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typescript/no-complex-return-type

Disallow complex inferred return types for arrow functions.

Targeted pattern scope​

This rule targets arrow functions with no explicit return type annotation when their returned expression is a type assertion around a complex expression.

Specifically, it matches asserted return expressions whose asserted value is a function expression, arrow function expression, object expression, or class expression.

What this rule reports​

This rule reports arrow functions with complex asserted return expressions when no explicit return type is declared.

Why this rule exists​

Complex asserted return expressions can hide the intended API shape.

An explicit return annotation makes the contract visible at the function boundary.

❌ Incorrect​

const create = () => (() => 1) as () => number;

✅ Correct​

const create = (): (() => number) => () => 1;

Behavior and migration notes​

This rule reports only and does not provide an autofix.

Migration is typically adding an explicit return type annotation to the arrow function.

Options​

This rule has no options.

Additional examples​

const create = () => ({ run() {} }) as { run(): void };
// ❌ reported: complex asserted return expression and no return annotation

const createTyped = (): { run(): void } => ({ run() {} });
// ✅ valid

ESLint flat config example​

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

export default [
 {
  plugins: { "etc-misc": etcMisc },
  rules: {
   "etc-misc/typescript/no-complex-return-type": "error",
  },
 },
];

When not to use it​

Disable this rule if your team allows complex inferred return types without explicit annotations.

Package documentation​

Rule catalog ID: R087

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.