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Here is an example how patches could be used to record the incremental updates and inverse apply them:. The generated patches are similar but not the same to the RFC JSON patch standard , except that the path property is an array, rather than a string. This makes processing patches easier.

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If you want to normalize to the official specification, patch. Anyway, this is what a bunch of patches and their inverse could look like:. For a more in-depth study, see Distributing patches and rebasing actions using Immer. Tip: Check this trick to compress patches produced over time. It is allowed to return Promise objects from recipes. This can be pretty useful for long running processes, that only produce the new object once the promise chain resolves. Note that produce itself even in the curried form will return a promise if the producer is async.

Warning: please note that the draft shouldn't be 'leaked' from the async process and stored else where. The draft will still be revoked as soon as the async process completes. It avoids the need to always create a function in order to work with drafts. Instead, one can create a draft, modify it, and at some time in the future finish the draft, in which case the next immutable state will be produced.

We could for example rewrite our above example as:. Note: finishDraft takes a patchListener as second argument, which can be used to record the patches, similarly to produce. It is not needed to return anything from a producer, as Immer will return the finalized version of the draft anyway. However, it is allowed to just return draft. It is also allowed to return arbitrarily other data from the producer function.

But only if you didn't modify the draft. This can be useful to produce an entirely new state. Some examples:. Note: It is not possible to return undefined this way, as it is indistinguishable from not updating the draft! Read on So, in general, one can replace the current state by just return ing a new value from the producer, rather than modifying the draft. There is a subtle edge case however: if you try to write a producer that wants to replace the current state with undefined :.

The problem is that in JavaScript a function that doesn't return anything also returns undefined! So immer cannot differentiate between those different cases. So, by default, Immer will assume that any producer that returns undefined just tried to modify the draft. However, to make it clear to Immer that you intentionally want to produce the value undefined , you can return the built-in token nothing :. Note that this problem is specific for the undefined value, any other value, including null , doesn't suffer from this issue.

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Draft mutations in Immer usually warrant a code block, since a return denotes an overwrite. Sometimes that can stretch code a little more than you might be comfortable with.

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In such cases, you can use javascripts void operator, which evaluates expressions and returns undefined. Immer exposes a named export original that will get the original object from the proxied instance inside produce or return undefined for unproxied values. A good example of when this can be useful is when searching for nodes in a tree-like state using strict equality. Immer automatically freezes any state trees that are modified using produce.

This protects against accidental modifications of the state tree outside of a producer. This comes with a performance impact, so it is recommended to disable this option in production. It is by default enabled. By default, it is turned on during local development and turned off in production. By default produce tries to use proxies for optimal performance. However, on older JavaScript engines Proxy is not available.

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In such cases, Immer will fallback to an ES5 compatible implementation which works identical, but is a bit slower. So the following imports are all correct, where the first is recommended:. Every other object must use the immerable symbol to mark itself as compatible with Immer. When one of these objects is mutated within a producer, its prototype is preserved between copies.

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For arrays, only numeric properties and the length property can be mutated. Custom properties are not preserved on arrays. When working with Date objects, you should always create a new Date instance instead of mutating an existing Date object. Built-in classes like Map and Set are not supported. As a workaround, you should clone them before mutating them in a producer:.

The Immer package ships with type definitions inside the package, which should be picked up by TypeScript and Flow out of the box and without further configuration. The TypeScript typings automatically remove readonly modifiers from your draft types and return a value that matches your original type. See this practical example:.

This ensures that the only place you can modify your state is in your produce callbacks. It even works recursively and with ReadonlyArray s! For curried reducers, the type is inferred from the first argument of recipe function, so make sure to type it. The Draft utility type can be used if the state argument type is immutable:.


Read the second part of the introduction blog. Here is a simple benchmark on the performance of Immer. This test takes 50, todo items and updates 5, of them. Freeze indicates that the state tree has been frozen after producing it. This is a development best practice, as it prevents developers from accidentally modifying the state tree. These tests were executed on Node 9. Use yarn test:perf to reproduce them locally. In your producers, make sure you're not treating this as the draft. Make sure you don't return any promises as state, because produce will actually invoke the promise and wait until it settles.

Q: Does Immer use structural sharing? So that my selectors can be memoized and such? Q: I can't rely on Proxies being present on my target environments. Can I use Immer? Q: Can I store Date objects, functions etc in my state tree when using Immer? Special thanks to Mendix, which supports its employees to experiment completely freely two full days a month, which formed the kick-start for this project. A significant part of my OSS work is unpaid. So donations are greatly appreciated :.

GitHub - immerjs/immer: Create the next immutable state by mutating the current one

Possible values are "lookup" and "best fit" ; the default is "best fit". For information about this option, see the Intl page. Possible values are "sort" and "search" ; the default is "sort". Possible values are: "base" : Only strings that differ in base letters compare as unequal. Other differences may also be taken into consideration. Possible values are true and false ; the default is false.

Implementations are not required to support this property. Possible values are "upper" , "lower" , or "false" use the locale's default ; the default is "false". Contributors to this page: mdnwebdocs-bot , wbamberg , kamoroso94 , aleonhard , paul. Last updated by: mdnwebdocs-bot , Mar 18, , PM. Learn the best of web development Get the latest and greatest from MDN delivered straight to your inbox.

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