reduce 是 JavaScript 数组对象上的一个高阶函数
它可以用来迭代数组中的所有元素,并返回一个单一的值。
其常用的语法为: array.reduce(callback[, initialValue])
其中,callback 是一个回调函数,它接受四个参数:累加器(初始值或上一次回调函数的返回值)、当前元素、当前索引、操作的数组本身。initialValue 是一个可选的初始值,如果提供了该值,则作为累加器的初始值,否则累加器的初始值为数组的第一个元素。 reduce 函数会从数组的第一个元素开始,依次对数组中的每个元素执行回调函数。回调函数的返回值将成为下一次回调函数的第一个参数(累加器)。最后,reduce 函数返回最终的累加结果。 以下是一个简单的 reduce 示例,用于计算数组中所有元素的和:
- const arr = [1, 2, 3, 4, 5];
- const sum = arr.reduce((accumulator, currentValue) => accumulator + currentValue);
- console.log(sum); // 15
在上面的代码中,reduce 函数从数组的第一个元素开始,计算累加值,返回最终的累加结果 15。 除了数组的求和,reduce 函数还可以用于其他各种用途,如数组求平均数、最大值、最小值等。此外,reduce 函数还可以与 map、filter、forEach 等函数组合使用,实现更加复杂的数据操作。
当然,以下是一些 reduce 的实际应用案例,帮助你更好地理解它的用法:
计算数组的平均数
- const arr = [1, 2, 3, 4, 5];
- const average = arr.reduce((accumulator, currentValue, index, array) => {
- accumulator += currentValue;
- if (index === array.length - 1) {
- return accumulator / array.length;
- } else {
- return accumulator;
- }
- });
- console.log(average); // 3
求数组的最大值
- const arr = [1, 2, 3, 4, 5];
- const max = arr.reduce((accumulator, currentValue) => Math.max(accumulator, currentValue));
- console.log(max); // 5
求数组的最小值
- const arr = [1, 2, 3, 4, 5];
- const min = arr.reduce((accumulator, currentValue) => Math.min(accumulator, currentValue));
- console.log(min); // 1
数组去重
- const arr = [1, 2, 3, 3, 4, 4, 5];
- const uniqueArr = arr.reduce((accumulator, currentValue) => {
- if (!accumulator.includes(currentValue)) {
- accumulator.push(currentValue);
- }
- return accumulator;
- }, []);
- console.log(uniqueArr); // [1, 2, 3, 4, 5]
计算数组中每个元素出现的次数
- const arr = [1, 2, 3, 3, 4, 4, 5];
- const countMap = arr.reduce((accumulator, currentValue) => {
- if (!accumulator[currentValue]) {
- accumulator[currentValue] = 1;
- } else {
- accumulator[currentValue]++;
- }
- return accumulator;
- }, {});
- console.log(countMap); // {1: 1, 2: 1, 3: 2, 4: 2, 5: 1}
实现数组分组
- const arr = [1, 2, 3, 4, 5];
- const result = arr.reduce((accumulator, currentValue) => {
- if (currentValue % 2 === 0) {
- accumulator.even.push(currentValue);
- } else {
- accumulator.odd.push(currentValue);
- }
- return accumulator;
- }, { even: [], odd: [] });
- console.log(result); // {even: [2, 4], odd: [1, 3, 5]}
计算数组中连续递增数字的长度
- const arr = [1, 2, 3, 5, 6, 7, 8, 9];
- const result = arr.reduce((accumulator, currentValue, index, array) => {
- if (index === 0 || currentValue !== array[index - 1] + 1) {
- accumulator.push([currentValue]);
- } else {
- accumulator[accumulator.length - 1].push(currentValue);
- }
- return accumulator;
- }, []);
- const maxLength = result.reduce((accumulator, currentValue) => Math.max(accumulator, currentValue.length), 0);
- console.log(maxLength); // 5
计算对象数组的属性总和
- const arr = [
- { name: 'Alice', age: 25 },
- { name: 'Bob', age: 30 },
- { name: 'Charlie', age: 35 },
- ];
- const result = arr.reduce((accumulator, currentValue) => accumulator + currentValue.age, 0);
- console.log(result); // 90
将对象数组转换为键值对对象
- const arr = [
- { name: 'Alice', age: 25 },
- { name: 'Bob', age: 30 },
- { name: 'Charlie', age: 35 },
- ];
- const result = arr.reduce((accumulator, currentValue) => {
- accumulator[currentValue.name] = currentValue.age;
- return accumulator;
- }, {});
- console.log(result); // {Alice: 25, Bob: 30, Charlie: 35}
计算数组中出现次数最多的元素
- const arr = [1, 2, 3, 4, 4, 4, 5, 5, 6, 6, 6, 6];
- const result = arr.reduce((accumulator, currentValue) => {
- accumulator[currentValue] = (accumulator[currentValue] || 0) + 1;
- return accumulator;
- }, {});
- const maxCount = Math.max(...Object.values(result));
- const mostFrequent = Object.keys(result).filter(key => result[key] === maxCount).map(Number);
- console.log(mostFrequent); // [6]
实现 Promise 串行执行
- const promise1 = () => Promise.resolve('one');
- const promise2 = (input) => Promise.resolve(input + ' two');
- const promise3 = (input) => Promise.resolve(input + ' three');
- const promises = [promise1, promise2, promise3];
- const result = promises.reduce((accumulator, currentValue) => {
- return accumulator.then(currentValue);
- }, Promise.resolve('start'));
- result.then(console.log); // 'one two three'
对象属性值求和
- const obj = {
- a: 1,
- b: 2,
- c: 3
- };
- const result = Object.values(obj).reduce((accumulator, currentValue) => accumulator + currentValue);
- console.log(result); // 6
按属性对数组分组
- const arr = [
- { id: 1, name: 'John' },
- { id: 2, name: 'Mary' },
- { id: 3, name: 'Bob' },
- { id: 4, name: 'Mary' }
- ];
- const result = arr.reduce((accumulator, currentValue) => {
- const key = currentValue.name;
- if (!accumulator[key]) {
- accumulator[key] = [];
- }
- accumulator[key].push(currentValue);
- return accumulator;
- }, {});
- console.log(result);
- /*
- {
- John: [{ id: 1, name: 'John' }],
- Mary: [
- { id: 2, name: 'Mary' },
- { id: 4, name: 'Mary' }
- ],
- Bob: [{ id: 3, name: 'Bob' }]
- }
- */
扁平化数组
- // 如果你有一个嵌套的数组,可以使用reduce将其扁平化成一个一维数组。例如:
- const nestedArray = [[1, 2], [3, 4], [5, 6]];
- const flattenedArray = nestedArray.reduce((acc, curr) => acc.concat(curr), []);
- console.log(flattenedArray); // [1, 2, 3, 4, 5, 6]
合并对象
- // 可以使用reduce将多个对象合并成一个对象。例如:
- const obj1 = { a: 1, b: 2 };
- const obj2 = { c: 3, d: 4 };
- const obj3 = { e: 5, f: 6 };
- const mergedObj = [obj1, obj2, obj3].reduce((acc, curr) => Object.assign(acc, curr), {});
- console.log(mergedObj); // {a: 1, b: 2, c: 3, d: 4, e: 5, f: 6}
以上就是JavaScript数组对象高阶函数reduce的妙用详解的详细内容,更多关于JavaScript数组对象reduce的资料请关注w3xue其它相关文章!