TS2366: Function lacks ending return statement - in CI

By Kaveh Alemi·Latchkey

A function declares a return type but a code path reaches the end without returning a value.

What this error means

Type-checking fails with TS2366 because not all branches return, yet the declared return type excludes undefined.

src/grade.ts(3,32): error TS2366: Function lacks ending return statement and return type does not include 'undefined'.

Common causes

How to fix it

Return on every path

1. Add a final return or a default/exhaustive branch

function grade(n: number): string {
  if (n >= 90) return "A"
  return "F"
}

Widen the return type if undefined is valid

1. Declare the return type as T | undefined when some paths legitimately return nothing

How to prevent it

• Keep noImplicitReturns on and make branching functions exhaustive or explicitly nullable.

Frequently asked questions

Related guides

References

• Node.js documentation
• npm CLI documentation
• GitHub Actions documentation