What’s the Return Type of a Function That Returns Nothing in TypeScript?

📌Intro

If a function doesn’t return anything, what’s its return type in TypeScript? Many developers guess undefinedbut the real answer is a bit more nuanced. Let’s break it down with examples.

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🚩 Case 1: Normal Functions (No Return)

When you don’t return anything from a normal function, TypeScript infers its return type as void.

function logMessage(message: string): void {
  console.log(message);
}

Even if you don’t annotate it, TypeScript will automatically infer it:

function logMessage(message: string) {
  console.log(message);
}
// inferred as: (message: string) => void

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At runtime, this function technically returns undefined, but TypeScript uses voidto say “ignore the return value.”

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🚩 Case 2: Async Functions (No Return)

For asyncfunctions, it’s just Promise<void>.

async function logMessage(message: string): Promise<void> {
  console.log(message);
}

Or with arrow functions:

const logMessage = async (message: string): Promise<void> => {
  console.log(message);
};

Even if you don’t write the annotation, TypeScript infers Promise<void>.

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🤔 Why Not undefined?

It’s tempting to write:

function logMessage(message: string): undefined {
  console.log(message);
}

But this will error:

Function lacks ending return statement and return type does not include 'undefined'

That’s because undefinedmeans the function must explicitly return undefined.

Example of correct undefinedreturn:

function doNothing(): undefined {
  return undefined; // ✅ works
}

But in most cases, what you want is void, not undefined.

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🏗 Real-Life Example

A logger utility:

function logger(message: string): void {
  console.log(`[LOG]: ${message}`);
}

async function saveAndLog(data: string): Promise<void> {
  // simulate saving
  await new Promise(res => setTimeout(res, 1000));
  logger(`Saved data: ${data}`);
}

Here:

• logger returns nothing → void
• saveAndLog is async → Promise<void>

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💡 Extras

• void= “I don’t care about the return value.”
• undefined= “This function must return undefined explicitly.”

👉 Always prefer voidunless you specifically want to enforce return undefined;.