This default scrolling behavior comes from the keydown event. In order to prevent it, you must handle the keydown event and prevent the default behavior, either by returning false from the event handler or calling event.preventDefault().
As a rule of thumb, think carefully before you prevent default behavior like spacebar scrolling. I use it all the time and I get extremely annoyed when it doesn’t work in a page.
But if you want to eat the key…
window.onkeydown = function(e) {
return e.keyCode !== 32;
};
According to the MDN web docs for KeyboardEvent#keyCode, keyCode is a deprecated property. Although it still works in most browsers, you are encouraged to use KeyboardEvent#key going forward, which is a more standardized string representation of a key. The key value for spacebar is literally the input value: " " (single space string). So if you wanted to be very careful to support all browsers, you could write:
window.onkeydown = function(e) {
return ev.keyCode !== 32 && ev.key !== " ";
}