js各种关键字

typeof: 把类型信息当作关键字返回，可以用来验证变量的类型。返回值有几种"number," "string," "boolean," "object," "function," 和 "undefined."但是对于null和arr返回的都是objct，故无法准确判断对象、数组的类型 num | 0  取整操作 num >> 1 取半操作 若是小数 则会把小数点去掉再取半

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