Immer documentation (#232)

Author: labkey-nicka

packages/components/docs/immer.md

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+# Immer
+
+[Immer](https://immerjs.github.io/immer/docs/introduction) is a JavaScript package that provides immutability for normal
+JavaScript objects, arrays, Sets, and Maps. After internal review by several of our frontend engineers we've elected
+to add Immer as a dependency and start using it for new development in lieu of [ImmutableJS](https://immutable-js.github.io/immutable-js/).
+That said, we do not plan on actively migrating all usages away from ImmutableJS and it will remain a dependency for
+the foreseeable future.
+
+This document intends to outline why we're moving to Immer, provide some links to good resources for Immer, and provide
+a couple of scenarios highlighting aspects of note.
+
+## Rationale for switching
+
+In April 2020 we took some time to start investigating how we could move
+away from ImmutableJS. We've utilized ImmutableJS since 2015
+to provide immutable data structures to work with on the client. At the time, it was one of the most well-supported
+immutablility packages out there. Alas, as things progressed ImmutableJS started to fall out of favor for
+several reasons.
+
+#### Disadvantages of ImmutableJS
+
+1. No longer actively developed. We use `v3.8.2` which was released in late 2016. `v4` has yet to get
+passed "release candidate" status with it's most recent candidate releasing in late 2018.
+1. API learning curve. Immutable provides all of its own data structures (e.g. `List`, `Map`, `Set`, etc) which are
+wholy different from native JS data structures. While the API for these structures is powerful, allowing for really
+complex mutations and iterations, it can be difficult to ramp up on understanding it all.
+1. Poorly constructed `Record`. A class we rely on heavily is `Immutable.Record`. Extending record and
+providing the correct typings annotations requires three declarations of each value. Additionally, due to the
+nature of ImmutableJS, the constructor isn't able to make any effectual modifications of what the user passes in,
+which lead to us using a `RecordType.create()` static method pattern. `Immutable.Record` was removed in `v4`.
+1. Difficult to debug. The Immutable data structures can be difficult to debug and generally requires the code to be
+modified to include `.toJS()` statements to understand what is actually held in a data structure.
+
+#### Advantages of Immer
+
+These are copied [directly from the website](https://immerjs.github.io/immer/docs/introduction#benefits):
+
+1. Immutability with normal JavaScript objects, arrays, Sets and Maps. No new APIs to learn!
+1. Strongly typed, no string based paths selectors etc.
+1. Structural sharing out of the box
+1. Object freezing out of the box
+1. Deep updates are a breeze
+1. Boilerplate reduction. Less noise, more concise code.
+1. First class support for patches
+1. Small: 3KB gzipped
+
+## Learning Immer
+
+This document intends to provide some specific insights about using Immer in our LabKey client-side code. As such, it expects
+the reader to have an understanding of why Immer exists, what Immer does, and how Immer does it.
+
+Before you read further it is **highly recommended** that you read (or watch) at least one the following:
+
+- [Immer's official documentation](https://immerjs.github.io/immer/docs/introduction) - read the docs! Most up-to-date and covers lots of topics.
+- [Introducing Immer: Immutability the easy way](https://hackernoon.com/introducing-immer-immutability-the-easy-way-9d73d8f71cb3) - written by the author of Immer
+- [Simplify Creating Immutable Data Trees With Immer](https://egghead.io/lessons/redux-simplify-creating-immutable-data-trees-with-immer) - egghead.io video tutorial
+
+## Scenarios
+
+This section focuses on a couple of scenarios to help get a better understanding. These were written against `v6.0.3` of
+Immer so things may have changed if you're working with a more current version.
+
+### Immutable class
+
+This scenario highlights declaring an immutable class in TypeScript using Immer. By the end we'll have an immutable class
+that is both compile-time and run-time safe. To keep the class simple we're going to define a `Circle` class defined only
+by its `radius`:
+
+```ts
+class Circle {
+    radius: number;
+
+    constructor(r: number) {
+        this.radius = r;
+    }
+}
+```
+
+#### Run-time safety
+
+This initial declaration is fully mutable. You can externally modify the radius after construction.
+
+```ts
+let circle = new Circle(5);
