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no-method-signature

Disallow interface method signatures.

Targeted pattern scope​

This rule targets TSMethodSignature nodes in TypeScript interfaces.

What this rule reports​

  • Method signatures inside interfaces, such as run(): void

Why this rule exists​

Readonly function-valued properties are more explicit and align with immutable interface design.

❌ Incorrect​

interface Service {
  run(input: string): number;
}

✅ Correct​

interface Service {
  readonly run: (input: string) => number;
}

Additional examples​

// ❌ Optional method signature
interface Formatter {
  format?(value: string): string;
}

// ✅ Optional readonly function property
interface Formatter {
  readonly format?: (value: string) => string;
}

ESLint flat config example​

import immutable from "eslint-plugin-immutable-2";

export default [
  {
    files: ["**/*.{ts,tsx,mts,cts}"],
    plugins: { immutable },
    rules: {
      "immutable/no-method-signature": "error",
    },
  },
];

When not to use it​

If your TypeScript style guide intentionally uses method signatures for readability (especially in public library interfaces with overload-heavy APIs), this rule may conflict with established conventions. In that case, keep a consistent method-signature style instead of mixing both forms.

Rule catalog ID: R907

Further reading​