Is this a suitable case in which using dynamic function creation would be justified?

Question

I'm currently developing a tutorial site for teaching the fundamentals of Web development (HTML, CSS, and JavaScript, for starters). I'd like a setup where I could give in-depth coverage of all sorts of topics and then provide a basic sandbox environment where the user could write code which solves the question asked at the end of each tutorial section.

For example, if I'd covered multiplication in a previous tutorial, and the user had just finished a lesson on functions being capable of returning values, I might request that they submit a function which returns the product of two parameters.

Is this not the perfect instance in which using dynamic function creation would be considered a good idea? Let's look at an example.

<script>
function check()
 {
eval('var f = ' + document.getElementById('user_code').value);
if (f(5, 10) == 50)
 {
// user properly wrote a function which
// returned the product of its parameters
 }
 }
</script>

Is this at all a bad idea? If so, please explain.

Answer 1

This sounds like it could work. However, the biggest challenge in your environment might be error handling. Students will surely make all sorts of errors:

• Compile time errors, that will be detected in eval()
• Run time errors, that will be detected when you call the function
• Undetectable run time errors, such as an infinite loop or a stack overflow

A more elaborate approach might parse the entered Javascript into a parse tree representation, then compare it to an expected parse tree. If it does not match, then point out what might be wrong and have the student try again. If it does match, then you can eval() and call the function, knowing that it will do what you expect.

Implementing a lexer and parser for Javascript in Javascript would be challenging but certainly not impossible.