JavaScript Array reduce()用法及代碼示例

const message = ["JavaScript ", "is ", "fun."];

// function to join each string elements
function joinStrings(accumulator, currentValue) {
  return accumulator + currentValue;
}

// reduce join each element of the string
let joinedString = message.reduce(joinStrings);
console.log(joinedString);

// Output: JavaScript is fun.

reduce() 語法

用法:

arr.reduce(callback(accumulator, currentValue), initialValue)

這裏，arr 是一個數組。

參數：

reduce() 方法包含：

• callback- 在每個數組元素上執行的函數(如果沒有，則第一個元素除外initialValue提供)。它吸收
  • accumulator - 它累積回調的返回值。
  • currentValue - 從數組傳遞的當前元素。
• initialValue(可選)- 將在第一次調用時傳遞給 callback() 的值。如果未提供，則第一個元素在第一次調用時充當 accumulator，而 callback() 不會在其上執行。

注意：調用reduce()在沒有的空數組上initialValue會拋出TypeError.

返回：

• 返回減少數組後產生的單個值。

示例 1：數組所有值的總和

const numbers = [1, 2, 3, 4, 5, 6];

function sum_reducer(accumulator, currentValue) {
  return accumulator + currentValue;
}

let sum = numbers.reduce(sum_reducer);
console.log(sum); // 21

// using arrow function
let summation = numbers.reduce(
  (accumulator, currentValue) => accumulator + currentValue
);
console.log(summation); // 21

輸出

21
21

示例 2：減去數組中的數字

const numbers = [1800, 50, 300, 20, 100];

// subtract all numbers from first number
// since 1st element is called as accumulator rather than currentValue
// 1800 - 50 - 300 - 20 - 100
let difference = numbers.reduce(
  (accumulator, currentValue) => accumulator - currentValue
);
console.log(difference); // 1330

const expenses = [1800, 2000, 3000, 5000, 500];
const salary = 15000;

// function that subtracts all array elements from given number
// 15000 - 1800 - 2000 - 3000 - 5000 - 500
let remaining = expenses.reduce(
  (accumulator, currentValue) => accumulator - currentValue,
  salary
);
console.log(remaining); // 2700

輸出

1330
2700

這個例子清楚地解釋了傳遞 initialValue 和不傳遞 initialValue 之間的區別。

示例 3：從數組中刪除重複項

let ageGroup = [18, 21, 1, 1, 51, 18, 21, 5, 18, 7, 10];
let uniqueAgeGroup = ageGroup.reduce(function (accumulator, currentValue) {
  if (accumulator.indexOf(currentValue) === -1) {
    accumulator.push(currentValue);
  }
  return accumulator;
}, []);

console.log(uniqueAgeGroup); // [ 18, 21, 1, 51, 5, 7, 10 ]

輸出

[
  18, 21,  1, 51,
   5,  7, 10
]

示例 4：按屬性對對象進行分組

let people = [
  { name: "John", age: 21 },
  { name: "Oliver", age: 55 },
  { name: "Michael", age: 55 },
  { name: "Dwight", age: 19 },
  { name: "Oscar", age: 21 },
  { name: "Kevin", age: 55 },
];

function groupBy(objectArray, property) {
  return objectArray.reduce(function (accumulator, currentObject) {
    let key = currentObject[property];
    if (!accumulator[key]) {
      accumulator[key] = [];
    }
    accumulator[key].push(currentObject);
    return accumulator;
  }, {});
}

let groupedPeople = groupBy(people, "age");
console.log(groupedPeople);

輸出

{
  '19': [ { name: 'Dwight', age: 19 } ],
  '21': [ { name: 'John', age: 21 }, { name: 'Oscar', age: 21 } ],
  '55': [
    { name: 'Oliver', age: 55 },
    { name: 'Michael', age: 55 },
    { name: 'Kevin', age: 55 }
  ]
}