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The do...while Loop

3:18 with Guil Hernandez

Use `do...while` to create a loop that executes code until the test condition evaluates to false.

Resources

do...while - MDN

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There's another type of loop in JavaScript that's closely related to the while loop. 0:00

It's called the do-while loop. 0:04

Now open the file do-while.js and copy the getRandomNumber 0:06

function from while.js and paste it inside this file. 0:12

In index.html, update the script tag source attribute 0:17

to js/do-while.js. The structure of a do-while loop looks like this. 0:22

The do statement comes first and is followed by a while condition. 0:29

Do-while repeats an action over and over again, while a condition is true. 0:35

In other words, the code you place inside the do code 0:41

block here will run as long as this while condition is true. 0:45

That sounds pretty much like a while loop, doesn't it? 0:49

It pretty much is. 0:51

But there's one important difference. 0:52

Do-while will always execute the code block 0:55

once before the condition is checked. 0:58

That's because the condition isn't tested until after the code block. 1:02

Let's compare the two. 1:05

Earlier, you used a while loop to log 10 random numbers to the console. 1:07

Here's the code you wrote. 1:12

Notice that with a while loop, 1:14

a condition gets checked at the very beginning, before this code block runs. 1:16

If the condition is true, then the loop runs. 1:21

If the condition is false, the loop does not run. 1:25

The condition might be false right at the beginning meaning the loop will never run. 1:27

Here's how you'd write this using do-while I'll again initialize 1:32

a counter variable to 0, to keep track of the number of times the loop runs. 1:38

Inside the do block, 1:44

I'll add the console.log statement displaying the random number. 1:46

Then increment the counter variable by 1. 1:51

With counter += 1, after each pass through the loop, 1:55

the while condition will be evaluated. 2:00

So between the parentheses I'll add counter is less than 10. 2:03

Remember, do while does not check the condition until 2:09

the code block has run once. 2:14

If this condition is true, it runs again and 2:16

again until the condition is no longer true. 2:18

In this case when counter is no longer less than 10 I'll save my file. 2:21

And when I preview index.html in the browser, 2:26

10 random numbers appear in the console. 2:29

Good. 2:31

So when might you use do-while over while? 2:34

Keep in mind that in many cases you can use do-while for 2:38

the exact purposes as while. 2:42

Use do-while when you need your loop to execute at least one time. 2:44

For examples, log at least one random number to the console, then check if 2:49

counter is less than 10 before running the loop again and logging more values. 2:54

Use while when you need to check a condition before performing an action. 2:59

For example check if counter is less than 10, before running the loop and 3:04

logging a value to the console. 3:09

Either way, you'll get the same output. 3:11

So the approach you use is mostly a matter of preference. 3:12

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