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直接看代码：

console.log(typeof 42);
  // expected output: "number"

  console.log(typeof 'blubber');
  // expected output: "string"

  console.log(typeof true);
  // expected output: "boolean"

  var test;
  console.log(typeof test);
  // expected output: "undefined"

  console.log(test);
  // expected output: undefined
  if(test != undefined){
      console.log("已定义")
  }else {
      console.log("未定义");// expected output: 未定义
  }
  if(typeof(test) != "undefined"){//typeof() 返回的值是字符串
      console.log("已定义");
  }else{
      console.log("未定义");// expected output: 未定义
  }

typeof() 返回的值是字符串