How override eval function in javascript?

Question

For example:

(function() {
      var proxied = window.eval;
      window.eval = function() {
        return proxied.apply(this, arguments);
      };
    })()         


        

Answer 1

eval is magic. Unlike a ‘real’ function, it can read and write local variables in the caller:

function foo() {
    var a= 1;
    eval('a+= 1');
    alert(a); // 2
}

Replace that eval with a proxied function and you've got a problem: the a+= 1 executes in the scope of the proxied function instead of foo. Depending on what's happening in the evaled code that could cause values to go missing, damage to the proxy's local, accidental globals, and so on.

It is, therefore, impossible to replace eval with a fully-working proxy. (For simple cases which don't need the locals, you can kind of get away with it.)