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Requires that private members are marked as readonly if they're never modified outside of the constructor (prefer-readonly)

This rule enforces that private members are marked as readonly if they're never modified outside of the constructor.

Rule Details

Member variables with the privacy private are never permitted to be modified outside of their declaring class. If that class never modifies their value, they may safely be marked as readonly.

Examples of incorrect code for this rule:

class Container {
  // These member variables could be marked as readonly
  private neverModifiedMember = true;
  private onlyModifiedInConstructor: number;

  public constructor(
    onlyModifiedInConstructor: number,
    // Private parameter properties can also be marked as readonly
    private neverModifiedParameter: string,
  ) {
    this.onlyModifiedInConstructor = onlyModifiedInConstructor;
  }
}

Examples of correct code for this rule:

class Container {
  // Public members might be modified externally
  public publicMember: boolean;

  // Protected members might be modified by child classes
  protected protectedMember: number;

  // This is modified later on by the class
  private modifiedLater = 'unchanged';

  public mutate() {
    this.modifiedLater = 'mutated';
  }
}

Options

This rule, in its default state, does not require any argument.

onlyInlineLambdas

You may pass "onlyInlineLambdas": true as a rule option within an object to restrict checking only to members immediately assigned a lambda value.

{
  "@typescript-eslint/prefer-readonly": ["error", { "onlyInlineLambdas": true }]
}

Example of correct code for the { "onlyInlineLambdas": true } options:

class Container {
  private neverModifiedPrivate = 'unchanged';
}

Example of incorrect code for the { "onlyInlineLambdas": true } options:

class Container {
  private onClick = () => {
    /* ... */
  };
}

Attributes

  • ✅ Recommended
  • 🔧 Fixable
  • 💭 Requires type information