Javascript chellenges, what is output of this code?

Question

I'm new with Javascript and I love Sololearn chellenges, but i have trouble with some questions in quiz.
I'm sorry if this is not where I need to post this stuff but I need to find out few things.

1st question: What is the output of this code?
var a=3, b=7;
if((a++ > 5) && (b > 4))
document.write(b);
else
document.write(a);

The answer is 4.
Well, I see that correct is ''else'' but what I don't know why is 4 instead of 3, in if part saying a++ but how is that affect if correct is else statement? I thought the correct answer would be 3.

2nd question: What is output of this code?
var a = [1,2,3,4,5];
var c = [];
c[0] = new Array(a);
a[c.length] = 3;
alert(c[0]);

Correct answer in this one says 1,3,3,4,5 and i wondered why, I need explanation with this.

This is maybe funny questions to those who has more experience so please don't laugh. :)

Answer 1

1st question: The answer is 4 because after comparing 'a'(which is currently 3) with 5, it is incremented by 1 using the '++' operator.

2nd question: var a is an array with [1,2,3,4,5].
Then var c is an empty array.
Then c[0] refers to the first index of array 'c' which is assigned with the value of array 'a'. Which makes 'c' a multidimensional array ---
c = [[1,2,3,4,5],undefined,undefined,...].
Then c.length refers to the length of array c which is 1.
Then 'a[c.lenght] =3' is equivalent to 'a[1] = 3'. That replaces 2 with 3 in array 'a'.
Thats why alert(c[0]) outputs 1,3,3,4,5.

I hope you understand. Feel free to ask any concepts you don't understand.

Answer 2

The answer should be 7.
First, x is assigned with the value of 7.
Then y is assigned with the value of x which is currently 7. Then x is incremented by 1 using the expression '++'.
(x++ and ++x are evaluated differently).
Then there is z which is equal to 'y modulo x'.(Ignore the ++ )
'%' is the modulo operator which basically gets the remainder of y/x. Which in this case is :
y/x == 7/8 then y%x ==7.
alert(z) == 7.

I think the '++' operator confuses you.
Basically it adds 1 to the variable. But it's execution depends on where it is placed(before or after the variable).

In the quiz, it was 'y=x++' which assigns x to y before adding 1. That's why 'y' is 7 and x only becomes 8 after.
If it was 'y=++x', 1 is added to x first then it is assigned to y.

I hope you get my long explanation.
PS: I'm not an expert. I'm still fairly new to JavaScript.😃

Answer 3

Thank you Jonathan for beautiful answer, I really love the way you explained. I hope you still getting notification on this one because I have more question I've gotten in quiz and I don't know how the correct answer is actually correct.

var x = 7;
var y = x++;
var z = y++ % x;
alert(z);

It seems like a nice simple code, I also put the code in console and try to figured how this concept works, but for now it doesn't make any sense to me.
I want you to know I really trying to understand every question before I post, but after hour of thinking I obviously need some expert opinion.

Answer 4

Again, very nice and detailed explanation. Now it make all sense, now I get it.
Big respect and big thank you, this really means a lot to me. :)