Arrays and objects play a vital role in various programming languages. This article will introduce to you the commonly used array traversal and object traversal methods in JavaScript, the differences between each method, and what to pay attention to when using them. matter.
With the continuous development of JS, there are more than ten traversal methods as of the ES7 specification. The following is a group of methods with similar functions to introduce the common traversal methods of arrays.
for, forEach, for...of
const list = [1, 2, 3, 4, 5, 6, 7, 8,, 10, 11];
for (let i = 0, len = list.length; i < len; i++) {
if (list[i] === 5) {
break; // 1 2 3 4
// continue; // 1 2 3 4 6 7 8 undefined 10 11
}
console.log(list[i]);
}
for (const item of list) {
if (item === 5) {
break; // 1 2 3 4
// continue; // 1 2 3 4 6 7 8 undefined 10 11
}
console.log(item);
}
list.forEach((item, index, arr) => {
if (item === 5) return;
console.log(index); // 0 1 2 3 5 6 7 9 10
console.log(item); // 1 2 3 4 6 7 8 10 11
});Summary
some、every
const list = [
{ name: '头部导航', backward: false },
{ name: '轮播', backward: true },
{ name: '页脚', backward: false },
];
const someBackward = list.some(item => item.backward);
// someBackward: true
const everyNewest = list.every(item => !item.backward);
// everyNewest: falseSummary
filter, map
const list = [
{ name: '头部导航', type: 'nav', id: 1 },,
{ name: '轮播', type: 'content', id: 2 },
{ name: '页脚', type: 'nav', id: 3 },
];
const resultList = list.filter(item => {
console.log(item);
return item.type === 'nav';
});
// resultList: [
// { name: '头部导航', type: 'nav', id: 1 },
// { name: '页脚', type: 'nav', id: 3 },
// ]
const newList = list.map(item => {
console.log(item);
return item.id;
});
// newList: [1, empty, 2, 3]
// list: [
// { name: '头部导航', type: 'nav', id: 1 },
// empty,
// { name: '轮播', type: 'content', id: 2 },
// { name: '页脚', type: 'nav', id: 3 },
// ]Summary
find、findIndex
const list = [
{ name: '头部导航', id: 1 },
{ name: '轮播', id: 2 },
{ name: '页脚', id: 3 },
];
const result = list.find((item) => item.id === 3);
// result: { name: '页脚', id: 3 }
result.name = '底部导航';
// list: [
// { name: '头部导航', id: 1 },
// { name: '轮播', id: 2 },
// { name: '底部导航', id: 3 },
// ]
const index = list.findIndex((item) => item.id === 3);
// index: 2
list[index].name // '底部导航';Summary
reduce, reduceRight
The reduce method receives two parameters, the first parameter is the callback function (callback), the second parameter The first parameter is the initial value (initialValue).
reduceRight method is completely consistent with it except that it is in the opposite direction of reduce execution (from right to left).
The callback function receives four parameters:
If no initial value is passed in, the reduce method will execute the callback function starting from index 1. If an initial value is passed in, the callback function will be executed cumulatively from index 0 and based on the initial value.
Calculate the sum of a certain attribute of the object array
const list = [
{ name: 'left', width: 20 },
{ name: 'center', width: 70 },
{ name: 'right', width: 10 },
];
const total = list.reduce((currentTotal, item) => {
return currentTotal + item.width;
}, 0);
// total: 100Deduplication of the object array, and count the number of repetitions of each item
const list = [
{ name: 'left', width: 20 },
{ name: 'right', width: 10 },
{ name: 'center', width: 70 },
{ name: 'right', width: 10 },
{ name: 'left', width: 20 },
{ name: 'right', width: 10 },
];
const repeatTime = {};
const result = list.reduce((array, item) => {
if (repeatTime[item.name]) {
repeatTime[item.name]++;
return array;
}
repeatTime[item.name] = 1;
return [...array, item];
}, []);
// repeatTime: { left: 2, right: 3, center: 1 }
// result: [
// { name: 'left', width: 20 },
// { name: 'right', width: 10 },
// { name: 'center', width: 70 },
// ]Getting the maximum/minimum value of the object array
const list = [
{ name: 'left', width: 20 },
{ name: 'right', width: 30 },
{ name: 'center', width: 70 },
{ name: 'top', width: 40 },
{ name: 'bottom', width: 20 },
];
const max = list.reduce((curItem, item) => {
return curItem.width >= item.width ? curItem : item;
});
const min = list.reduce((curItem, item) => {
return curItem.width <= item.width ? curItem : item;
});
// max: { name: "center", width: 70 }
// min: { name: "left", width: 20 }reduce is very powerful. For more tricks, I recommend checking out this article "25 Advanced Array Reduce Usage You Have to Know"
Having said so much, what is the difference in performance between these traversal methods? Let’s try it in the Chrome browser. I execute each loop 10 times, remove the maximum and minimum values, and take the average to reduce the error.
