Loops

For loop

Syntax

for (initialExpression; condition; incremetExpression) {
 statement;
}

Standard for loop

const fruitBasket = ['banana', 'pear', 'guava'];
for (let i = 0; i < fruitBasket.length; i++) {
  console.log('There\'s a ' + fruitBasket[i] + ' in the basket.');
}

Negative increment expression

if needed … has a better performance

for (let i = fruitBasket.length - 1; i >= 0; i--) {
  console.log('There\'s a ' + fruitBasket[i] + ' in the basket.');
}

For-of loop

preferable to the standard for loop

for (let fruit of fruitBasket) {
  console.log('There\'s a ' + fruit + ' in the basket.');
}

Note

Can be used with any iterable object.
Iterable objects contain the Symbol.iterator property.
Watch out for the Symbol.iterator key in this console.log:

const emptyArray = [];
console.log(emptyArray);

ForEach loop

The forEach loop takes in a callback (here with three arguments):

• currentValue: refers to the current item in the array
• index: the position of the item within the array (zero-based index)
• array: refers to the array you are looping over (can be omitted)

Syntax

array.forEach((currentValue, index, array) => {
  // your statement here
});

// you can omit index and array most of the time:
array.forEach(currentValue => {
  // your statement here
})

ForEach with an arrow function

array.forEach(currentValue => console.log(currentValue));

All the loops with an example

// for loop
for (let i = 0; i < fruitBasket.length; i++) {
  console.log(fruitBasket);
};

// for...of loop
for (let fruit of fruitBasket) {
  console.log(fruit);
};

// forEach loop
fruitBasket.forEach(function (fruit) {
  console.log(fruit);
});
// forEach with an arrow function:
fruitBasket.forEach(fruit => console.log(fruit));

Excercises

const people = [
  { firstName: 'Benjamin', lastName: 'Franklin', yearOfDeath: '1790'},
  { firstName: 'Thomas', lastName: 'Edison', yearOfDeath: '1931'},
  { firstName: 'Franklin', lastName: 'Roosevelt', yearOfDeath: '1945'},
  { firstName: 'Napoleon', lastName: 'Bonaparte', yearOfDeath: '1821'},
  { firstName: 'Abraham', lastName: 'Lincoln', yearOfDeath: '1865'},
  { firstName: 'Mother', lastName: 'Theresa', yearOfDeath: '1997'},
  { firstName: 'Mahatma', lastName: 'Gandhi', yearOfDeath: '1948'},
  { firstName: 'Winston', lastName: 'Churchill', yearOfDeath: '1965'},
  { firstName: 'Charles', lastName: 'Darwin', yearOfDeath: '1882'},
  { firstName: 'Albert', lastName: 'Einstein', yearOfDeath: '1995'},
  { firstName: 'Pablo', lastName: 'Picasso', yearOfDeath: '1973'},
  { firstName: 'Ludwig', lastName: 'van Beethoven', yearOfDeath: '1827'},
  { firstName: 'Walt', lastName: 'Disney', yearOfDeath: '1966'},
  { firstName: 'Henry', lastName: 'Ford', yearOfDeath: '1947'},
  { firstName: 'Steve', lastName: 'JObs', yearOfDeath: '2012'},
]

// Test for exercises
const theFirstName = people[0].firstName;
console.log(theFirstName);

// Excercise 1 (all variants)
// for (let i = 0; i < people.length; i++) {
//   console.log(people[i].firstName);
// }

// for (let i = people.length - 1; i >= 0; i--) {
//   console.log(people[i].firstName);
// }

// for(let person of people) {
//   console.log(person.firstName);
// }

people.forEach(person => {
console.log(person.firstName)
});

// Excercise 2
const firstNames = [];
people.forEach(person => {
  firstNames.push(person.firstName)
});
console.log(firstNames);

// Excercise 3
const diedAfter1950 = [];
people.forEach(person => {
  if (person.yearOfDeath > 1950) {
    diedAfter1950.push(person);
  }
});
console.log(diedAfter1950);

// Excercise 4
// if sth. cannot be found, index should be -1
let darwinIndex = -1;
people.forEach((person, index) => {
  if (person.firstName === 'Charles' && person.lastName === 'Darwin') {
    darwinIndex = index;
  }
});
console.log(darwinIndex);

// Alternative:
people.forEach((person, index) => {
  if (person.firstName === 'Charles' && person.lastName === 'Darwin') {
    console.log(index);
  }
});