is-immutable-type

is-immutable-type

Test the immutability of TypeScript types.

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Installation

# Install with npm
npm install is-immutable-type

# Install with yarn
yarn add is-immutable-type

# Install with pnpm
pnpm add is-immutable-type

Usage

import { getTypeImmutability, Immutability, isReadonlyDeep, isUnknown } from "is-immutable-type";
import type ts from "typescript";

function foo(checker: ts.TypeChecker, node: ts.Node) {
  const nodeType = checker.getTypeAtLocation(node);
  const constrainedNodeType = checker.getBaseConstraintOfType(nodeType);
  const immutability = getTypeImmutability(checker, constrainedNodeType);

  if (isUnknown(immutability)) {
    console.log("`immutability` is `Unknown`").
  } else if (isReadonlyDeep(immutability)) {
    console.log("`immutability` is `ReadonlyDeep` or `Immutable`").
  } else {
    console.log("`immutability` is `ReadonlyShallow` or `Mutable`").
  }
}

Tip: You can also use comparator expressions (such as > and <) to compare Immutability.
Note: Immutability.Unknown will always return false when used in a comparator expression. This includes === - use isUnknown() if you need to test if a value is Unknown.

Immutability

Definitions

• Immutable: Everything is deeply read only and nothing can be modified.
• ReadonlyDeep: The data is deeply immutable but methods are not.
• ReadonlyShallow: The data is shallowly immutable, but at least one deep value is not.
• Mutable: The data is shallowly mutable.
• Unknown: We couldn't determine the immutability of the type.

Note: Internally Unknown may also be used to mean that we are still calculating the immutability of the type. This shouldn't be exposed to the outside world though.

Overrides

Sometimes we cannot correctly tell what a type's immutability is supposed to be just by analyzing its type makeup. One common reason for this is because methods may modify internal state and we cannot tell this just by the method's type. For this reason, we allow types to be overridden.

To override a type, pass an overrides array of all the override objects you want to use to your function call.
You can either override a type by name or by a regex pattern.
You must specify a to property with the new immutability value that should be used.
Additionally you may specify a from property which will make it so the override will only be applied if the calculated immutability is between the to and from values (inclusively).

Example 1

Always treat ReadonlyArrays as Immutable.

[{ name: "ReadonlyArray", to: Immutability.Immutable }]

Example 2

Treat ReadonlyArrays as Immutable instead of ReadonlyDeep. But if the instance type was calculated as ReadonlyShallow, it will stay as such.

[{ name: "ReadonlyArray", to: Immutability.Immutable, from: Immutability.ReadonlyDeep }]

Default Overrides

By default the following types are overridden to be Mutable:

• Map
• Set
• Date
• URL
• URLSearchParams

If you know of any other TypeScript types that need to be overridden, please open an issue.

Note: When providing custom overrides, the default ones will not be used. Be sure to include the default overrides in your custom overrides if you don't want to lose them. You can obtain them with getDefaultOverrides().

Another Use for Overrides

Currently due to limitations in TypeScript, it is impossible to write a utility type that will transform any given type to an immutable version of it in all cases. (See this issue)

One popular implementation of such a utility type is type-fest's ReadonlyDeep type. If you want this library to treat types wrapped in ReadonlyDeep as immutable regardless, you can provide an override stating as such.

[{ pattern: /^ReadonlyDeep<.+>$/u, to: Immutability.Immutable }]

Note here the inclusion of ReadonlyObjectDeep, this comes from the internal workings of ReadonlyDeep.

Limitations (when it comes to overrides)

Primitives

Currently we cannot override primitives or aliases of primitives.

For example, if we have the following code:

type A = string;
type B = A;

We cannot override A, B or string here.

The reason we cannot override the aliases is because, internally, TypeScript discards both A and B and just uses string in their place.

Caching

By default we use a global cache to speed up the calculation of multiple types' immutability. This prevents us from needing to calculate the immutability of the same types over and over again.

However, this cache assumes you are always using the same type checker. If you need to use multiple (such as in a testing environment), this can lead to issues. To prevent this, you can provide a custom cache (by passing a WeakMap) to be used or have a temporary cache be used (by passing false).

Making ReadonlyDeep types Immutable

Many types that you may expect to be immutable (including those defined internally by TypeScript itself) are not written with immutable methods and thus are not reported as immutable by this library. Luckily it is quite easy to make such type immutable. Just simply wrap them in Readonly.

Example

These types are ReadonlyDeep:

type Foo = ReadonlySet<string>;
type Bar = ReadonlyMap<string, number>;

While these types are Immutable:

type Foo = Readonly<ReadonlySet<string>>;
type Bar = Readonly<ReadonlyMap<string, number>>;

However it should be noted that this does not work for arrays. TypeScript will treat Readonly<Array<T>> exactly the same as ReadonlyArray<T> and as a consequence Readonly<ReadonlyArray<T>> is also treated the same.

In order to get around this, we need to slightly tweak the Readonly definition like so:

type ImmutableShallow<T extends {}> = {
  readonly [P in keyof T & {}]: T[P];
};

Now the follow will correctly be marked as Immutable.

type Foo = ImmutableShallow<readonly string[]>;
type Bar = ImmutableShallow<ReadonlyArray<string>>;

Note: ImmutableShallow<string[]> will also be marked as immutable but the type will still have methods such as push and pop. Be sure to pass a readonly array to ImmutableShallow to prevent this.

Changelog

1.2.5 (2023-03-11)

Bug Fixes

• support for recursive types in the form type Foo = ReadonlyArray<Foo> (#88) (a2582a7)