Some interpreted languages provide an eval function that you may use however insecure it may be. In fact they can even be converted into JavaScript functions. The parser is often preceded by a separate lexical analyser which creates tokens from the sequence of input characters.
In fact they can even be converted into JavaScript functions.
A Tiny JavaScript Expression Parser. So now we have. Expression e new Expression1 pi. When parsing an expression via mathparse expr mathjs generates an expression tree and returns the root node of the tree.