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                >[danger]說一下for...in 和 for...of的區別? for...of遍歷獲取的是對象的鍵值, for...in獲取的是對象的鍵名; for...in會遍歷對象的整個原型鏈, 性能非常差不推薦使用,而for...of只遍歷當前對象不會遍歷原型鏈; 對于數組的遍歷,for...in會返回數組中所有可枚舉的屬性(包括原型鏈上可枚舉的屬性),for...of只返回數組的下標對應的屬性值; 總結:for...in循環主要是為了遍歷對象而生,不適用遍歷數組; for....of循環可以用來遍歷數組、類數組對象、字符串、Set、Map以及Generator對象
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