var list = Array(100000).fill(1)
console.time('for');
for (let index = 0, len = list.length; index < len; index++) {
}
console.timeEnd('for');
// for: 2.427642822265625 ms
console.time('every');
list.every(() => { return true })
console.timeEnd('every')
// some: 2.751708984375 ms
console.time('some');
list.some(() => { return false })
console.timeEnd('some')
// some: 2.786590576171875 ms
console.time('foreach');
list.forEach(() => {})
console.timeEnd('foreach');
// foreach: 3.126708984375 ms
console.time('map');
list.map(() => {})
console.timeEnd('map');
// map: 3.743743896484375 ms
console.time('forof');
for (let index of list) {
}
console.timeEnd('forof')
// forof: 6.33380126953125 msIt can be seen from the printing results that the for loop is the fastest and the for of loop is the slowest
Commonly used traversal termination and performance table comparison
| 是否可终止 | ||||
|---|---|---|---|---|
| ** | break | continue | return | 性能（ms） |
| for | 终止 ✅ | 跳出本次循环 ✅ | ❌ | 2.42 |
| forEach | ❌ | ❌ | ❌ | 3.12 |
| map | ❌ | ❌ | ❌ | 3.74 |
| for of | 终止 ✅ | 跳出本次循环 ✅ | ❌ | 6.33 |
| some | ❌ | ❌ | return true ✅ | 2.78 |
| every | ❌ | ❌ | return false ✅ | 2.75 |
最后，不同浏览器内核 也会有些差异,有兴趣的同学也可以尝试一下。
在对象遍历中，经常需要遍历对象的键、值，ES5 提供了 for...in 用来遍历对象，然而其涉及对象属性的“可枚举属性”、原型链属性等，下面将从 Object 对象本质探寻各种遍历对象的方法，并区分常用方法的一些特点。
for in
Object.prototype.fun = () => {};const obj = { 2: 'a', 1: 'b' };for (const i in obj) { console.log(i, ':', obj[i]);}// 1: b// 2: a// fun : () => {} Object 原型链上扩展的方法也被遍历出来for (const i in obj) { if (Object.prototype.hasOwnProperty.call(obj, i)) { console.log(i, ':', obj[i]); }}// name : a 不属于自身的属性将被 hasOwnProperty 过滤小结
使用 for in 循环时，返回的是所有能够通过对象访问的、可枚举的属性，既包括存在于实例中的属性，也包括存在于原型中的实例。如果只需要获取对象的实例属性，可以使用 hasOwnProperty 进行过滤。
使用时，要使用(const x in a)而不是(x in a)后者将会创建一个全局变量。
for in 的循环顺序，参考【JavaScript 权威指南】（第七版）6.6.1。
Object.keys
Object.prototype.fun = () => {};const str = 'ab';console.log(Object.keys(str));// ['0', '1']const arr = ['a', 'b'];console.log(Object.keys(arr));// ['0', '1']const obj = { 1: 'b', 0: 'a' };console.log(Object.keys(obj));// ['0', '1']小结
用于获取对象自身所有的可枚举的属性值，但不包括原型中的属性，然后返回一个由属性名组成的数组。
Object.values
Object.prototype.fun = () => {};const str = 'ab';console.log(Object.values(str));// ['a', 'b']const arr = ['a', 'b'];console.log(Object.values(arr));// ['a', 'b']const obj = { 1: 'b', 0: 'a' };console.log(Object.values(obj));// ['a', 'b']小结
用于获取对象自身所有的可枚举的属性值，但不包括原型中的属性，然后返回一个由属性值组成的数组。
Object.entries
const str = 'ab';for (const [key, value] of Object.entries(str)) { console.log(`${key}: ${value}`);}// 0: a// 1: bconst arr = ['a', 'b'];for (const [key, value] of Object.entries(arr)) { console.log(`${key}: ${value}`);}// 0: a// 1: bconst obj = { 1: 'b', 0: 'a' };for (const [key, value] of Object.entries(obj)) { console.log(`${key}: ${value}`);}// 0: a// 1: b小结
用于获取对象自身所有的可枚举的属性值，但不包括原型中的属性，然后返回二维数组。每一个子数组由对象的属性名、属性值组成。可以同时拿到属性名与属性值的方法。
Object.getOwnPropertyNames
Object.prototype.fun = () => {};Array.prototype.fun = () => {};const str = 'ab';console.log(Object.getOwnPropertyNames(str));// ['0', '1', 'length']const arr = ['a', 'b'];console.log(Object.getOwnPropertyNames(arr));// ['0', '1', 'length']const obj = { 1: 'b', 0: 'a' };console.log(Object.getOwnPropertyNames(obj));// ['0', '1']小结
用于获取对象自身所有的可枚举的属性值，但不包括原型中的属性，然后返回一个由属性名组成的数组。
我们对比了多种常用遍历的方法的差异，在了解了这些之后，我们在使用的时候需要好好思考一下，就能知道那个方法是最合适的。欢迎大家纠正补充。
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