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.

var 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.prototype.hasOwnProperty() method, like this:

var foo = {};

// Define the bar property

foo.bar = 'moon';
console.log(foo.bar); // "moon"

// Test to be sure bar exists before accessing it

if (foo.hasOwnProperty('bar')) {
  console.log(foo.bar);
}