complete/require-capital-read-only

🚫 This rule is disabled in the ✅ recommended config.

📝 Requires maps/sets/arrays with a capital letter to be read-only.

💭 This rule requires type information.

Rule Details

In TypeScript:

• Variables are usually typed with camelCase names.
• Static constants are usually typed with SCREAMING_SNAKE_CASE names.

Thus, when a map, set, array, or object is typed with a capital letter, it is heavily implied that it is acting as a static constant. Since maps, sets, and arrays are writable by default, it is common to explicitly type these kinds of constants with ReadonlyMap<Foo, Bar> or ReadonlySet<Foo> or readonly Foo[] or Readonly<Foo> to enforce that they can never be modified later on, which prevents bugs. However, it can be difficult to remember to do this every single time.

Use this rule to ensure that the SCREAMING_SNAKE_CASE variables are read-only throughout your codebase to keep things as safe as possible.

Also see the require-capital-const-assertions rule.

// Bad
const MY_MAP = new Map([
  [1, 2],
  [3, 4],
  [5, 6],
]);

// Good
const MY_MAP: ReadonlyMap<number, number> = new Map([
  [1, 2],
  [3, 4],
  [5, 6],
]);

// Bad
const MY_SET = new Set([1, 2, 3]);

// Good
const MY_SET: ReadonlySet<number> = new Set([1, 2, 3]);

// Bad
const MY_ARRAY = [1, 2, 3];

// Good
const MY_ARRAY: readonly number[] = [1, 2, 3];

// Bad
const MY_OBJECT = {
  foo: 123,
  bar: 456,
};

// Good
interface Foo {
  foo: number;
  bar: number;
}
const MY_OBJECT: Readonly<Foo> = {
  foo: 123,
  bar: 456,
};

Options

This rule is not configurable.

Resources

• How to use this rule
• Rule source
• Test source