Sam Wight

What's the void type in TypeScript?

One of the more confusing types in the TypeScript universe is the void type. The most common place it’s used is as the return type for a callback function. For example, the type of the callback you pass to the Array forEach function is this:

type CallbackFn<T> = 
	(value: T, index: number, array: readonly T[]) => void;

The forEach callback accepts three arguments: the current value (T), the current index, and the array (readonly T[]). Its return type is the void type.

Most people think that the void type means ‘returns undefined’, and that void and undefined are interchangeable. For example, TypeScript accepts both of these callback functions as valid:

const logNumbersVoid = (num: number): void => {
	console.log(num);
	return undefined;
}

const logNumbersUndefined = (num: number): undefined => {
	console.log(num);
	return undefined;
}

// TypeScript doesn't complain about either of these.
[1, 2, 3].forEach(logNumbersVoid);
[1, 2, 3].forEach(logNumbersUndefined);

These two functions (logNumbersVoid and logNumbersUndefined) are the same, except for their return type. Both accept a number as their parameter, log the number, and return undefined. The first function’s return type is void, and the second one is undefined.

TypeScript allows us to use both of these as the callback to the forEach function. So, it seems like void and undefined are doing the same thing.

However, there are some cases where void allows some things that undefined does not:

const logNumbersVoid = (num: number): void {
	console.log(num);

	// Note that we're returning `num` in both functions.
	return num;
}

const logNumbersUndefined = (num: number): undefined {
	console.log(num);
	
	// TypeScript complains about this but not the other one???
	return num;
}

Here, we change both functions to return num instead of undefined. TypeScript complains about the return statement in logNumbersUndefined, but doesn’t complain about the return from the void function.

What’s going on here? Why are we allowed to return a number from a void function but not from an undefined function? Let’s dig deeper.

Where does void come from?

Several constructs in TypeScript have similarly named constructs in JavaScript. For example, TypeScript’s typeof operator comes from the typeof operator in JavaScript, working similarly to it in type definitions. It’s useful to understand the JavaScript version of these TypeScript constructs. Knowing the behavior in JavaScript can help us predict how they’ll behave in TypeScript.

Just like typeof, TypeScript’s void type has a counterpart in JavaScript: the void keyword.

The void keyword in JavaScript is a keyword that can be put before any expression (something evaluating to a value). JavaScript will evaluate the expression and then return the value undefined for the entire expression. So in the example below, “It returned undefined!” would be logged to the console:

if (void "helloWorld" === undefined) {
	console.log("It returned undefined!");
}

void works similarly to typeof in that it evaluates the expression to the right of it. However, void throws away the result of the expression. In this way, void sort of acts like a trash can: the value of any expression you give it will not be accessible again. Hold on to that analogy for a minute.

Use of void

It might seem like the void keyword doesn’t have much of a use case now, and you’d be right in thinking that. But before ES5 JavaScript, it had an important use case: getting the value undefined.

In JavaScript, there are two versions of undefined: the value and the variable. For whatever goddamn reason, undefined is actually a global variable in JavaScript, not a reserved word. By default, this variable points to the value undefined. Tricky, I know.

Before the ES5 standard, any script could modify the contents of the undefined variable. Running something like window.undefined = "HAHA FUCK YOU" could potentially screw up a large part of most working programs (e.g., conditions like variableName === undefined would return false).

Because of this, many JavaScript developers would use the expression void 0 to obtain the undefined value. Contrary to the undefined variable, void is a reserved word that cannot be modified or changed. This meant that variableName === void 0 would always return true if variableName was undefined, even if someone re-assigned the undefined variable.

The ES5 standard changed undefined so that it was read-only, so thankfully these kinds of bugs / exploits no longer exist. The resolution of this issue removed one of the main use cases for void. As a result, most JS programmers don’t know about it.

void in TypeScript

The TypeScript Handbook describes void like this:

void represents the return value of functions which don’t return a value. It’s the inferred type any time a function doesn’t have any return statements, or doesn’t return any explicit value from those return statements.

So void is a type used when our functions don’t have any return value. If we don’t have any return statements in the function, TypeScript will automatically assume that our function returns void. This explains the typing of the Array.prototype.forEach callback: forEach isn’t concerned with the result of the callback, only the parameters it needs to pass to it.

If void is inferred by TypeScript only when nothing is returned from the function, why could we return undefined in the example at the beginning of the post? The handbook goes on:

In JavaScript, a function that doesn’t return any value will implicitly return the value undefined. However, void and undefined are not the same thing in TypeScript.

In JavaScript, any function that doesn’t return a value automatically returns undefined. As consumers of a function, we have no way to tell whether the return of undefined was explicit (return undefined;) or implicit (no return statement). Therefore, TypeScript allows us to return undefined from a void function. As consumers of that function, we can’t tell the difference between the two.

But hold on a second, why does the handbook say that void and undefined are different? Scrolling down to the bottom of the page, we read this:

Contextual typing with a return type of void does not force functions to not return something. Another way to say this is a contextual function type with a void return type (type vf = () => void), when implemented, can return any other value, but it will be ignored.

This is saying that when we have an explicit return type of void (like in both of our functions before), we can return any type from our function. In other words, any type is assignable to void when we’re returning from a function.

This is not the case with a function with a return type of undefined: TypeScript will force us to either return nothing, or explicitly return undefined. Contrary to this, void doesn’t care. We can return implicit undefined, explicit undefined, or any explicit value we want.

Remember what we said about JavaScript’s void being a trash can? TypeScript’s void is the same way, but as a type. Once you assign something to void, TypeScript won’t let you use it again. You can put stuff into it, but you can’t get stuff back out. Once you tell TypeScript that something is void, it throws out whatever type it was before and starts treating it like nothing. It’s a trash can.

So that’s how void works in TypeScript. If you have any questions or comments, feel free to comment on the post below. I also have a newsletter you can subscribe to (I only send one email a week) if you’re interested in more posts like this. Last, feel free to follow me on Twitter to keep up-to-date with the latest things I’m working on.

#typescript #code