SyntaxError: new keyword cannot be used with an optional chain

The JavaScript exception "new keyword cannot be used with an optional chain" occurs when the constructor of a new expression is an optional chain, or if there's an optional chain between the constructor and the parenthesized list of arguments.

Message

SyntaxError: Invalid optional chain from new expression (V8-based)
SyntaxError: new keyword cannot be used with an optional chain (Firefox)
SyntaxError: Cannot call constructor in an optional chain. (Safari)

Error type

SyntaxError

What went wrong?

There are two ways to get this error. The first one is if the constructor expression is an optional chain expression, like this:

js
new Intl?.DateTimeFormat();
Number?.[parseMethod]`Hello, world!`;

The second one is if ?. occurs between the constructor and the arguments list, like this:

js
new Intl.DateTimeFormat?.();

Optional new is specifically forbidden because its syntax is complicated (new with and without arguments), and the result is unclear (it would be the only case where new does not evaluate to an object value). You need to translate the optional chaining to its underlying condition (see optional chaining for more information).

js
const result =
  Intl.DateTimeFormat === null || Intl.DateTimeFormat === undefined
    ? undefined
    : new Intl.DateTimeFormat();

Remember that optional chaining only short-circuits within a parenthesized unit. If you parenthesize your constructor expression, the optional chaining will not cause an error, because now the constructor does not short-circuit and the result is clear (the constructor will produce undefined and then cause the new expression to throw).

js
new (Intl?.DateTimeFormat)(); // Throws if Intl?.DateTimeFormat is undefined

However this is a bit nonsensical anyway because optional chaining prevents errors inside the property access chain, but is then guaranteed to generate an error when calling new. You would probably still want to use a conditional check.

Note that optional chaining is only forbidden as the constructor expression. You can use optional chaining inside the argument list, or use optional chaining on the new expression as a whole.

js
new Intl.DateTimeFormat(navigator?.languages);
new Intl.DateTimeFormat().resolvedOptions?.();

Note that there's no needs to use ?. on the new expression itself: new a()?.b, because new is guaranteed to produce a non-nullish object value.

See also