Frozen Moment

Immutability for Moment.js

If you build large applications that use Moment.js, you've probably been surprised at some point by the mutability of moments. Things like moment.startOf("day") change the date of your original moment (instead of returning a new moment). Unfortunately, this leads to subtle bugs if you pass moments around and then start to do math on them while expecting other places to still have the old value.

Or maybe you're smarter than me, and yet you still wish that Moment had an immutable API. It gets annoying to keep typing moment.clone() all the time.

Either way, this plugin is for you.

API Reference: What does it do?

Frozen Moment is a plugin for Moment.js. With Frozen Moment, all of your normal moments will still work the same way they always have -- so you won't need to adopt immutability throughout your entire codebase all at once. Frozen Moment simply adds a new method to every moment instance:

Methods on Moment and Frozen Moment instances

moment().freeze()

Returns a "frozen" copy of the original moment. "Frozen" moments will behave exactly like normal moments, but all of the methods that would normally change the value of a frozen moment will instead return a new frozen moment.

Basically, frozen moments will automatically call moment.clone() before you try to call any of Moment's setters or manipulation functions. You'll also get a new instance every time you change a frozen moment's locale.

For performance and compatibility reasons, frozen moments are not made immutable with Object.freeze. If you want to shoot yourself in the foot by manually meddling with your frozen moment's internal data, go right ahead. That said, frozen moments will be immutable as long as you only use Moment's documented API methods.

Frozen moments attempt to play nice with other Moment.js plugins, assuming that the Frozen Moment plugin is loaded last and/or moment.frozen.autowrap() is called after the last Moment plugin has been initialized. That said, we cannot guarantee that every plugin will behave correctly. If you have problems using Frozen Moment with any other Moment plugin, please open an issue and we'll work with the other plugin maintainer to resolve the incompatibility.

Frozen moments do not have the freeze() method -- only regular moments do.

frozenMoment.thaw()

Returns a normal (un-frozen) copy of a frozen moment.

Regular old moments do not have a thaw() method -- only frozen moments do.

moment().isFrozen() / frozenMoment.isFrozen()

Returns true if called on a frozen moment, and false if called on a standard moment.

Note that moment.isMoment() will return true for frozen moments and normal moments alike.

Global configuration

moment.frozen.fn

This is the prototype for all frozen moment instances. It inherits from moment.fn, which is the prototype used for all Moment instances created by the core library.

moment.frozen.unwrap(methodNames...)

Removes all existing wrappers for the named moment prototype function(s), and whitelists those method names so that wrappers will not be re-created by subsequent calls to moment.frozen.autowrap(). This is a mechanism for performance-optimizing plugin authors to whitelist methods that do not mutate the moment instance, so that Frozen Moment will not automatically clone a new instance every time those methods are invoked.

moment.frozen.autowrap()

Re-generates wrappers for all functions on the Moment prototype that have not been explicitly whitelisted. Some plugin authors may want to call this after adding mutation methods to the Moment prototype, so that their users will not need to load their plugin before Frozen Moment. Alternatively, application authors may wish to call this after loading their Moment plugins, to ensure that all plugin methods are properly wrapped for immutable behavior.

TODO

Frozen Moment should generally work, and it has been used by a few folks in production applications. The current v0.4 release is a reasonably stable implementation of the core ideas. This code has inspired and informed Moment's implementation of immutability, which is coming in version 3.0.

Our maintainer is manually running our unit tests in a variety of browsers (IE 8 and the current releases of Chrome, Firefox, and Safari) to ensure broad runtime compatibility, in addition to our CI build with Node.

Pull requests are enthusiastically welcomed for improvements on our current to-do list. If you have other ideas for new features, it's often a good idea to get our feedback on your plans before you bother writing the code. In any event, please remember to submit unit tests to verify your changes.

Historical Note

The original version of Frozen Moment was a full-fledged fork of Moment.js. It is no longer maintained.