+circle.radius = 10; // radius now 10
+```
+
+Let's try to use Immer on this class and see if it works:
+
+```ts
+import { produce } from 'immer';
+
+let circle = produce(new Circle(5), () => {});
+circle.radius = 10; // circle.radius is now 10! I thought using produce made it immutable!
+```
+
+To make a class immutable with Immer you first annotate the class with a [Symbol](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol)
+provided by Immer called `immerable`:
+
+```ts
+import { immerable } from 'immer';
+
+class Circle {
+    [immerable] = true;
+
+    radius: number;
+
+    constructor(r: number) {
+        this.radius = r;
+    }
+}
+```
+
+What does this symbol do? To paraphrase the Immer docs:
+
+> Classes 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.
+
+Now this class is ready to be used with Immer. Let's try again using `produce`:
+
+```ts
+import { produce } from 'immer';
+
+// without using produce the instance is still mutable
+let circle = new Circle(5);
+circle.radius = 20; // radius now 20.
+
+// with produce the instance is now immutable
+circle = produce(new Circle(5), () => {});
+circle.radius = 10; // non-strict mode: fails silently. Radius still 5.
+circle.radius = 10; // strict mode: Run-time error: Cannot assign to read only property 'radius' of object '#<Circle>'
+```
+
+Instances of this class declared via `produce` can only be mutated via `produce`. When an instance is passed through
+Immer's `produce` function it will [freeze](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze)
+the object (when `immer.setAutoFreeze(true)`). Depending on the strict mode, any attempts to explicitly modify the object
+will either fail to modify or throw a run-time error.
+
+Now, let's actually make an update to the immutable instance with Immer:
+
+```ts
+import { produce } from 'immer';
+
+// Create the initial instance
+const circle = produce(new Circle(5), () => {});
+
+// Mutate and copy from `produce`
+const newCircle = produce(circle, (draft) => {
+    draft.radius = 10;
+});
+console.log(circle.radius);    // 5
+console.log(newCircle.radius); // 10
+```
+
+The instance is now immutable, a mutated copy can be made via `produce`, and we have run-time safety via `Object.freeze`.
+
+#### Compile-time safety
+
+Immer results in run-time safety from mutations to your objects, but used in conjunction with Typescript you can
+also get compile-time safety. This has the advantages of catching errors earlier and applying to all code paths,
+even those not covered by tests.
+
+To get started, let's first declare all the properties on the `Circle` class as [`readonly`](https://www.typescriptlang.org/docs/handbook/classes.html#readonly-modifier).
+
+```ts
+import { immerable } from 'immer';
+
+class Circle {
+    [immerable] = true;
+
+    readonly radius: number;
+
+    constructor(r: number) {
+        this.radius = r;
+    }
+}
+```
+
+The `radius` property is now read-only so if we attempt to modify it directly we receive an error:
+
+```ts
+import { produce } from 'immer';
+
+let circle = new Circle(5);
+circle.radius = 5; // Error: TS2540: Cannot assign to 'radius' because it is a read-only property.
+
+// Same for the produced version
+let circle = produce(new Circle(5), () => {});
+circle.radius = 5; // Error: TS2540: Cannot assign to 'radius' because it is a read-only property.
+```
+
+This gives us compile-time safety against invalid writes. The next feature we can use is the `Draft` utility from
+Immer. `Draft` To quote the docs:
+
+> The `Draft` utility type can be used if the state argument type is immutable.
+
+```ts
+import { Draft, produce } from 'immer';
+
+// Without "Draft"
+let circle = produce(new Circle(5), (draft: Circle) => {
+    draft.radius = 10; // Error: TS2540: Cannot assign to 'radius' because it is a read-only property.
+});
+
+// With "Draft"
+let circle = produce(new Circle(5), (draft: Draft<Circle>) => {
+    draft.radius = 10; // OK!
+});
+```
+
+If you'd like to have your class instances be immutable without requiring use of `produce` you can directly call
+`Object.freeze` at the end of the constructor.
+
+```ts
+import { immerable } from 'immer';
+
+class Circle {
+    [immerable] = true;
+
+    readonly radius: number;
+
+    constructor(r: number) {
+        this.radius = r;
+
+        // Optionally, freeze the instance. Still works with produce but doesn't require it to have
+        // an immutable instance via construction.
+        Object.freeze(this);
+    }
+}
+```
+
+Now we have an immutable class that is compile-time safe, run-time safe, and can be utilized by Immer.