How did we come to this answer

Question

var x = 0;
for (; x <= 20; x += 2) {
document.write(x);
}

Why is this 2? (; x += 2)

Answer 1

var x = 0;

for ( ; x <= 20; x += 2 )
{
document.write(x);
}

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Q. Why is this 2 ( x += 2 )

A. 2 is the increment, there is no needed of explanation, 2 is simply the increment of x variable. The loop will prints the 2's multiple, it will ends to 20!

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Please use an appropriate syntax, your code is too confusing.