Question mark before dot in javascript / react [duplicate]

Question

I know what ternary operator is in React.

When I'm developing react native app I encounter this kind of syntax that is covered by my eslint as unexpected token

ESLint: Parsing error: Unexpected token .

It goes like this:

const routeName = route.state?.routes[route.state.index]?.name ?? INITIAL_ROUTE_NAME;

What does that mean? It uses null coalescing operator in the end, however I can't understand what does the question mark do before a dot.

I know it is a correct syntax because it is a template from expo and they're very popular in react-native development community.

Can anyone help me explain?

Answer 1

That's optional chaining: MDN

The optional chaining operator ?. permits reading the value of a property located deep within a chain of connected objects without having to expressly validate that each reference in the chain is valid. The ?. operator functions similarly to the . chaining operator, except that instead of causing an error if a reference is nullish (null or undefined), the expression short-circuits with a return value of undefined. When used with function calls, it returns undefined if the given function does not exist.

This results in shorter and simpler expressions when accessing chained properties when the possibility exists that a reference may be missing. It can also be helpful while exploring the content of an object when there's no known guarantee as to which properties are required.