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Friday, 16 June, 2017 UTC

循環陣列

Summary

這裡有一些方法在 JavaScript 中可以循環陣列。我們將從典型例子並且補充標準規範。

while

let index = 0; const array = [1,2,3,4,5,6]; while (index < array.length) { console.log(array[index]); index++; }

for（典型）

const array = [1,2,3,4,5,6]; for (let index = 0; index < array.length, index++) { console.log(array[index]); }

forEach

const array = [1,2,3,4,5,6]; array.forEach(function(current_value, index, array) { console.log(`At index ${index} in array ${array} the value is ${current_value}`); }); // => undefined

map

最後的 construct 很有用，然而，它不會回傳一個新的陣列，這可能對你指定的情況是不合乎的。map 透過對每個元素應用一個函式，並回傳新的陣列來解決問題。

const array = [1,2,3,4,5,6]; const square = x => Math.pow(x, 2); const squares = array.map(square); console.log(`Original array: ${array}`); console.log(`Squared array: ${array}`);

map 完整個宣告方式：.map(current_value, index, array)。

reduce

根據 MDN：

reduce() 方法對累加器和數組中的每個元素（從左到右）應用一個函式，將其減少為一個值。

const array = [1,2,3,4,5,6]; const sum = (x, y) => x + y; const array_sum = array.reduce(sum, 0); console.log(`The sum of array: ${array} is ${array_sum}`);

filter

基於一個布林函式對陣列進行過濾。

const array = [1,2,3,4,5,6]; const even = x => x % 2 === 0; const even_array = array.filter(even); console.log(`Even numbers in array ${array}: ${even_array}`);

every

取得一個陣列，並且想要測試每個元素中是否滿足給定的條件？

const array = [1,2,3,4,5,6]; const under_seven = x => x < 7; if (array.every(under_seven)) { console.log('Every element in the array is less than 7'); } else { console.log('At least one element in the array was bigger than 7'); }

some

測試至少一個元素是否 match 我們的布林函式。

const array = [1,2,3,9,5,6,4]; const over_seven = x => x > 7; if (array.some(over_seven)) { console.log('At least one element bigger than 7 was found'); } else { console.log('No element bigger than 7 was found'); }

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