JavaScript Error Handling

JavaScript – ReferenceError: reference to undefined property “x”

Today, as we continue through the next thrilling segment in our JavaScript Error Handling series, we’re examining the Reference to Undefined Property error with a fine-toothed comb. The Reference to Undefined Property error can only appear when strict mode is enabled, and will occur when the code attempts to access an object property that simply doesn’t exist.

Below we’ll take a look at a few examples to show where Reference to Undefined Property errors might appear, then take a closer look at how to deal with these little errors when they pop up. Let’s get started!

The Technical Rundown

  • All JavaScript error objects are descendants of the Error object, or an inherited object therein.
  • The ReferenceError object is inherited from the Error object.
  • The Reference to Undefined Property error is a specific type of ReferenceError object.

When Should You Use It?

To properly examine Reference to Undefined Property errors we first need to understand what strict mode means in the context of JavaScript. Put simply, strict mode is a toggle which forces JavaScript to behave in a slightly altered manner, typically by opting into less secure limitations placed on upon the code, opening up execution to more dangers and exploits. However, in some cases it may be necessary to enable strict mode, and in such cases, it’s entirely possible to produce a Reference to Undefined Property error.

The Reference to Undefined Property error itself is rather straightforward: It will be thrown anytime a call is made on a property that hasn’t been defined for the referenced object. A call to a property is always in the form of object.property, such as foo.bar.

As an example, we’ll enable strict mode below, then create the greeting object and attempt to call an undefined languageproperty of it:

As expected, the greeting.text property is displayed properly in the console, but the call to the undefined greeting.language property causes an expected Reference to Undefined Property error:

It’s important to note that as JavaScript evolves and modern browsers adopt the latest versions, not all errors that once existed continue to be necessary or relevant, and thus they may be deprecated. The Reference to Undefined Property error is one such example; while older browsers will still produce the above error output by catching the Reference to Undefined Property error, most modern browsers simply ignore it, as if strict mode wasn’t enabled. Unfortunately, there’s no programmatic work around for this, other than ensuring there are no references to undefined properties within your code.

Executing the exact same code as above on Chrome 55 or Firefox 50, for example, outputs the greeting.text value, but then also outputs Language is undefined:

This behavior is identical to simply not using strict mode at all in an older browser where the Reference to Undefined Property error can still be thrown. JavaScript is effectively silently ignoring the improper reference to the undefined property.

Another safe practice to ensure no Reference to Undefined Property errors appear when your code is executed in older browsers is to use the Object.prototype.hasOwnProperty() method on the instance of the object in question. This method accepts one parameter, which is the name or symbol of the property to test and ensure it exists as part of the referenced object.

As a simple example, we can modify our above code by including an if statement that checks if the .language property is actually defined on the greeting object:

Since greeting has no .language property, the second output is not produced, nor is a Reference to Undefined Propertyerror thrown.

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