default Mac keyboard shortcut in safari, I want to overwrite them. such as cmd+s, cmd+n

let isMeta = false;
    window.onkeydown = e => {
        if(e.key == "Meta"){
            isMeta = true;
        }else if(e.key == 's' && isMeta){
            e.preventDefault();
            console.log('s');
        }else if(e.key == 'n' && isMeta){
            e.preventDefault();
            console.log('n');
        }
    };
    window.onkeyup = e => {
        if(e.key == "Meta"){
            isMeta = false;
        }
    };

testing result: cmd+s succeeded, when I press the shortcut, saving action is prevented and s is logged. but ctrl+n will call default opening new tab and n is not logged.

I wonder why this happens, why one works but the other.