Need explanation... Can someone explain to me this code, step by step?

Question

I understand what modulus is and how many iterations there are. But how does this code behave. What's the first step, second one and so on...

var a=[9,5,3];
for (var i=1; i<3; i++) {
a[0]%=a[i];
a[0]+=a[2];
}

document.write (a[0]);

Answer 1

You can divide the loop structure into four components:
1. initialization
2. checking condition
3. loop body
4. iteration

Now, for the given code, initial value for x is 1. The loop will stop if x becomes 5 or more.
x += x means, x = x+x
Also, before going to the next step each time, x will be x+1 because of the x++ part.

Detailed breakdown:

Step 1:
-----------
initial value: x = 1
condition: is x less than 5? Yes! So the loop body will be executed.
loop body: x = x+x = 1+1 = 2
iteration: x = x+1 = 2+1 = 3

Step 2:
----------
initial value: x = 3
condition: is x less than 5? Yes! So the loop body will be executed.
loop body: x = x+x = 3+3 = 6
iteration: x = x+1 = 6+1 = 7

Step 3:
----------
initial value: x =7
condition: is x less than 5? No :( So, execution will stop here. Loop body won't be executed.

Output: 7

Hope it helps :)

Answer 2

a[0] %= a[i] is equivalent to a[0] = a[0] % a[i]
Same rule goes for += as well.

Breakdown:

Step 1:
i = 1
a[0] = a[0] % a[1] = 9%5 = 4
a[0] = a[0] + a[2] = 4+3 = 7

Step 2:
i=2
a[0] = a[0] % a[2] = 7% 3 = 1
a[0] = a[0] + a[2] = 1+3 = 4

Output: 4

Answer 3

@Shamima Yasmin Thank you for your effort! Now I see.

Answer 4

@Shamima Yasmin Well of course. Thanks a lot. This clears a lot. I must go through js loops lesson once more. Especially part about iterations.
I thought iteration is just for the loop to go to the next step until certain condition is met, not realising that it also adds one to the value because incrementation.

Answer 5

wow

Answer 6

@Shamima Yasmin Can you explain one more thing, like previous example by steps, but now this one?

var x=1;
for (; x<5; x++) {
x+=x;
}

output: ?

How come the answer is 7?

Answer 7

thanks