Don't use the ternary operator then, it requires a third argument. You would need to reassign max to max if you don't want it to change (max = (max < b) ? b : max).

An if-statement is much more clear:

if (max < b) max = b;

And if you need it to be an expression, you can (ab)use the short-circuit-evaluation of AND:

(max < b) && (max = b)

Btw, if you want to avoid repeating variable names (or expressions?), you could use the maximum function:

max = Math.max(max, b);