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This feature is well established and works across many devices and browser versions. It’s been available across browsers since 2015年7月.
keys() 方法會回傳一個包含陣列中的每一個索引之鍵（keys）的新 Array Iterator 物件。
keys()
Array Iterator
const array1 = ["a", "b", "c"]; const iterator = array1.keys(); for (const key of iterator) { console.log(key); } // Expected output: 0 // Expected output: 1 // Expected output: 2
arr.keys()
一個新的 Array 迭代器（iterator）物件。
Array
var arr = ["a", , "c"]; var sparseKeys = Object.keys(arr); var denseKeys = [...arr.keys()]; console.log(sparseKeys); // ['0', '2'] console.log(denseKeys); // [0, 1, 2]
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Array.prototype.values()
Array.prototype.entries()