islam3zzat

// ...

function updateUserName(user, { firstName, lastName }) {
  const { name } = user;
  const newName = {
    firstName: firstName || name.firstName,
    lastName: lastName || name.lastName,
  };
  return { ...user, name: newName };
}

islam3zzat

// ...

function updateUserName(user, { firstName, lastName }) {
  const { name } = user;
  if (firstName) name.firstName = firstName;
  if (lastName) name.lastName = lastName;
  return { ...user, name };
}

function saveUser(users, updatedUser) {

islam3zzat

function userByMatcher({ key, value }) {
  return user => user[key] === value;
}

function updateUserName(user, { firstName, lastName }) {
  // <~ #1 mutating argument, bad idea
  if (firstName) user.name.firstName = firstName;
  if (lastName) user.name.lastName = lastName;
  return user;
}

islam3zzat

// Tail call optimization

// this function call itself n times
function f(x) {
  // with every new call, new stack frame is created
  // there is a limit, this limit varies from engine to another, but there is a limit
  // this simple function breaks on chrome at f(100000)
  if (x < 1) {
    return 0
  }