Loops are programming tools that repeat a block of code until a condition is met. We learned how to iterate through arrays using their numerical indexing, but the key-value pairs in objects aren’t ordered! JavaScript has given us alternative solution for iterating through objects with the for...in syntax .

for...in will execute a given block of code for each property in an object.

let spaceship = {
  crew: {
    captain: { 
      name: 'Lily', 
      degree: 'Computer Engineering', 
      cheerTeam() { console.log('You got this!') } 
    },
    'chief officer': { 
      name: 'Dan', 
      degree: 'Aerospace Engineering', 
      agree() { console.log('I agree, captain!') } 
    },
    medic: { 
      name: 'Clementine', 
      degree: 'Physics', 
      announce() { console.log(`Jets on!`) } },
    translator: {
      name: 'Shauna', 
      degree: 'Conservation Science', 
      powerFuel() { console.log('The tank is full!') } 
    }
  }
}; 

// for...in
for (let crewMember in spaceship.crew) {
  console.log(`${crewMember}: ${spaceship.crew[crewMember].name}`);
}

Our for...in will iterate through each element of the spaceship.crew object. In each iteration, the variable crewMember is set to one of spaceship.crew‘s keys, enabling us to log a list of crew members’ role and name.