Why is this an issue?

readonly fields can only be assigned in a class constructor. If a class has a field that’s not marked readonly but is only set in the constructor, it could cause confusion about the field’s intended use. To avoid confusion, such fields should be marked readonly to make their intended use explicit, and to prevent future maintainers from inadvertently changing their use.

How to fix it

Mark the given field with the readonly modifier.

Code examples

For JavaScript and TypeScript, {rule:javascript:S2933} reports one issue per class and uses secondary locations for each field that should be marked readonly.

Noncompliant code example

class Person { // Noncompliant: Mark this member as `readonly`.
  private birthYear: number;

  constructor(birthYear: number) {
    this.birthYear = birthYear;
  }
}

Compliant solution

class Person {
  private readonly birthYear: number;

  constructor(birthYear: number) {
    this.birthYear = birthYear;
  }
}

When more than one field in the same class should be marked readonly, the issue remains on the class and each affected field is listed as a secondary location.

class Person { // Noncompliant: Mark these members as `readonly`.
  private birthYear: number;
  private age: number;

  constructor(birthYear: number, age: number) {
    this.birthYear = birthYear;
    this.age = age;
  }
}

class Person {
  private readonly birthYear: number;
  private readonly age: number;

  constructor(birthYear: number, age: number) {
    this.birthYear = birthYear;
    this.age = age;
  }
}

Resources

Documentation