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typescript/define-function-in-one-statement

Require defining function properties in a single statement.

Targeted pattern scope​

This rule matches assignment expressions where:

  • the left-hand side is a member expression whose object resolves to one locally declared function, function expression, or arrow function, and
  • the assignment operator is =.

What this rule reports​

This rule reports property definitions attached to callable values, such as function name() {}; name.handler = value. Properties on ordinary objects and logical assignments are not reported.

Why this rule exists​

The intended style is to define callable values and attached properties together in one expression (commonly via Object.assign).

❌ Incorrect​

function load() {}
load.cache = new Map();

✅ Correct​

const f = Object.assign(() => {}, { x: 1 });
const handlers = {};
handlers.load = () => loadData();

Deprecated​

  • Lifecycle: Deprecated and frozen.
  • Deprecated since: v3.0.0
  • Available until: v4.0.0
  • Replacement: None.
  • Alternative API: Object.assign.

Behavior and migration notes​

This rule is deprecated without replacement because its style policy is too narrow and error-prone. It reports only and does not provide an autofix.

Options​

This rule has no options.

Additional examples​

const factory = Object.assign(() => 1, { cache: new Map() });
// ✅ valid

const obj = {};
obj.handler = () => 1;
// ✅ valid: obj is not callable

ESLint flat config example​

import etcMisc from "eslint-plugin-etc-misc";

export default [
 {
  plugins: { "etc-misc": etcMisc },
  rules: {
   "etc-misc/typescript/define-function-in-one-statement": "error",
  },
 },
];

When not to use it​

Disable this rule if function property assignment across statements is accepted in your codebase.

Package documentation​

Rule catalog ID: R110

Further reading​

Adoption resources​

  • Start at warning level in CI, then move to error after cleanup.
  • Use focused codemods/autofix batches per package or directory.