The JavaScript warning "reference to undefined property" occurs when a script attempted to access an object property which doesn't exist.
Message
ReferenceError: reference to undefined property "x" (Firefox)
Error type
(Firefox only) ReferenceError warning which is reported only if
javascript.options.strict preference is set to true.
What went wrong?
The script attempted to access an object property which doesn't exist. There are two ways to access properties; see the property accessors reference page to learn more about them.
Examples
Invalid cases
In this case, the property bar is an undefined property, so a
ReferenceError will occur.
const foo = {};
foo.bar; // ReferenceError: reference to undefined property "bar"
Valid cases
To avoid the error, you need to either add a definition for bar to the
object or check for the existence of the bar property before trying to
access it; ways to do that include using the in operator,
or the Object.hasOwn method, like this:
const foo = {};
// Define the bar property
foo.bar = "moon";
console.log(foo.bar); // "moon"
// Test to be sure bar exists before accessing it
if (Object.hasOwn(foo, "bar")) {
console.log(foo.bar);
}