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JavaScript JavaScript Loops, Arrays and Objects Simplify Repetitive Tasks with Loops Review For Loops and Loop Exiting

Wally Bryan
Wally Bryan
4,350 Points
Posted by Wally Bryan
Wally Bryan
4,350 Points

Is this a flaw that has been overlooked in JS? or am I just dumb hahah

here is the code, very basic, 

for (var j = 0; j <= 100; j += 10) {
  console.log( j );
}

prints 0,10,20,30,40,50,60,70,80,90,100 :) (very simple) -

But isn't this wrong?

The program runs - create J, J is 0, Is J less than or equal to 100? yes (currently); so add 10 to J, J is now the value of 10.

But the program prints 0 (Zero) first? At the point that the program is told to print anything, the value of J is now 10, not 0

P.s.

I could have missed something, so be gentle with my limited knowledge :P but this seems flawed if i have not missed something.

Jennifer Nordell
seal-mask
.a{fill-rule:evenodd;}techdegree
Jennifer Nordell
Treehouse Teacher
Jennifer Nordell
.a{fill-rule:evenodd;}techdegree
Jennifer Nordell
Treehouse Teacher

Hi, Wally! I added some markdown to your code so that it's a little more readable. If you're interested in learning how to do this, take a look at the Markdown Cheatsheet link at the :arrow_down: bottom of the "Add an Answer" section! :sparkles:

1 Answer

Jason Anders
MOD
Jason Anders
Treehouse Moderator 145,860 Points
Jason Anders
Jason Anders
Treehouse Moderator 145,860 Points

Hey Wally.

Well, you're not dumb, and this is actually a very good question. The reason it prints out 0 first is because of the way the for loop works... it's not as linear as you'd think.

The "For Loop"

  1. The first thing that happens is the variable is created and the value assigned. Here it's called j and the initial value of Zero is assigned.
  2. The loop now checks to see if the condition is met (a true or false response). Here, it's checking to see if j is less than or equal to 100. Right now it's zero, so the condition receives True.
  3. Because the loop received a True response... now here where many get tripped up... the loop executes the code block in the curly braces. Here, it's the console.log() method, and because the value is still zero (the increment hasn't run yet), the console logs out 0.
  4. Once the code block has been executed, the final part of the for loop is run... here incrementing the value of j by 10, and the loop starts over again.
  5. This time the variable isn't created, this only happens the first time, instead the value is changed to the new value of the previous iteration's increment.

I hope this makes sense and helps! :)

Keep Coding! :dizzy: