// 强制转换为Boolean 用 !!
var bool = !!"c";
console.log(typeof bool); // boolean

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用法：如果你想判断name为不为空~~~~

  一般这样写hasName = name? true : false，换种写法hasName = !!name

  !!相当于Boolean()
  var a = 1;
  !!a ==Boolean(a)

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   // 强制转换为Number 用 +

var num = +"1234";
console.log(typeof num); // number

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// 强制转换为String 用 ""+ var str = ""+ 1234; console.log(typeof str); // string