85.JavaScript中如何判断对象是否为空

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一般使用Object.keys()来判断对象是否为空

const obj = {};

Object.keys(obj).length === 0  // true 则为空对象

Object.keys()方法会返回一个由一个给定对象的自身可枚举属性组成的数组。

new Object();

new String();
new Number();
new Boolean();
new Array();
new RegExp();
new Function();
new Date();
new Error();

function badEmptyCheck(value) {
    return Object.keys(value).length === 0;
}

badEmptyCheck(new String()); // true
badEmptyCheck(new Number()); // true
badEmptyCheck(new Boolean()); // true
badEmptyCheck(new Array()); // true
badEmptyCheck(new RegExp()); // true
badEmptyCheck(new Function()); // true
badEmptyCheck(new Date()); // true
badEmptyCheck(new Error();); // true

function goodEmptyCheck(value) {
    return Object.keys(value).length === 0 && value.constructor === Object;
}

goodEmptyCheck(new String()); // false
goodEmptyCheck(new Number()); // false
goodEmptyCheck(new Boolean()); // false
goodEmptyCheck(new Array()); // false
goodEmptyCheck(new RegExp()); // false
goodEmptyCheck(new Function()); // false
goodEmptyCheck(new Date()); // false
goodEmptyCheck(new Error();); // false

Object.keys(null).length === 0;   // TypeError

Object.keys(undefined).length === 0;  // TypeError

function goodEmptyCheck(value) {
    return value && Object.keys(value).length === 0 && value.constructor === Object;
}

getOwnPropertySymbols 和 getOwnPropertyNames

Reflect.ownKeys()

const a = null
const b = undefined
console.log(!a) // true
console.log(!b) // true

通过ES6语法 Object.keys() 进行判断



const a = {
  name: 'a'
}
const b = {}
console.log(Object.keys(a).length === 0) // false
console.log(Object.keys(b).length === 0) // true



const a = {
  name: 'a'
}
const b = {}
console.log(JSON.stringify(a) === '{}') // false
console.log(JSON.stringify(b) === '{}') // true


const a = {
  name: 'a'
}
const b = {}
console.log(Object.getOwnPropertyNames(a).length === 0) // false
console.log(Object.getOwnPropertyNames(b).length === 0) // true


const a = { [Symbol()]: 'a' }

console.log(a) // { [Symbol()]: 'a' }
console.log(JSON.stringify(a) === '{}') // true
console.log(Object.keys(a).length === 0) // true
console.log(Object.getOwnPropertyNames(a).length === 0) // true

const a = { [Symbol()]: 'a' }

console.log(a) // { [Symbol()]: 'a' }
console.log(JSON.stringify(a) === '{}') // true
console.log(Object.keys(a).length === 0) // true
console.log(Object.getOwnPropertyNames(a).length === 0) // true

getOwnPropertySymbols
console.log(Object.getOwnPropertySymbols(a).length === 0) // false

const a = { [Symbol()]: 'a' }
const b = { a: 'a' }
const c = {}

console.log(Object.getOwnPropertyNames(a).length === 0 && Object.getOwnPropertySymbols(a).length === 0) // false
console.log(Object.getOwnPropertyNames(b).length === 0 && Object.getOwnPropertySymbols(b).length === 0)  // false
console.log(Object.getOwnPropertyNames(c).length === 0 && Object.getOwnPropertySymbols(c).length === 0)  // true

const a = { [Symbol()]: 'a' }
const b = { a: 'a' }
const c = {}
console.log(Reflect.ownKeys(a).length === 0) // false
console.log(Reflect.ownKeys(b).length === 0) // false
console.log(Reflect.ownKeys(c).length === 0) // true