Reading a non-existent property on an object always returns undefined. Doing so is usually an error; either in the name of the property or the type of the variable being accessed.

If an attempt is made to access properties of a primitive, the primitive is automatically encased in a primitive-wrapper object for the operation. But being "promoted" to an object doesn't mean that the primitive will actually have properties to access. The wrapper object still won't have the non-existent property and undefined will be returned instead.

This rule raises an issue when an attempt is made to access properties of a primitive. Thus this rule should only be activated when you don't use monkey patching for standard objects, like Number, Boolean and String.

Noncompliant Code Example

x = 42;
y = x.length;   // Noncompliant, Number type doesn't have "length" property

Exceptions

The Ember framework introduces a few extensions to String. Since it is a widely used package, the following String properties will not trigger this rule even though they are not built-in: