let arr = [1,2,3,4];
arr.reduce(function(pre,cur){return pre + cur}); // return 10

const userList = [
 {
 id: 1,
 username: 'john',
 sex: 1,
 email: 'john@163.com'
 },
 {
 id: 2,
 username: 'jerry',
 sex: 1,
 email: 'jerry@163.com'
 },
 {
 id: 3,
 username: 'nancy',
 sex: 0,
 email: ''
 }
];

function keyByUsernameReducer(acc, person) {
 return {...acc, [person.id]: person};
}
const userObj = peopleArr.reduce(keyByUsernameReducer, {});
console.log(userObj);

const fileLines = [
 'Inspector Algar,Inspector Bardle,Mr. Barker,Inspector Barton',
 'Inspector Baynes,Inspector Bradstreet,Inspector Sam Brown',
 'Monsieur Dubugue,Birdy Edwards,Inspector Forbes,Inspector Forrester',
 'Inspector Gregory,Inspector Tobias Gregson,Inspector Hill',
 'Inspector Stanley Hopkins,Inspector Athelney Jones'
];

function splitLineReducer(acc, line) {
 return acc.concat(line.split(/,/g));
}
const investigators = fileLines.reduce(splitLineReducer, []);
console.log(investigators);
// [
// "Inspector Algar",
// "Inspector Bardle",
// "Mr. Barker",
// "Inspector Barton",
// "Inspector Baynes",
// "Inspector Bradstreet",
// "Inspector Sam Brown",
// "Monsieur Dubugue",
// "Birdy Edwards",
// "Inspector Forbes",
// "Inspector Forrester",
// "Inspector Gregory",
// "Inspector Tobias Gregson",
// "Inspector Hill",
// "Inspector Stanley Hopkins",
// "Inspector Athelney Jones"
// ]

Array.prototype.flatMap = function(f) {
 const reducer = (acc, item) => acc.concat(f(item));
 return this.reduce(reducer, []);
}

const arr = ["今天天气不错", "", "早上好"]

const arr1 = arr.map(s => s.split(""))
// [["今", "天", "天", "气", "不", "错"],[""],["早", "上", "好"]]

const arr2 = arr.flatMap(s => s.split(''));
// ["今", "天", "天", "气", "不", "错", "", "早", "上", "好"]

const readings = [0.3, 1.2, 3.4, 0.2, 3.2, 5.5, 0.4];
const maxReading = readings.reduce((x, y) => Math.max(x, y), Number.MIN_VALUE);
const minReading = readings.reduce((x, y) => Math.min(x, y), Number.MAX_VALUE);
console.log({minReading, maxReading});
// {minReading: 0.2, maxReading: 5.5}

const readings = [0.3, 1.2, 3.4, 0.2, 3.2, 5.5, 0.4];
function minMaxReducer(acc, reading) {
 return {
  minReading: Math.min(acc.minReading, reading),
  maxReading: Math.max(acc.maxReading, reading),
 };
}
const initMinMax = {
 minReading: Number.MAX_VALUE,
 maxReading: Number.MIN_VALUE,
};
const minMax = readings.reduce(minMaxReducer, initMinMax);
console.log(minMax);
// {minReading: 0.2, maxReading: 5.5}

function notEmptyEmail(x) {
 return !!x.email
}

function notEmptyEmailUsername(a, b) {
 return a "htmlcode">

function notEmptyEmail(x) {
 return !!x.email
}

function notEmptyEmailUsername(usernameAcc, person){
 return (notEmptyEmail(person))
  "color: #ff0000">按顺序运行异步函数


我们可以做的另一件事.reduce()是按顺序运行promises（而不是并行）。如果您对API请求有速率限制，或者您需要将每个prmise的结果传递到下一个promise，reduce可以帮助到你。
举一个例子，假设我们想要为userList数组中的每个人获取消息。


function fetchMessages(username) {
 return fetch(`https://example.com/api/messages/${username}`)
  .then(response => response.json());
}

function getUsername(person) {
 return person.username;
}

async function chainedFetchMessages(p, username) {
 // In this function, p is a promise. We wait for it to finish,
 // then run fetchMessages().
 const obj = await p;
 const data = await fetchMessages(username);
 return { ...obj, [username]: data};
}

const msgObj = userList
 .map(getUsername)
 .reduce(chainedFetchMessages, Promise.resolve({}))
 .then(console.log);
// {glestrade: [ … ], mholmes: [ … ], iadler: [ … ]}


async函数返回一个 Promise 对象，可以使用then方法添加回调函数。当函数执行的时候，一旦遇到await就会先返回，等到异步操作完成，再接着执行函数体内后面的语句。
请注意，在此我们传递Promise作为初始值Promise.resolve()，我们的第一个API调用将立即运行。
下面是不使用async语法糖的版本


function fetchMessages(username) {
 return fetch(`https://example.com/api/messages/${username}`)
  .then(response => response.json());
}

function getUsername(person) {
 return person.username;
}

function chainedFetchMessages(p, username) {
 // In this function, p is a promise. We wait for it to finish,
 // then run fetchMessages().
 return p.then((obj)=>{
  return fetchMessages(username).then(data=>{
   return {
    ...obj,
    [username]: data
   }
  })
 })
}

const msgObj = peopleArr
 .map(getUsername)
 .reduce(chainedFetchMessages, Promise.resolve({}))
 .then(console.log);
// {glestrade: [ … ], mholmes: [ … ], iadler: [ … ]}


总结
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