This is a bit troublesome!

  function f() {var x = 99; return function(a,b) {return a(b)};}
  f()(eval, 'console.log(x);')
  ReferenceError: x is not defined
  function f() {var x = 99; return function(a,b) {return eval(b)};}
  f()(eval, 'console.log(x);')
  99
  undefined