gridContainer.width = width * CELL_LENGTH; // 1
gridContainer.style.width = width * CELL_LENGTH; // 2
gridContainer.style.width = width * CELL_LENGTH + "px"; // 3

I just figured out that the code in cases 1 and 2 are wrong. The problem is js doesn't complain about
either of them. No errors in console. Nothing!

How should I know or figure out things like this?? When there's no error and I don't know why it doesn't working other than trying different syntax until it works!

I used console and dev tools to figure it out as well but div.width seems to just adding another property to div that's useless for browser.
However for the second case, It just refuses to assign wrong syntax value to div.style.width without any complaint