In my last post I wrote about how to detect a JavaScript array, so I thought it would be a good idea to see how jQuery does it.

Here's the jQuery.isArray method (in 1.6.2):

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isArray: Array.isArray || function( obj ) {
    return jQuery.type(obj) === "array";
}

It uses the native Array.isArray method if it exists, if not it will find out the type of whatever is passed in using the jQuery.type method and see if that equals the string "array".

But come on, that's not much of a learning experience not to mention it makes for a pretty short blog post.

So instead, I looked into the jQuery.type method to see how that works.

jQuery.type

It looks something like this:

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// Save a reference to some core methods
toString = Object.prototype.toString
...
// [[Class]] -> type pairs
class2type = {};
...
type: function( obj ) {
    return obj == null ?
        String( obj ) :
        class2type[ toString.call(obj) ] || "object";
}
...
// Populate the class2type map
jQuery.each(
    "Boolean Number String Function Array Date RegExp Object".split(" "),
    function(i, name) {
    class2type[ "[object " + name + "]" ] = name.toLowerCase();
})

(Distant pieces of code are separated with three dots)

Breaking it down

• Line 3 - A reference to the toString method of Object.
• Line 6 - An internal object (map) that will contain all types as strings.
• Line 15-19 - For each member in the array we're passing in, we'll create a property in the object and set its value to the name of the array member in lowercase.
• Line 8-12 - The method will check if the argument passed in is null or undefined, if so, it will return the string "null" or "undefined" (by calling the String constructor without the new keyword, which is allowed for some constructors). If it's something else, we'll call the toString method on the argument and look to see if the class2type object has a value for it, if not, we'll just return the string "object".

To give you a better overall picture, here's how the class2type object would look after page load:

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var class2type = {
    "[object Array]": "array",
    "[object Boolean]": "boolean",
    "[object Date]": "date",
    "[object Function]": "function",
    "[object Number]": "number",
    "[object Object]": "object",
    "[object RegExp]": "regexp",
    "[object String]": "string"
};

Epiphany

That's why we call the toString method of Object when we're looking for the property name. Remember how this method returns this sort of a "[object Constructor]" string pattern?

So, if we did jQuery.type("hello") we'd actually be doing something like class2type["[object String]"], which would return the corresponding value "string"

And that's how the jQuery.type method works.

It's not that complicated when you have an "Epiphany" section, huh?

Sources:

jQuery.type - jQuery API Documentation

jQuery 1.6.2 Source Code