JavaScript reduce()的使用
语法
arr.reduce(callback(accumulator, currentValue, index, array), initialValue)
参数
callback
执行数组中每个值 (如果没有提供 initialValue则第一个值除外)的函数，包含四个参数：
accumulator
累计器累计回调的返回值; 它是上一次调用回调时返回的累积值，或initialValue（见于下方）。
currentValue
数组中正在处理的元素。
index
数组中正在处理的当前元素的索引。 如果提供了initialValue，则起始索引号为0，否则从索引1起始。
array
调用reduce()的数组
initialValue
作为第一次调用 callback函数时的第一个参数的值。 如果没有提供初始值，则将使用数组中的第一个元素。 在没有初始值的空数组上调用 reduce 将报错。
返回值
函数累计处理的结果
描述
reduce为数组中的每一个元素依次执行callback函数，不包括数组中被删除或从未被赋值的元素，接受四个参数：
- accumulator 累计器
- currentValue 当前值
- currentIndex 当前索引
- array 数组
回调函数第一次执行时，accumulator 和currentValue的取值有两种情况：如果调用reduce()时提供了initialValue，accumulator取值为initialValue，currentValue取数组中的第一个值；如果没有提供 initialValue，那么accumulator取数组中的第一个值，currentValue取数组中的第二个值。
如果数组为空且没有提供initialValue，会抛出TypeError 。如果数组仅有一个元素（无论位置如何）并且没有提供initialValue， 或者有提供initialValue但是数组为空，那么此唯一值将被返回并且callback不会被执行。
提供初始值通常更安全，正如下面的例子，如果没有提供initialValue，则可能有四种输出：
var maxCallback = ( acc, cur ) => Math.max( acc.x, cur.x );
var maxCallback2 = ( max, cur ) => Math.max( max, cur );
// reduce() 没有初始值
[ { x: 2 }, { x: 22 }, { x: 42 } ].reduce( maxCallback ); // NaN
[ { x: 2 }, { x: 22 } ].reduce( maxCallback ); // 22
[ { x: 2 } ].reduce( maxCallback ); // { x: 2 }
[ ].reduce( maxCallback ); // TypeError
// map/reduce; 这是更好的方案，即使传入空数组或更大数组也可正常执行
[ { x: 22 }, { x: 42 } ].map( el => el.x )
.reduce( maxCallback2, -Infinity );
reduce() 如何运行
假如运行下段reduce()代码：
[0, 1, 2, 3, 4].reduce(function(accumulator, currentValue, currentIndex, array){
return accumulator + currentValue;
});
callback 被调用四次，每次调用的参数和返回值如下表：
| callback | accumulator | currentValue | currentIndex | array | return value |
|---|---|---|---|---|---|
| first call | 0 | 1 | 1 | [0, 1, 2, 3, 4] | 1 |
| second call | 1 | 2 | 2 | [0, 1, 2, 3, 4] | 3 |
| third call | 3 | 3 | 3 | [0, 1, 2, 3, 4] | 6 |
| fourth call | 6 | 4 | 4 | [0, 1, 2, 3, 4] | 10 |
由reduce返回的值将是最后一次回调返回值（10）。
你还可以使用箭头函数来代替完整的函数。 下面的代码将产生与上面的代码相同的输出：
[0, 1, 2, 3, 4].reduce((prev, curr) => prev + curr );
如果你打算提供一个初始值作为reduce()方法的第二个参数，以下是运行过程及结果：
[0, 1, 2, 3, 4].reduce((accumulator, currentValue, currentIndex, array) => {
return accumulator + currentValue
}, 10)
| callback | accumulator | currentValue | currentIndex | array | return value |
|---|---|---|---|---|---|
| first call | 10 | 0 | 0 | [0, 1, 2, 3, 4] | 10 |
| second call | 10 | 1 | 1 | [0, 1, 2, 3, 4] | 11 |
| third call | 11 | 2 | 2 | [0, 1, 2, 3, 4] | 13 |
| fourth call | 13 | 3 | 3 | [0, 1, 2, 3, 4] | 16 |
| fifth call | 16 | 4 | 4 | [0, 1, 2, 3, 4] | 20 |
这种情况下reduce()返回的值是20。
例子
数组里所有值的和
var sum = [0, 1, 2, 3].reduce(function (accumulator, currentValue) {
return accumulator + currentValue;
}, 0);
// 和为 6
你也可以写成箭头函数的形式：
var total = [ 0, 1, 2, 3 ].reduce(
( acc, cur ) => acc + cur,
0
);
累加对象数组里的值
要累加对象数组中包含的值，必须提供初始值，以便各个item正确通过你的函数。
var initialValue = 0;
var sum = [{x: 1}, {x:2}, {x:3}].reduce(function (accumulator, currentValue) {
return accumulator + currentValue.x;
},initialValue)
console.log(sum) // logs 6
你也可以写成箭头函数的形式：
var initialValue = 0;
var sum = [{x: 1}, {x:2}, {x:3}].reduce(
(accumulator, currentValue) => accumulator + currentValue.x
,initialValue
);
console.log(sum) // logs 6
将二维数组转化为一维
var flattened = [[0, 1], [2, 3], [4, 5]].reduce(
function(a, b) {
return a.concat(b);
},
[]
);
// flattened is [0, 1, 2, 3, 4, 5]
你也可以写成箭头函数的形式：
var flattened = [[0, 1], [2, 3], [4, 5]].reduce(
( acc, cur ) => acc.concat(cur),
[]
);
计算数组中每个元素出现的次数
var names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];
var countedNames = names.reduce(function (allNames, name) {
if (name in allNames) {
allNames[name]++;
}
else {
allNames[name] = 1;
}
return allNames;
}, {});
// countedNames is:
// { 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }
按属性对object分类
var people = [
{ name: 'Alice', age: 21 },
{ name: 'Max', age: 20 },
{ name: 'Jane', age: 20 }
];
function groupBy(objectArray, property) {
