Running code with eval() from user input

Question

So I'm a bit of a noob when it comes to web langauges, but I've read online that I can use the eval() method to basically run code as a string.

My question is, can I return a string for the eval function to be equal to? And can I RUN an actual function inside the string?
Example:
var x = eval("function run(){ var input = "test"; return input;}");

// My hope/want is that x would be equal to "test"
* the idea is that the user writes code, and I evaluate it to see if the resulted input is what was expected. *

Answer 1

Your example code string evaluate only the declaration of the function, but don't run it...
You'll need something like:

user_input = "function run() { var input = 'test'; return input;}";
var x = eval(user_input+" run();");

... or, if you don't know the name of user function to run and/or want let him start his code by his own way, encapsulate the user code in a self running anonymized function:

user_input = "function run() { var input = 'test'; return input;}";
var x = eval("("+user_input+" )();");

user_input = "function a() { return 'test'; } function b() { input=a(); return input; } b();";
var x = eval("(function(){ "+user_input+" })();");

Answer 2

@visph Thanks, it worked like a charm!