From PHP development I know that eval is evil and I've recently read What constitutes “Proper use” of the javascript Eval feature? and Don't be eval. The only proper use of eval I've read is Ajax.

I'm currently developing a visualization tool that lets users see how polynomials can interpolate functions:

• Example
• Code on GitHub

I use eval for evaluation of arbitrary functions. Is this a legitimate use of eval? How could I get rid of eval?

I want the user to be able to execute any function of the following forms:

1. a x^i with a,i in R
2. sin, cos, tan
3. b^x with b in R
4. any combination that you can get by
   • adding (e.g. x^2 + x^3 + sin(x)),
   • multiplying (e.g. sin(x)*x^2) or
   • inserting (e.g. sin(x^2))