Several methods of checking JS to judge empty objects

Knowledge preparation

In the inventory JS Let's first understand the following three methods before judging the space .

Object.keys

Object.keys()

Method takes object as parameter , Returns an array containing the matching properties and methods in the object

 var obj = {
 name: "cxy",
 age: "19"
 }; 
 var objArray = Object.getOwnPropertyNames(obj);
 console.log(objArray)


You can see

objArray

For the return value , And the return value is an array with the contents of the attributes in the object

Object.getOwnPropertyNames

Object.getOwnPropertyNames()

Methods also take objects as parameters , Returns an array containing the matching properties and methods in the object

You will ask questions here , What's the difference between the two ？

Object.getOwnPropertyNames()

All properties can be returned , and

Object.keys()

Only enumerable properties can be returned , EH ？ Here we are confused again

Enumerable properties

What is it again? ？ Don't worry , Let me explain what an enumerable attribute is

Enumerable properties

Enumerable or non enumerable attributes are passed through enumerable flags inside objects

enumerable

To make a distinction , By default , We

obj.name = "cxy"

After adding a property to the object , Its enumerable flags

enumerable

by

ture

, And when the value is

false

It can't be enumerated , When we do

for

,

Object.keys()

,

JSON.stringify()

When enumerable attributes cannot be found , We can understand that the non enumerable attribute is

stealth

Of

Now let's take the one above

Object.getOwnPropertyNames

and

Object.keys()

Give practical examples , We go through

defineProperty

To add

age

attribute , Because this method can set the enumeration flag , I'm going to set it to

false

, You can see the following two different return results

var stuObj = {
 name: "cxy"
}
Object.defineProperty(stuObj, 'age', {
 value: "18",
 enumerable: false
});
console.log(Object.keys(stuObj))
console.log(Object.getOwnPropertyNames(stuObj))


hasOwnProperty

hasOwnProperty()

It is used to judge whether an object contains a certain attribute , Its parameter is attribute name

 var stuObj = {
 name: "cxy"
 }
 console.log(stuObj.hasOwnProperty('name'))


But one thing to pay attention to here is ,

hasOwnProperty()

Judge

Inheritance attribute

Will return

false

, Inherited properties are the properties that the object inherits from the prototype object , for instance

toString

Method of checking blank

JSON.stringify

Sentenced to empty

This method is relatively simple , Use

JSON.stringify

Convert objects to strings , Then, the Boolean value of whether the object is empty can be obtained by judging whether it is equal to

let obj = {
 name: "cxy"
}
console.log(JSON.stringify(obj) == '{}')


for in

Sentenced to empty

Use

for in

It can return when the cycle is triggered

false

When the loop is not triggered, it means that the object is null and returns

ture

let forNull = (items) => {
 for (let item in items) {
 return false
 }
 return true
}


Object.getOwnPropertyNames

Sentenced to empty

The above mentioned

Object.getOwnPropertyNames

, Of the array that will be returned

length

As a basis for judgment .

let stuArray = Object.getOwnPropertyNames(obj)
console.log(stuArray.length === 0)


Object.keys()

Sentenced to empty

Same as the previous method , Use an array as a basis for judgment

let stuArray = Object.getOwnPropertyNames(obj)
console.log(stuArray.length === 0)


hasOwnProperty

Sentenced to empty

Use

hasOwnProperty

It's using for The loop judges the element and returns

false

Description cannot be empty , Otherwise, it is empty

let forNull = (items) => {
 for (let item in items) {
 if(items.hasOwnProperty(item)){
 return false
 }
 }
 return true
}