摘要：
0.undefined 本身的介绍undefinedis a property of theglobal object, i.e. it is a variable in global scope. JavaScript 1.8.5noteStarting in JavaScript 1.8.5 (Firefox 4),undefinedis non-writable, as per the ECMAScript 5 specification.1.关于undefined值的判断：有 a:x== undefined,b: x===undefined,c:typeof x == 'und 阅读全文
