type-declaration-immutability.md

Enforce the immutability of types based on patterns (functional/type-declaration-immutability)

💼🚫 This rule is enabled in the following configs: ☑️ lite, noMutations, ✅ recommended, 🔒 strict. This rule is disabled in the disableTypeChecked config.

🔧💡 This rule is automatically fixable by the --fix CLI option and manually fixable by editor suggestions.

💭 This rule requires type information.

Require type alias declarations and interfaces that imply some level of immutability to comply to it.

Rule Details

This rule enforces rules on type immutability based on the type's name.

For details on what the different levels of immutability mean, see the immutability section of is-immutable-type.

❌ Incorrect

/* eslint functional/type-declaration-immutability: "error" */

type ReadonlyElement = {
  id: number;
  data: string[];
};

type ReadonlyDeepElement = Readonly<{
  id: number;
  data: string[];
}>;

type MutableElement = Readonly<{
  id: number;
  data: ReadonlyArray<string>;
}>;

✅ Correct

/* eslint functional/type-declaration-immutability: "error" */

type ReadonlyElement = Readonly<{
  id: number;
  data: string[];
}>;

type ReadonlyDeepElement = Readonly<{
  id: number;
  data: ReadonlyArray<string>;
}>;

type MutableElement = {
  readonly id: number;
  data: ReadonlyArray<string>;
};

Settings

This rule can leverage shared settings to configure immutability settings.

See the immutability docs.

Options

This rule accepts an options object of the following type:

type Options = {
  rules: Array<{
    identifiers: string | string[];
    immutability: "Mutable" | "ReadonlyShallow" | "ReadonlyDeep" | "Immutable";
    comparator?: "Less" | "AtMost" | "Exactly" | "AtLeast" | "More";
    fixer?:
      | { pattern: string; replace: string }
      | Array<{ pattern: string; replace: string }>
      | false;
    suggestions?: Array<{ pattern: string; replace: string }> | false;
  }>;
  ignoreInterfaces: boolean;
  ignoreIdentifierPattern: string[] | string;
};

Default Options

const defaults = {
  rules: [
    {
      identifiers: "^(?!I?Mutable).+",
      immutability: "Immutable",
      comparator: "AtLeast",
      fixer: false,
      suggestions: false,
    },
  ],
  ignoreInterfaces: false,
};

Preset Overrides

recommended and lite

const recommendedAndLiteOptions = {
  rules: [
    {
      identifiers: "I?Immutable.+",
      immutability: "Immutable",
      comparator: "AtLeast",
    },
    {
      identifiers: "I?ReadonlyDeep.+",
      immutability: "ReadonlyDeep",
      comparator: "AtLeast",
    },
    {
      identifiers: "I?Readonly.+",
      immutability: "ReadonlyShallow",
      comparator: "AtLeast",
      fixer: [
        {
          pattern: "^(Array|Map|Set)<(.+)>$",
          replace: "Readonly$1<$2>",
        },
        {
          pattern: "^(.+)$",
          replace: "Readonly<$1>",
        },
      ],
    },
    {
      identifiers: "I?Mutable.+",
      immutability: "Mutable",
      comparator: "AtMost",
      fixer: [
        {
          pattern: "^Readonly(Array|Map|Set)<(.+)>$",
          replace: "$1<$2>",
        },
        {
          pattern: "^Readonly<(.+)>$",
          replace: "$1",
        },
      ],
    },
  ],
};

rules

An array of rules to enforce immutability by.

These rules should be sorted by precedence as each type declaration will only enforce the first matching rule to it.

identifiers

A regex pattern or an array of regex patterns that are used to match against the name of the type declarations.

immutability

The level of immutability to compare against. This value will be compared to the calculated immutability using the comparator.

comparator

The comparator to use to compare the calculated immutability to the desired immutability. This can be thought of as <, <=, ==, >= or >.

fixer

Configure the fixer for this rule to work with your setup. If not set, or set to false, the fixer will be disabled.

suggestions

Configure any suggestions for this rule to work with your setup.

ignoreInterfaces

A boolean to specify whether interfaces should be exempt from these rules. false by default.

ignoreIdentifierPattern

This option takes a RegExp string or an array of RegExp strings. It allows for the ability to ignore violations based on a type's name.