Void and Never

Intro to Void and Never
Intro to Void and Never
Not all functions return meaningful values. TypeScript has two special types for these situations: void and never.

The void Type

void means "this function doesn't return a value." It performs a side effect like logging, updating state, or sending data.
function logMessage(message: string): void {
	console.log(message)
	// No return statement (or return;)
}
Common side effects:
  • Logging to console
  • Updating DOM elements
  • Sending analytics events
  • Writing to a database
Most functions should return a value (be "pure"). Functions with void return types are the exceptionβ€”they exist to cause side effects. When you see void, you know the function is doing something beyond computing a result.

The never Type

never means "this function never returns." It either:
  1. Throws an error (execution stops)
  2. Runs forever (infinite loop)
function throwError(message: string): never {
	throw new Error(message)
}

function infiniteLoop(): never {
	while (true) {
		// Never exits
	}
}

Why Does This Matter?

TypeScript uses never for exhaustiveness checking. If you handle all possible cases, the code should be unreachable:
type Status = 'pending' | 'success' | 'error'

function handleStatus(status: Status) {
	switch (status) {
		case 'pending':
			return 'Loading...'
		case 'success':
			return 'Done!'
		case 'error':
			return 'Failed!'
		default:
			// If we handled all cases, this is unreachable
			const _exhaustive: never = status
			return _exhaustive
	}
}
In this exercise, you'll work with both void and never.
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