Write code for humans!

Not for machines

Humans prefer this

Functional programming

          
            const kmOfItalianDrivers = users
              .filter(u => u.country === 'ITALY' && 
                           u.hasDriverLicence &&
                           u.age >= 18
              )
              .flatMap(u => u.cars)
              .reduce((totalKm, car) => totalKm + car.km, 0);

Machines prefer this

Imperative code

            
              let kmOfItalianDrivers = 0;

              for (let i = 0; i < users.length; i++) {
                if (users[i].country === 'ITALY' &&
                    users[i].hasDriverLicence &&
                    users[i].age >= 18) {
                      for (let j = 0; j < users[i].cars.length; j++) {
                        kmOfItalianDrivers += users[i].cars[j].km;
                      }
                }
              }
            
              return kmOfItalianDrivers;

Why humans prefer
functional programming

It focuses on what you want to achieve
Fewer lines of code
It encourages code reusability (composing map, flatMap, filter, reduce etc.)
It hides implementation details

Why machines prefer
imperative programming

It focuses on how you want to achieve it
You write what the machine should do
Better performance (not always)

When you write code
remember:

Someone will review it
Someone should be able to understand it
Someone will take inspiration from that
Someone will build on top of it

Hardware is cheap,
developers are expensive

It's better to save hours spent by developers instead of milliseconds spent by machines
You should help your team building things quickly