Let's talk about the optional (?.) operator in JavaScript

This article will introduce you to the optional (?.) operator in JavaScript. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.

How to use null (null and undefined) check to access the nested properties of an object? Suppose we have to access the interface from the backend Access user details.

You can use the nested ternary operator:

const userName = response ? (response.data ? (response.data.user ? response.data.user.name : null) : null) : null;

or use if for null value checking:

let userName = null;
if(response && response.data && response.data.user){
  userName = response.data.user.name;
}

Or better yet The && conditions that make it a single line link, like this:

const userName = response && response.data && response.data.user && response.data.user.name;

What the above codes have in common is that links can sometimes be very verbose and become harder to format and read. This is why the ?. operator was proposed. Let’s change it to ?. and reconstruct the above code:

const userName = response?.data?.user?.name;

is very nice.

Syntax

##?. The syntax was introduced in ES2020 and its usage is as follows:

obj.val?.pro  // 如果`val`存在，则返回`obj.val.prop`，否则返回 `undefined`。

obj.func?.(args) // 如果 obj.func 存在，则返回 `obj.func?.(args)`，否则返回 `undefined`。

obj.arr?.[index] // 如果 obj.arr 存在，则返回 `obj.arr?.[index]`，否则返回 `undefined`。

Use?.Operator

user object:

const user = {
  name: "前端小智",
  age: 21,
  homeaddress: {
    country: "中国"
  },
  hobbies: [{name: "敲代码"}, {name: "洗碗"}],
  getFirstName: function(){
    return this.name;
  }
}

Properties

console.log(user.homeaddress.country); 
// 中国

console.log(user.officeaddress.country); 
// throws error "Uncaught TypeError: Cannot read property 'country' of undefined"

?. instead. Access non-existent attributes:

console.log(user.officeaddress?.country); 
// undefined

Method

console.log(user.getFirstName());

console.log(user.getLastName()); 
// throws error "Uncaught TypeError: user.getLastName is not a function";

?. instead to access non-existent methods Method:

console.log(user.getLastName?.()); 
// "undefined"

Array

console.log(user.hobbies[0].name); 
// "敲代码"

console.log(user.hobbies[3].name); 
// throws error "Uncaught TypeError: Cannot read property 'name' of undefined"

Access non-existent array: <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:js;toolbar:false;">console.log(user.dislikes?.[0]?.name); // &quot;undefined&quot;</pre><div class="contentsignin">Copy after login</div></div>

??<strong></strong> OperatorWe know

Operation symbol If the object does not exist, it will just return undefined. During development, it may not return undefined but return a default value. In this case, we can use double Question?? Operator. is a bit abstract, let’s take a direct example:

const country = user.officeaddress?.country;
console.log(country);
// undefined

Need to return the default value:

const country = user.officeaddress?.country ?? "中国";
console.log(country);
// 中国

Author: Ashish Lahoti

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