Question

Given a Javascript function, is it possible to verify that the function has no side effects; i.e., that the function does not change the value of any variables that are declared outside the function’s scope?

Answer 1

You would need to write an effect analysis for JavaScript, which would require an effect semantics for JavaScript. Some of the things to consider:

• does the code access primitive operations that have side effects? (e.g. memory writes, disk writes, file IO, updating any global state?)
• if the code does indeed write to variables, do they escape local scope? (i.e. is it in the ST or IO monad?)
• is any communication done via shared variables?

People have written type systems for impure languages to statically determine if side effects are present. Ben Lippmeier’s thesis covers a lot of the ground.