var a = [];
a['name'] = 'Hubert'; // No! Don't do this!
</code>This technically works because Arrays are Objects and you think everything is fine and dandy, but it won't do what you want and will burn you.
=Iterating over Maps and Lists=
Iterate over a map like this:
<code> for (var k in object) {
f(object[k]);
}
</code>Iterate over a list like this:
<code> for (var ii = 0; ii < list.length; ii++) {
f(list[ii]);
}
</code>Note that there are a bunch of provided iterators for Array, like Array.each.
If you try to use <tt>for (var k in ...)</tt> syntax to iterate over an Array, you'll pick up a whole pile of keys you didn't intend to and it won't work. If you try to use <tt>for (var ii = 0; ...)</tt> syntax to iterate over an Object, it won't work at all.
<b>If you consistently treat Arrays as lists and Objects as maps and use the corresponding iterators, everything will always work properly.</b>
=Sparse Arrays=
One wrench in this mess is that Arrays aren't precisely like lists, because they do have indexes and may be sparse:
<code> var a = []; a[2] = 1;
console.log(a); // [undefined, undefined, 1]
</code>The correct way to deal with this (as implemented by the provided iterators) is:
<code> for (var ii = 0; ii < list.length; ii++) {
if (list[ii] == undefined) {
continue;
}
f(list[ii]);
}
</code>
=Ordered Maps=
If you need an ordered map, you need to have a map for key-value associations and a list for key order. Don't try to build an ordered map using one Object or one Array. This generally applies for other complicated datatypes, as well; you need to build them out of more than one primitive.
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