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// x has not been declared before
if (typeof x === ‘undefined‘) { // evaluates to true without errors
// these statements execute
}
//如果未定义变量，那么会出现引用错误
if(x === undefined){ // throws a ReferenceError
}标签：
原文地址：http://www.cnblogs.com/micro-chen/p/5871250.html
