jquery原代码：isType()

class2type = {};
toString = Object.prototype.toString;
type: function( obj ) {
		return obj == null ?
			String( obj ) :
			class2type[ toString.call(obj) ] || "object";
	},

	isFunction: function( obj ) {
		return jQuery.type(obj) === "function";
	},

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

	// A crude way of determining if an object is a window
	isWindow: function( obj ) {
		return obj && typeof obj === "object" && "setInterval" in obj;
	},

	isNaN: function( obj ) {
		return obj == null || !rdigit.test( obj ) || isNaN( obj );
	},