Return Early, Return Often

The cognitive load introduced by having a single return statement in a function (normally accompanied by a result variable) is often underappreciated, but luckily with a little refactoring it's fairly easy to resolve.

I was working in a project recently when I came across some code similar to this (domain has been changed for privacy, but the logic is the same):

const isDuplicateId = (id: string) => {
    let duplicate = false;

    const employee = getEmployee(id);

    if (employee !== null) {
        duplicate = true;
    } else {
        const manager = getManager(id);

        if (manager !== null) {
            duplicate = true;
        } else {
            const admin = getAdmin(id);

            if (admin !== null) {
                duplicate = true;
            }
        }
    }

    return duplicate;
};

It's not particularly difficult to read, but the single return statement muddies the waters slightly.

In the first if block, it isn't clear that true is going to be the return value just from that statement alone, because there isn't a return at that point. The reader also need to consider the else statement, and check further down the method to make sure that duplicate isn't modified anywhere else. The same applies for the two other if statements.

This means that the task of mentally tracing the code and figuring out how it behaves is made a lot more difficult - if you just want to see how it behaves when employee isn't null, the reader needs to read the whole method just to check that it isn't doing anything.

With some return statements, this code can be made a lot more manageable.

const isDuplicateId = (id: string) => {
    const employee = getEmployee(id);

    if (employee !== null) {
        return true;
    }

    const manager = getManager(id);

    if (manager !== null) {
        return true;
    }

    const admin = getAdmin(id);

    if (admin !== null) {
        return true;
    }

    return false;
};

Now the code requires much less reading in order to understand specific behaviours. If you want to see how the code behaves when employee isn't null, you only need to read the first 3 lines of code (excluding whitespace).

Return Early, Return Often

Aug 06, 2019