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The JavaScript Array Methods Handbook

健忘的 Sean 常常需要查看的陣列處理方法小抄。

What will this method return

  • 單純遞迴執行:forEach
  • 回傳一個新的陣列:mapfilterconcat
  • 回傳一個布林值:someeveryincludes
  • 回傳該元素:find
  • 回傳該索引:findIndexindexOf
  • 回傳一個整數值:reduceunshift

forEach

Array.prototype.forEach()

將陣列裡的每個元素傳入,並「執行」給定的函式一次。

1const array1 = ['a', 'b', 'c'];
2
3array1.forEach((element) => console.log(element));
4
5// Expected output: "a"
6// Expected output: "b"
7// Expected output: "c"
8

map

Array.prototype.map()

原陣列的每一個元素經過指定的函式「運算」後,回傳新的結果陣列。

1const array1 = [1, 2, 3, 4];
2
3// Pass a function to map
4const result = array1.map((x) => x * 2);
5
6console.log(result);
7// Expected output: Array [2, 4, 6, 8]
8

filter

Array.prototype.filter()

原陣列經過指定的函式「過濾」後,回傳新的結果陣列。

1const words = ['Sean', 'Damao', 'Paul', 'Iris', 'Julia'];
2
3const result = words.filter((word) => word.length > 4);
4
5console.log(result);
6// Expected output: Array ["Damao", "Julia"]
7

concat

合併兩個或多個陣列,回傳一個新的陣列。

Array.prototype.concat()

1const array1 = ['a', 'b', 'c'];
2const array2 = ['d', 'e', 'f', 'g'];
3const array3 = array1.concat(array2);
4
5console.log(array3);
6// Expected output: Array ["a", "b", "c", "d", "e", "f", "g"]
7

some

Array.prototype.some()

測試陣列中,是否有「至少一個」元素符合指定的條件函式,是的話會回傳 true 否則回傳 false

1const array1 = [1, 2, 3, 5, 7];
2
3// Checks whether an element is even
4const isEven = (element) => element % 2 === 0;
5
6console.log(array1.some(isEven));
7// Expected output: true
8

every

Array.prototype.every()

回傳是否陣列中的「所有」元素都符合條件。

1const array1 = [2, 4, 6, 8, 10];
2
3// Checks whether all elements are even
4const areEven = (element) => element % 2 === 0;
5
6console.log(array1.every(areEven));
7// Expected output: true
8

includes

Array.prototype.includes()

回傳陣列是否「包含」指定的元素。

1const pets = ['cat', 'dog'];
2
3console.log(pets.includes('cat'));
4// Expected output: true
5
6console.log(pets.includes('pig'));
7// Expected output: false
8

find

Array.prototype.find()

回傳「第一個」滿足條件的元素,如果通通不符合則回傳 undefined

1const array1 = [5, 6, 8, 100, 44];
2
3const found = array1.find((element) => element > 10);
4
5console.log(found);
6// Expected output: 100
7

findIndex

Array.prototype.findIndex()

回傳「第一個」滿足條件的元素之「索引」,如果找不到則回傳 -1

1const array1 = [5, 6, 8, 100, 44];
2
3const isLargeNumber = (element) => element > 8;
4
5console.log(array1.findIndex(isLargeNumber));
6// Expected output: 3
7

indexOf

Array.prototype.indexOf()

回傳「指定的元素」在陣列中「第一個」被找到的「索引」,如果找不到則回傳 -1

1const beasts = ['squirrel', 'seal', 'cat', 'duck', 'rabbit'];
2
3console.log(beasts.indexOf('squirrel'));
4// Expected output: 0
5
6// Start from index 2
7console.log(beasts.indexOf('seal', 2));
8// Expected output: 4
9
10console.log(beasts.indexOf('pig'));
11// Expected output: -1
12

reduce

Array.prototype.reduce()

將陣列中的每一項元素「由左至右」傳入「累加器」,最後回傳一個值。

1const array1 = [1, 2, 3, 4];
2
3// 0 + 1 + 2 + 3 + 4
4const initialValue = 0;
5const sumWithInitial = array1.reduce(
6  (accumulator, currentValue) => accumulator + currentValue,
7  initialValue,
8);
9
10console.log(sumWithInitial);
11// Expected output: 10
12

unshift

Array.prototype.unshift()

unshift() 會增加一個或多個元素到陣列的開頭,並且回傳陣列的新長度。

1const array1 = [1, 2, 3];
2
3console.log(array1.unshift(4, 5));
4// Expected output: 5
5
6console.log(array1);
7// Expected output: Array [4, 5, 1, 2, 3]
8

References