I prefer early return pattern.

Which would look like this:

    const user = {
        firstName: "Seán",
        lastName: "Barry",
        email: "my.address@email.com",
        number: "00447123456789",
    };

    if (!user) {
        throw new Error("User must be defined.");
    }

    if (!user.firstName) {
        throw new Error("User's first name must be defined");
    }

    if (typeof user.firstName !== "string") {
        throw new Error("User's first name must be a string");
    }

    return user;