Yes you can go wild nesting ternaries. I find this version to be fairly readable:
var foo = (
bar === 'a' ? 1 : // if
bar === 'b' ? 2 : // else if
bar === 'c' ? 3 : // else if
null // else
);
but that’s not a widely shared opinion, and you should probably stick to if/else or switch when working on a team.