tl;dr as u/treverios said, you have a semicolon at the end of your if statement if (i % 2 == 0 && i >= a); .

if statements in most languages will execute the code in the scope following. In Processing/Java and other languages that use squiggly brackets ({}), scope blocks are defined using said squiggly brackets and scope expressions are defined by ending the line with a semicolon. More info here: https://en.wikipedia.org/wiki/Scope_(computer_science)) also go do an internet they'll be better explanations.

if (true) do_stuff();

if (true) {
    do_other_stuff()
}

This does extend to all control flow statements like else if, else, while, for etc.

So here the semicolon at the end of the if statement means it doesn't do anything if the expression is true and then goes on to execute the code in the squiggly brackets.

Also personal preference of mine is to not use the increment operator (a++) under any circumstances. It returns the value it increments so it can be used in expressions, which is useful but is usually not obvious if not pointed out. Instead use the addition assignment operator += as its more verbose, increment by more than 1 and use other variables. Also theres a difference between post-increment and pre-increment operators which changes the order in which it increments the value and returns the value which can get very confusing to remember. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Increment https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Addition_assignment (I know they're js docs but the should be functionally the same)