js各種關鍵字

typeof: 把類型信息當作關鍵字返回，可以用來驗證變量的類型。返回值有幾種"number," "string," "boolean," "object," "function," 和 "undefined."但是對於null和arr返回的都是objct，故無法準確判斷對象、數組的類型 num | 0  取整操作 num >> 1 取半操作 若是小數 則會把小數點去掉再取半

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