typeofを改善したtypeOf()関数 ref: http://qiita.com/Hiraku/items/87e5d1cdaaa475c80cc2

typeOfの戻り値

console.log(typeOf(undefined));         //'undefined'
console.log(typeOf(null));              //'null'
console.log(typeOf("str"));             //'string'
console.log(typeOf(new String("str"))); //'String'
console.log(typeOf(true));              //'boolean'
console.log(typeOf(new Boolean(true))); //'Boolean'
console.log(typeOf(1));                 //'number'
console.log(typeOf(NaN));               //'NaN'
console.log(typeOf(Infinity));          //'Infinity'
console.log(typeOf(-Infinity));         //'-Infinity'
console.log(typeOf(new Number(1)));     //'Number'
console.log(typeOf(function(){}));      //'function'
console.log(typeOf([]));                //'Array'
console.log(typeOf(/aaa/));             //'RegExp'
console.log(typeOf(new Date));          //'Date'
console.log(typeOf(document));          //'Document'
console.log(typeOf({}));                //'Object'
console.log(typeOf(Object.create(null)));   //'Object'
console.log(typeOf(new function Hoge(){})); //'Hoge' IEでは'Object'

typeof演算子の改良版

function typeOf(x) {
    if (x === null) return 'null';
    if (x == null) return 'undefined';
    var type = typeof x, c = x.constructor;
    if (type === 'number') {
        if (isNaN(x)) return 'NaN';
        if (!isFinite(x))
            return x === Infinity ? 'Infinity' : '-Infinity';
    }
    if (type === 'object') {
        return c && c.name ? c.name :
            Object.prototype.toString.call(x).slice(8, -1);
    }
    return type;
}