JS logic empty allocation double question mark syntax, double vertical bar syntax and optional chain syntax

The syntax of the optional chain allows developers to access deeper nested object properties , Don't worry about whether the attribute really exists . in other words , If the optional chain encounters null or undefined Value , Through a short circuit （short-circuit） Calculation , return undefined, And never report a mistake .

Logical null assignment operator is only used when null value or undefined （null  or undefined） The value is assigned to a

Double question mark ： When the previous value is null 、undefined when , Return the following value , Otherwise, take the previous value

user.userinfo.age ?? 0

Double vertical bar ： As long as the previous value is converted to a Boolean value of false when , Take the following value （ Such as ：undefined、null、false、 An empty string 、 The number 0）

reflection ： Double question marks are more appropriate when variables are unknown Whether to define perhaps Assignment or not The scenario used

?. Optional chain : If it means

let test = {
    value: {
        a: '1222'
    }
}

If we want to achieve test In existence , return test.value, Otherwise return to undefined
It needs to be written like this ：
let myvalue = test ?  test. value : undefined

If you use the optional chain Syntax , It can be abbreviated as ：
let myvalue = test ?. value

If you want to go back test.value.a , It can be abbreviated as ：
let a = test ?.value?.a || 10    // If test.value.a If it doesn't exist ( by null or undefined) , Then return to 10 Fixed value

Reference resources ：

Know optional chain and double question mark Syntax _ Front end discovery blog -CSDN Blog

vue Project support js New syntax optional chain “?.“ And logical empty allocation （ Double question mark ）“??“_ The dove .Luna The blog of -CSDN Blog _vue Double question mark