Functions should have a single level of abstraction

I discovered this tip in Uncle Bob's book Clean Code.

It is my favourite tip because it is:

• simple to reason about
• easy to apply to your work

Example

(Not a real world example - I have changed things for brevity)

/**
 * Multiple levels of abstraction; we have:
 * (a) function calls
 * (b) property assignments
 * (c) a conditional statement
 */
async function createUser(userDto: UserDto) {
  const user = new User();
  user.name = userDto.name;
  user.email = userDto.email;
  user.createdAt = Date.now();

  if (userDto.country === "UK") {
    logger.logActivity(`User created in the UK: ${user.id}`);
  } else {
    logger.logActivity(`User created somewhere in the world: ${user.id}`);
  }

  db.add(user);
  await db.save();
}

/**
 * Single level of abstraction: we only have function calls
 */
async function createUser(userDto: UserDto) {
  const user = User.createFromDto(userDto);
  logUserCreated(user);
  createUserInDb(user);
}

This results in code that is much easier to read and maintain.