Assuming w has not been previously defined, the following JS code gives ReferenceError: w is not defined:

w?.y;

whereas this code simply returns undefined:

let w; w?.y;

Why does the ?. operator not treat not defined as undefined?

CodePudding user response：

This is mentioned in the documentation

Optional chaining cannot be used on a non-declared root object, but can be used with an undefined root object.

which means you have to declare the root object, in this case, it's w

CodePudding user response：

The difference is between trying to access a (potentially) non-existent variable vs a (potentially) non-existent property.

From MDN:

On reference errors:

The ReferenceError object represents an error when a variable that doesn't exist (or hasn't yet been initialized) in the current scope is referenced.

This is the case in your first example:

const value = w?.y;
//            ^ ReferenceError occurs here
console.log(value);

On optional chaining (?.):

The ?. operator is like 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.

This is the case in your second example:

let w;
const value = w?.y;
//            ^ Reference exists (but it is undefined)
console.log(value);

CodePudding user response：

I don't know about ecmascript-6. I'm curious what ?. does. Anyways, in js, not defined basically means that its not declared. undefined is just a placeholder.