JavaScript

TS2366: Not All Code Paths Return: What It Means and How to Fix It

Error reference

TS2366: Not All Code Paths Return

Language

JavaScript

Severity

Medium

When it happens

TypeScript error TS2366 is reported when a function is declared to return a specific type but has at least one code path that reaches the end of the function without a return statement.

Potential fixes

Add a final return statement that covers all unhandled cases. Consider throwing an error for truly unreachable paths to make the intent explicit. Use a switch with an exhaustive check or a lookup table to eliminate branching.

Deep dive

TypeScript error TS2366 is reported when a function is declared to return a specific type but has at least one code path that reaches the end of the function without a return statement. The function would implicitly return undefined on that path.

Why it happens

•A conditional chain does not cover all possible inputs with a return statement.
•A switch statement is missing a default case that returns a value.
•An if/else chain handles some cases but not all.
•Early returns in try/catch blocks leave the end of the function reachable.

Potential fixes

Add a final return statement that covers all unhandled cases. Consider throwing an error for truly unreachable paths to make the intent explicit. Use a switch with an exhaustive check or a lookup table to eliminate branching.

Examples

JavaScript

Code that triggers the error

function getLabel(code: number): string {
  if (code === 200) return "OK";
  if (code === 404) return "Not Found";
  // missing: no return for other codes
}

Error output

error TS2366: Function lacks ending return statement and return type does not include 'undefined'.

Fixed code

function getLabel(code: number): string {
  if (code === 200) return "OK";
  if (code === 404) return "Not Found";
  return "Unknown";
}

Practice in English

How would you explain a TS2366: Not All Code Paths Return to a fellow dev? Choose the right phrase:

"I hit an error...

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