return objectArray.reduce(function (acc, obj) {
var key = obj[property];
if (!acc[key]) {
acc[key] = [];
}
acc[key].push(obj);
return acc;
}, {});
}
var groupedPeople = groupBy(people, 'age');
// groupedPeople is:
// {
// 20: [
// { name: 'Max', age: 20 },
// { name: 'Jane', age: 20 }
// ],
// 21: [{ name: 'Alice', age: 21 }]
// }
使用扩展运算符和initialValue绑定包含在对象数组中的数组
// friends - 对象数组
// where object field "books" - list of favorite books
var friends = [{
name: 'Anna',
books: ['Bible', 'Harry Potter'],
age: 21
}, {
name: 'Bob',
books: ['War and peace', 'Romeo and Juliet'],
age: 26
}, {
name: 'Alice',
books: ['The Lord of the Rings', 'The Shining'],
age: 18
}];
// allbooks - list which will contain all friends' books +
// additional list contained in initialValue
var allbooks = friends.reduce(function(prev, curr) {
return [...prev, ...curr.books];
}, ['Alphabet']);
// allbooks = [
// 'Alphabet', 'Bible', 'Harry Potter', 'War and peace',
// 'Romeo and Juliet', 'The Lord of the Rings',
// 'The Shining'
// ]
数组去重
let myArray = ['a', 'b', 'a', 'b', 'c', 'e', 'e', 'c', 'd', 'd', 'd', 'd']
let myOrderedArray = myArray.reduce(function (accumulator, currentValue) {
if (accumulator.indexOf(currentValue) === -1) {
accumulator.push(currentValue)
}
return accumulator
}, [])
console.log(myOrderedArray)
let arr = [1,2,1,2,3,5,4,5,3,4,4,4,4];
let result = arr.sort().reduce((init, current) => {
if(init.length === 0 || init[init.length-1] !== current) {
init.push(current);
}
return init;
}, []);
console.log(result); //[1,2,3,4,5]
按顺序运行Promise
/**
* Runs promises from array of functions that can return promises
* in chained manner
*
* @param {array} arr - promise arr
* @return {Object} promise object
*/
function runPromiseInSequence(arr, input) {
return arr.reduce(
(promiseChain, currentFunction) => promiseChain.then(currentFunction),
Promise.resolve(input)
);
}
// promise function 1
function p1(a) {
return new Promise((resolve, reject) => {
resolve(a * 5);
});
}
// promise function 2
function p2(a) {
return new Promise((resolve, reject) => {
resolve(a * 2);
});
}
// function 3 - will be wrapped in a resolved promise by .then()
function f3(a) {
return a * 3;
}
// promise function 4
function p4(a) {
return new Promise((resolve, reject) => {
resolve(a * 4);
});
}
const promiseArr = [p1, p2, f3, p4];
runPromiseInSequence(promiseArr, 10)
.then(console.log); // 1200
功能型函数管道
// Building-blocks to use for composition
const double = x => x + x;
const triple = x => 3 * x;
const quadruple = x => 4 * x;
// Function composition enabling pipe functionality
const pipe = (...functions) => input => functions.reduce(
(acc, fn) => fn(acc),
input
);
// Composed functions for multiplication of specific values
const multiply6 = pipe(double, triple);
const multiply9 = pipe(triple, triple);
const multiply16 = pipe(quadruple, quadruple);
const multiply24 = pipe(double, triple, quadruple);
// Usage
multiply6(6); // 36
multiply9(9); // 81
multiply16(16); // 256
multiply24(10); // 240
使用 reduce实现map
if (!Array.prototype.mapUsingReduce) {
Array.prototype.mapUsingReduce = function(callback, thisArg) {
return this.reduce(function(mappedArray, currentValue, index, array) {
mappedArray[index] = callback.call(thisArg, currentValue, index, array)
return mappedArray
}, [])
}
}
[1, 2, , 3].mapUsingReduce(
(currentValue, index, array) => currentValue + index + array.length
) // [5, 7, , 10]
Polyfill
// Production steps of ECMA-262, Edition 5, 15.4.4.21
// Reference: http://es5.github.io/#x15.4.4.21
// https://tc39.github.io/ecma262/#sec-array.prototype.reduce
if (!Array.prototype.reduce) {
Object.defineProperty(Array.prototype, 'reduce', {
value: function(callback /*, initialValue*/) {
if (this === null) {
throw new TypeError( 'Array.prototype.reduce ' +
'called on null or undefined' );
}
if (typeof callback !== 'function') {
throw new TypeError( callback +
' is not a function');
}
// 1. Let O be ? ToObject(this value).
