js中reduce方法的常用应用场景

reduce() 方法可以用来迭代数组并且执行一个回调函数，将数组中的元素聚合成一个单独的值。它可以被用于一系列的操作，如累加求和，计算平均值和查找最大值或最小值等。以下是reduce() 方法的几个应用场景和相应的示例：

求和或求积：

可以使用reduce方法来对数组中的所有元素进行求和或求积。

let numbers = [1, 2, 3, 4];
let sum = numbers.reduce((previous, current) => previous + current, 0);
console.log(sum); // 结果为10let product = numbers.reduce((previous, current) => previous * current, 1);
console.log(product); // 结果为24

找出数组中的最大值

const numbers = [10, 20, 30, 40];
const max = numbers.reduce((accumulator, currentValue) => Math.max(accumulator, currentValue));
console.log(max); // Output: 40

计算平均值

const numbers = [10, 20, 30, 40];
const average = numbers.reduce((accumulator, currentValue, index, array) => {accumulator += currentValue;if (index === array.length - 1) {return accumulator / array.length;} else {return accumulator;}
});
console.log(average); // Output: 25

计数：

可以使用reduce方法来计算数组中特定元素的出现次数。

let animals = ['dog', 'cat', 'dog', 'cat', 'bird', 'cat'];
let count = animals.reduce((previous, animal) => {if(animal in previous) {previous[animal]++;} else {previous[animal] = 1;}return previous;
}, {});
console.log(count); // 结果为 { dog: 2, cat: 3, bird: 1 }

const str = 'hello world';
const charCount = str.split('').reduce((result, char) => {if (!result[char]) {result[char] = 1;} else {result[char]++;}return result;
}, {});
console.log(charCount); // Output: { h: 1, e: 1, l: 3, o: 2, ' ': 1, w: 1, r: 1, d: 1 }

筛选：

可以使用reduce方法来筛选出满足条件的元素。

let numbers = [1, 2, 3, 4, 5, 6];
let evenNumbers = numbers.reduce((previous, number) => {if(number % 2 == 0) {previous.push(number);}return previous;
}, []);
console.log(evenNumbers); // 结果为 [2, 4, 6]

// 筛选出所有年龄大于 20 岁的人
const people = [{ name: 'Amy', age: 23 },{ name: 'Bob', age: 18 },{ name: 'Charlie', age: 27 },{ name: 'David', age: 30 },{ name: 'Emily', age: 21 }
];
const result = people.reduce((accumulator, currentValue) => {if (currentValue.age > 20) {accumulator.push(currentValue);}return accumulator;
}, []);
console.log(result); // Output: [{ name: 'Amy', age: 23 }, { name: 'Charlie', age: 27 }, { name: 'David', age: 30 }, { name: 'Emily', age: 21 }]

// 筛选出所有姓氏为“张”的人的名字和年龄
const people = [{ name: '张三', age: 23 },{ name: '李四', age: 18 },{ name: '张五', age: 27 },{ name: '王六', age: 30 },{ name: '张七', age: 21 }
];
const result = people.reduce((accumulator, currentValue) => {if (currentValue.name.startsWith('张')) {accumulator.push({ name: currentValue.name, age: currentValue.age });}return accumulator;
}, []);
console.log(result); // Output: [{ name: '张三', age: 23 }, { name: '张五', age: 27 }, { name: '张七', age: 21 }]

// 将数组对象按照条件筛选成多个数组对象
const people = [{ name: 'Amy', country: 'USA' },{ name: 'Bob', country: 'Canada' },{ name: 'Charlie', country: 'USA' },{ name: 'David', country: 'Canada' },{ name: 'Emily', country: 'USA' }
];
const result = people.reduce((accumulator, currentValue) => {const country = currentValue.country;if (!accumulator[country]) {accumulator[country] = [];}accumulator[country].push(currentValue);return accumulator;
}, {});
console.log(result);
/*{USA: [{ name: 'Amy', country: 'USA' },{ name: 'Charlie', country: 'USA' },{ name: 'Emily', country: 'USA' }],Canada: [{ name: 'Bob', country: 'Canada' },{ name: 'David', country: 'Canada' }]
}
*/

扁平化数组：

可以使用reduce方法来将多维数组变为一维。

let arrays = [[1, 2, 3], [4, 5], [6]];
let flatten = arrays.reduce((previous, current) => [...previous, ...current], []);
console.log(flatten); // 结果为 [1, 2, 3, 4, 5, 6]