ReferenceError: reference to undefined property "x"
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.
js
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:
js
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);
}