While Loops & Do While Loops

In the last video, we looked at for loops, which are used to iterate over a block of code until a condition is met. A while and do while loop does the same thing using a different syntax. They work a bit different as well.

While Loop Syntax

The syntax for a while loop is very similar in many languages. The syntax is:

while ([conditionExpression]) {
  statement
}

There are a few differences between the for loop, which we have already talked about, and the while loop.

In a while loop, the variable is initialized before the loop runs and it is initialized outside of the loop.

Let's look at a simple example and print Number 1-10, like we did with the for loop.

let i = 0;

while (i <= 20) {
  console.log('Number ' + i);
  i++;
}

For many cases, you could use a while or a for loop and achieve the same result, but a rule that many programmers follow is to use a for loop when you know the number of times you want to run the loop and use a while loop when the number of times the loop will run is unknown. That is not a mandatory convention, but something that a lot of people do.

Looping Over Arrays

We can loop over arrays with while loops as well

const arr = [10, 20, 30, 40];

let i = 0;

while (i < arr.length) {
  console.log(arr[i]);
  i++;
}

Nested While Loops

We can nest while loops to create a loop that runs a loop inside of a loop.

let i = 1;

while (i <= 10) {
  console.log('Number ' + i);
  let j = 1;
  while (j <= 10) {
    console.log(i + ' * ' + j + ' = ' + i * j);
    j++;
  }
  i++;
}

Do While Loops

The do while loop is a little different from the while loop. The do while loop will always run at least once, even if the condition is false.

So the answer to the question, "when would I want to use a do while loop?" is when you always want to run the block of code at least once.

The syntax for a do while loop is:

do {
  statement
} while ([conditionExpression]);

Let's look at a simple example:

let i = 1;

do {
  console.log('Number ' + i);
  i++;
} while (i <= 20);

Now let's change the i value to 21. This means that the condition expression is never met, however, the console.log() will run at least once, even if the condition is false.

let i = 21;

do {
  console.log('Number ' + i);
  i++;
} while (i <= 20);

You can also use the break and continue statements with while and do while loops.