isType methods

istype.js

var o = {};

function getIsType(type) {
    return function (o) {
        return /* undefined */ typeof o === type ||
            /* null */ o === null && type === "null" ||
            Object.prototype.toString.call(o).match(/\s+(\w+)/)[1].toLowerCase() === type;
    };
}

var types = "String Number Date Object Array RegExp Function Null Undefined Boolean".split(" ");

for (var i in types) {
    if (types.hasOwnProperty(i)) {
        o["is" + types[i]] = getIsType(types[i].toLowerCase());
    }
}

// o.isString("")  === true
// o.isNull(null)  === true
// o.isUndefined() === true