var o = Object(this);
// 2. Let len be ? ToLength(? Get(O, "length")).
var len = o.length >>> 0;
// Steps 3, 4, 5, 6, 7
var k = 0;
var value;
if (arguments.length >= 2) {
value = arguments[1];
} else {
while (k < len && !(k in o)) {
k++;
}
// 3. If len is 0 and initialValue is not present,
// throw a TypeError exception.
if (k >= len) {
throw new TypeError( 'Reduce of empty array ' +
'with no initial value' );
}
value = o[k++];
}
// 8. Repeat, while k < len
while (k < len) {
// a. Let Pk be ! ToString(k).
// b. Let kPresent be ? HasProperty(O, Pk).
// c. If kPresent is true, then
// i. Let kValue be ? Get(O, Pk).
// ii. Let accumulator be ? Call(
// callbackfn, undefined,
// « accumulator, kValue, k, O »).
if (k in o) {
value = callback(value, o[k], k, o);
}
// d. Increase k by 1.
k++;
}
// 9. Return accumulator.
return value;
}
});
}
如果您需要兼容不支持Object.defineProperty的JavaScript引擎，那么最好不要 polyfill Array.prototype方法，因为你无法使其成为不可枚举的。
求数组里所有的和
<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <title></title> </head> <body> <div id="app"> <h2 v-cloak>{{totalsum}}</h2> </div> <script type="text/javascript"> const app = new Vue({ el: 'app', data: { array: [1,2,3,4,5,6,7,8,9], }, computed:{ totalsum: function () { return this.array.reduce((accumulator, currentValue) => accumulator + currentValue, 0 ); } }; }) </script> <style> [v-colak]{ display: none; } </style> </body> </html>
累加对象数组的值
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
</head>
<body>
<div id="app">
<h1 v-cloak>{{totalPrice}}</h1>
</div>
<script src="js/vue.js" type="text/javascript" charset="utf-8"></script>
<script type="text/javascript">
const app = new Vue({
el: '#app',
data: {
meats:[
{id: '1', name: 'beef', price: 48},
{id: '2', name: 'Lamb', price: 55},
{id: '3', name: 'pork', price: 38}
]
},
computed:{
totalPrice: function () {
return this.meats.reduce((accumulator,currentValue )=> accumulator + currentValue.price,
0
);
}
}
});
</script>
<style>
[v-colak]{
display: none;
}
</style>
</body>
</html>
按属性对Object分类
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
</head>
<body>
<div id="app">
<ul>
<h2>男生</h2>
<li v-for="item in resultObj['男']">name:{{item.name}} age:{{item.age}} gender:{{item.gender}}</li>
</ul>
<ul>
<h2>女生</h2>
<li v-for="item in resultObj['女']">name:{{item.name}} age:{{item.age}} gender:{{item.gender}}</li>
</ul>
</div>
<script src="js/vue.js" type="text/javascript" charset="utf-8"></script>
<script type="text/javascript">
const app = new Vue({
el: '#app',
data:{
people: [
{name: 'Alice', age: 22,gender:'女'},
{name: 'Kangkang', age: 23,gender: '男'},
{name: 'LingDa', age: 26,gender:'女'},
{name: 'Argio' ,age: 23,gender:'女'},
{name: 'DuoLa' ,age: 27,gender:'女'}
],
resultObj:{}
},
mounted() {
this.peopleGroup();
},
methods:{
peopleGroup: function (){
this.resultObj = this.people.reduce(function(acc,obj){
var key = obj['gender']
if(!acc[key]){
acc[key] = []
}
acc[key].push(obj)
return acc;
},{});
//console.log(result);
//男:[{name: 'Kangkang', age: 23,gender: '男'}]
//女：[
// {name: 'Alice', age: 22,gender:'女'},
// {name: 'LingDa', age: 26,gender:'女'},
// {name: 'Argio' ,age: 23,gender:'女'},
// {name: 'DuoLa' ,age: 27,gender:'女'}
// ]
}
}
})
</script>
</body>
</html>
使用扩展运算符和initialValue绑定包含在对象数组中的数组
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
