Loops

while

A while loop repeatedly executes a code block as long as a specified condition is true.

var i = 0;
while (i < 5) {
  console.log("i is " + i);
   i++;
}

// Will print out:
// >i is 0
// >i is 1
// >i is 2
// >i is 3
// >i is 4

The parts of a while loop

while (CONDITION) {
  //CODE
}

for

A for loop is a fancy while loop.

for (var i = 0; i < 5; i++) {
  console.log("i is " + i);
}

// Will _also_ print out:
// >i is 0
// >i is 1
// >i is 2
// >i is 3
// >i is 4

The parts of a for loop

for (VARIABLE DECLARATION; CONDITION; UPDATE) {
  //CODE
}

In other words, you declare a variable and test to see if that variable passes a condition in order to run the code block. The update statement runs after the code block is executed.

Very commonly, you will use it to loop through an array.

var foods = ["pizza", "tacos", "ice cream"];
for (var i = 0; i < foods.length; i++) {
  console.log("i like " + foods[i]);
}

// Will print out:
// >i like pizza
// >i like tacos
// >i like ice cream

for...in

A for...in loop is similar to a for loop, but good for looping through all the key-value pairs in an Object.

var car = {
  wheels: 4,
  doors: 2,
  seats: 5
};
for (var thing in car) {
  console.log("my car has " + car[thing] + " " + thing);
}

// Will print out:
// > my car has 4 wheels
// > my car has 2 doors
// > my car has 5 seats

Exercises

1. Implement Fizz Buzz. Loop from 1 to 100. If the number is divible by both 3 and 5, print "fizzbuzz". Otherwise, if the number if divisible by 3, print "fizz", or, if the number is divisible by 5, print "buzz". If none of the above are true, print the number. This is a very common interview question!

2. Use a for...in loop to examine the phoneBook Object below and print out the names of all the people who share the phone number "333-333-3333".

var phoneBook = {
  "Abe": "111-111-1111",
  "Bob": "222-222-2222",
  "Cam": "333-333-3333",
  "Dan": "444-444-4444",
  "Ern": "555-555-5555",
  "Fry": "111-111-1111",
  "Gil": "222-222-2222",
  "Hal": "333-333-3333",
  "Ike": "444-444-4444",
  "Jim": "555-555-5555",
  "Kip": "111-111-1111",
  "Liv": "222-222-2222",
  "Mia": "333-333-3333",
  "Nik": "444-444-4444",
  "Oli": "555-555-5555",
  "Pam": "111-111-1111",
  "Qiq": "222-222-2222",
  "Rob": "333-333-3333",
  "Stu": "444-444-4444",
  "Tad": "555-555-5555",
  "Uwe": "111-111-1111",
  "Val": "222-222-2222",
  "Wil": "333-333-3333",
  "Xiu": "444-444-4444",
  "Yam": "555-555-5555",
  "Zed": "111-111-1111"
};