Simple type checking in JavaScript.

type.js

(function (root) {

    var type = function (o) {

        // handle null in old IE
        if (o === null) {
            return 'null';
        }

        // handle DOM elements
        if (o && (o.nodeType === 1 || o.nodeType === 9)) {
            return 'element';
        }

        var s = Object.prototype.toString.call(o);
        var type = s.match(/\[object (.*?)\]/)[1].toLowerCase();

        // handle NaN and Infinity
        if (type === 'number') {
            if (isNaN(o)) {
                return 'nan';
            }
            if (!isFinite(o)) {
                return 'infinity';
            }
        }

        return type;
    };

    var types = [
        'Null',
        'Undefined',
        'Object',
        'Array',
        'String',
        'Number',
        'Boolean',
        'Function',
        'RegExp',
        'Element',
        'NaN',
        'Infinite'
    ];

    var generateMethod = function (t) {
        type['is' + t] = function (o) {
            return type(o) === t.toLowerCase();
        };
    };

    for (var i = 0; i < types.length; i++) {
        generateMethod(types[i]);
    }

    if (typeof exports === "object" && exports) {
        exports = type;
    }
    else if (typeof define === "function" && define.amd) {
        define(type);
    }
    else {
        root.type = type;
    }

})(this);

Nicely done!

Checkout http://jsperf.com/type-cecking for some performance comparisons…

Jon, I listend to you awesome talk at JSConf.eu 2013 and have been using the type() method you proposed in your presentation in some of my hobby projects since then (most usually to assert parameter values).

I'd love to see this as an npm plugin, that i don't need to copy it in every project! ;)