Errors: Undefined prop

Message

ReferenceError: reference to undefined property "x" (Firefox)

Error type

ReferenceError warning in strict mode only.

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.

Errors about references to undefined properties occur in strict mode code only. In non-strict code, accesses to properties that don't exist are silently ignored.

Examples

Invalid cases

In this case, the property bar is an undefined property, so a ReferenceError will occur.

'use strict'; 

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; one way to do that is to use the Object.prototype.hasOwnProperty() method), like this:

'use strict';

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);
}

See also

• Strict mode