never vs void | Engineering Interview Kit

TL;DR

void: Used when a function successfully finishes executing but does not return a value (it implicitly returns undefined).
never: Used when a function never successfully finishes executing. It either crashes (throws an error) or runs in an infinite loop.

Mental Model

How It Works

`void`

This is the standard return type for side-effect functions, like console.log wrappers or DOM manipulation functions. Even though you don’t write return, JavaScript engines implicitly return undefined.

`never`

This represents a state that shouldn’t exist. It is the “bottom type” in TypeScript. Because a never function never returns, TypeScript knows that any code written after a call to a never function is unreachable.

Example

// --- VOID ---
// This function finishes successfully, but returns nothing.
function logMessage(msg: string): void {
    console.log(msg);
}

// --- NEVER ---
// This function throws an error, interrupting the flow. It never returns.
function throwError(errorMsg: string): never {
    throw new Error(errorMsg);
}

// This function loops forever. It never returns.
function keepRunning(): never {
    while (true) {
        console.log("Running...");
    }
}

Common Interview Questions

Can you assign a value to a variable of type `never`?

No. You cannot assign anything to never (not even any). However, you can assign never to any other type. This makes it incredibly useful for Exhaustive Checking in switch statements (ensuring you handled all possible union types).

If a function returns `undefined`, should its type be `void` or `undefined`?

If the function doesn’t have a return statement, it should be void. If the function explicitly has return undefined;, its type can be undefined. Generally, void is preferred for callbacks and functions meant to be executed for side effects.

TL;DR

Mental Model

How It Works

void

never