JavaScript 的 Array 用法整理

函數用法

forEach()

• The forEach() method calls a function for each element in an array.
• Params: currentValue, index(Optional), arr(Optional)
• Return Value: undefined.
• 會直接改變原陣列。

基本用法：

array.forEach((currentValue, index, arr) => {});

第三個參數很少用到。

const arr = [1, 2, 3, 4, 5];
let sum = 0;
arr.forEach((cur) => {
  sum += cur;
})
> sum: 15

const arr = [1, 2, 3, 4, 5];
arr.forEach((cur, idx) => {
  arr[idx] *= 10;
})

filter()

• The forEach() method calls a function for each element in an array.
• Params: currentValue, index(Optional), arr(Optional)
• Return: Array.
• 回傳的陣列是新的陣列，不會改變原本的陣列。

const ages = [18, 25, 30];
const newAges = ages.filter((cur) => { return cur > 18; })
> newAges: [25, 30]
> ages: [18, 25, 30]

every()

• The every() method executes a function for each array element.
• The every() method returns true if the function returns true for all elements.
• Params: currentValue, index(Optional), arr(Optional)
• Return: Bool.

const ages = [18, 25, 30];
const allOver18 = ages.every((cur) => cur > 18);
const allOver16 = ages.every((cur) => cur > 16);
> allOver18: false
> allOver16: true

includes()

• Return : Bool.
• Params: searchValue, startIdx(Optional)
• Syntax:

array.includes(searchValue, start);

const fruits = ["Banana", "Orange", "Apple", "Mango"];
const hasApple = fruits.includes("fruits");
> hasApple: true