Why would I need to freeze an object in JavaScript?

Question

It is not clear to me when anyone would need to use Object.freeze in JavaScript. MDN and MSDN don't give real life examples when it is useful. I get it that an attempt to change such an object at runtime means a crash. The question is rather, when would I appreciate this crash?

To me the immutability is a design time constraint which is supposed to be guaranteed by the type checker.

So is there any point in having a runtime crash in a dynamically typed language, besides detecting a violation better later than never?

Answer 1

The Object.freeze function does the following:

• Makes the object non-extensible, so that new properties cannot be added to it.
• Sets the configurable attribute to false for all properties of the object. When - configurable is false, the property attributes cannot be changed and the property cannot be deleted.
• Sets the writable attribute to false for all data properties of the object. When writable is false, the data property value cannot be changed.

That's the what part, but why would anyone do this?

Well, in the object-oriented paradigm, the notion exists that an existing API contains certain elements that are not intended to be extended, modified, or re-used outside of their current context. The final keyword in various languages is the most suitable analogy of this. Even in languages that are not compiled and therefore easily modified, it still exists, i.e. PHP, and in this case, JavaScript.

Answer 2

You can use this when you have an object representing a logically immutable data structure, especially if:

• Changing the properties of the object or altering its "duck type" could lead to bad behavior elsewhere in your application
• The object is similar to a mutable type or otherwise looks mutable, and you want programmers to be warned on attempting to change it rather than obtain undefined behavior.

As an API author, this may be exactly the behavior you want. For example, you may have an internally cached structure that represents a canonical server response that you provide to the user of your API by reference but still use internally for a variety of purposes. Your users can reference this structure, but altering it may result in your API having undefined behavior. In this case, you want an exception to be thrown if your users attempt to modify it.