ops[operation] || function() { return null; }
You need this function for other operation values beside "+-*/" and it would return null.
For example, if you get "m" or "w" as an operation, the compiler will give errors since m or w are not a method so OR function is needed to return null.

ops[operation] || function() { return null; }
You need this function for other operation values beside "+-*/" and it would return null.