var name = 'world!'; // console.log(typeof name) (function () { console.log(this.name, name, typeof name, typeof name === undefined) if(typeof name === undefined){ var name = 'a!' console.log('hello ' + name) } else{ var name = 'b!' console.log('hello ' + name) } })();
运行结果是
"world!" undefined "undefined" false
hello b!
这里有两个陷阱
第一个是变量提升后 name 值为 undefined
第二个是 typeof name 为字符串 "undefined"