Any code paths that do not have explicit returns will return undefined. It is recommended to replace any implicit dead-ends that return undefined with a return null statement.

As a convention, undefined signals that an unexpected value has been produced as the result of a logical error in the program. A null return, on the other hand, signals that the input to a function was incorrect, or a value was 'not found'.

Having this distinction in code helps you figure out if something was caused because of a logical error, or malformed input to a function call.

Bad Practice

function getUser(name) {
    if (userExists(name)) {
        return userDb.get(name);
    }
    // whoops! Implicit undefined return
}

function readFile(fileName) {
    if (fs.existsSync(fileName)) {
        return fs.readFileSync(fileName, 'utf-8');
    }

    return; // Implicit undefined return.
}

Recommended

function getUser(name) {
    if (userExists(name)) {
        return userDb.get(name);
    }

    // `null` indicates there is no such user.
    return null;
}

function readFile(fileName) {
    if (fs.existsSync(fileName)) {
        return fs.readFileSync(fileName, 'utf-8');
    }

    // `null` indicates there is no such file.
    return null;
}

References

Stack Overflow: When is null or undefined used in JavaScript?

CIDARLAB / 3DuF

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