<style type="text/css">
[v-cloak]{
display: none;
}
</style>
</head>
<body>
<div id="app" v-cloak>
<ul>
<h2>合并后的结果：</h2>
<li v-for="item in booksArray">{{item}}</li>
</ul>
</div>
<script src="js/vue.js" type="text/javascript" charset="utf-8"></script>
<script type="text/javascript">
const app = new Vue({
el:'#app',
data:{
booksArray:[],
friends: [
{
name: 'Anna',
books: ['Bible', 'Harry Potter'],
age: 21
}, {
name: 'Bob',
books: ['War and peace', 'Romeo and Juliet'],
age: 26
}, {
name: 'Alice',
books: ['The Lord of the Rings', 'The Shining'],
age: 18
}
]
},
mounted() {
this.groupBy();
},
methods:{
groupBy(){
this.booksArray = this.friends.reduce((acc,arr)=>
[...acc ,...arr.books],
["How is steel made"],
)
//['How is steel made',Bible', 'Harry Potter','War and peace', 'Romeo and Juliet','The Lord of the Rings', 'The Shining']
/**
* 扩展运算符的使用
var arr1 =['a','b','c']
var arr2 =['d','e','f']
// ES5 合并
var es5Arr = arr1.concat(arr2);
console.log(es5Arr) //['a','b','c','d','e','f']
// Es6 合并
var es6Arr = [...arr1,...arr2];
console.log(es6Arr)//['a','b','c','d','e','f']
*/
},
},
})
</script>
</body>
</html>
数组去重
<!DOCTYPE html> <html> <head> <meta charset="utf-8" /> <title></title> </head> <body> <div id="app"> <ul> <li v-for="item in myarray">{{item}}</li> </ul> <ul> <h2>去重后</h2> <li v-for="item in unrepeatarray">{{item}}</li> </ul> </div> <script src="js/vue.js" type="text/javascript" charset="utf-8"></script> <script type="text/javascript"> const app = new Vue({ el:'#app', data:{ myarray: ['a','b','c','a','b','d','e','f','e','b','d','g',], unrepeatarray: [] }, mounted() { this.repeat(); }, methods:{ repeat(){ this.unrepeatarray = this.myarray.reduce( (accumulator, currentValue)=>{ if(accumulator.indexOf(currentValue)=== -1){ accumulator.push(currentValue); } return accumulator },[]) } }, }) </script> </body> </html>
对象数组去重(根据对象内部某一个属性)
<!DOCTYPE html> <html> <head> <meta charset="utf-8" /> <title></title> </head> <body> <div id="app"> <h2>{{jsonMyArray}}</h2> </div> <script src="js/vue.js" type="text/javascript" charset="utf-8"></script> <script type="text/javascript"> const app = new Vue({ el:'#app', data:{ myarray: [ {id:'001',contract:'20201010',date:'2020-10-10',customer:'LingDa'}, {id:'001',contract:'20201010',date:'2020-10-10',customer:'LingDa'}, {id:'002',contract:'20201012',date:'2020-11-12',customer:'BluseLi'}, {id:'002',contract:'20201012',date:'2020-11-12',customer:'BluseLi'}, {id:'003',contract:'20201013',date:'2020-12-22',customer:'GaoJie'} ], jsonMyArray: Object }, mounted() { this.repeat(); }, methods:{ repeat(){ function depduplication(array){ return array.reduce((accumulator, currentValue)=>{ if(accumulator.length === 0 || (accumulator.length >0 && accumulator[accumulator.length - 1].id != currentValue.id)){ accumulator.push(currentValue) } return accumulator; },[]); } this.jsonMyArray = JSON.stringify(depduplication(this.myarray)) //console.log(this.jsonMyArray) /** * 注意：确保去重的对象属性(id)是排好序的 * [ {"id":"001","contract":"20201010","date":"2020-10-10","customer":"LingDa"}, {"id":"002","contract":"20201012","date":"2020-11-12","customer":"BluseLi"}, {"id":"003","contract":"20201013","date":"2020-12-22","customer":"GaoJie"} ] */ } }, }) </script> </body> </html>
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