Original subtitles

Alright now let's

take a look at the third kind of loop we have in JavaScript.

That is a do-while loop. Do-while loops

are very similar to y loops, but they're slightly different. It's

easier to show you in code. So I'm going to rewrite this logic

using a do-while loop. Just like the y loops, we have to declare

our loop variable externally, so we declare i and

initialize it to 0. Okay, now if we save the changes,

we're going to get an error identifier has already been declared.

Because I have declared i here, so I can not redeclare

it here. This is different than the i that we have in the for loop,

again, this is all about scoping, and I'm going to talk about that later in the course.

So, to make this work I'm going to temporarily

comment out these few lines, and rewrite this

logic using a do while loop. So we add the do statement

do statement here, then a code block, in this block we should

have our statements. So I'm going to borrow them

from our while loop, copy these, paste them here,

and then finally at the end

of this block we add the while statement along with

our condition. That is i less than or equal to 5,

followed by a semi colon. Now you might be wondering

what is the difference between a while loop, and a do-while loop,

do-while loops are always executed at least once.

Even if this condition evaluates to false. Let me show you

what I mean. So I'm going to temporarily comment out these few

lines. And bring back our while loop.

Also, we don't need this for block for now, so let's comment it out,

now if you save the changes, we get 1, 3, 5 on

the console. However, if I change i

to 9, we are not going to see anything, because the first time

we try to execute this while loop, this condition evaluates

to false. So these statements are never executed.

So save the changes, look, there's nothing in the console. So in

while loops, this condition is evaluated ahead of time.

At the beginning of every iteration. In contrast

in do-while loops,

this condition is evaluated at the end. And

that means these statements are always executed

at least once. Even if the condition is false.

So let's try this, I'm going to comment out this while loop,

and change i to 9, just like before.

Save the changes, we get 9 on the console,

why? Because in our do-while loop here,

on line 15, we check to see if this is an odd-number it is,

and display it on the console. Next, we increment i by 1, so i,

is 10. Then, the condition is evaluated, of course it's

false. So our loop will terminate. Now, realistically

we're not going to use this do-while a lot in programming,

there are situations you may want to use this, but in practical terms,

most of the time you will be using a for or while loop. Just

be aware of the difference between a while loop and a do while loop.