JavaScript for entertainment 1. Inspired by freetonik.

jsfe-1-1.js

/*
 * By default, `eval` evaluates code in the current context.
 */
var x = 0;

function foo() {
  var x = 5;
  eval('x = 100');
  return x;
}

x; // 0
foo(); // 100
x; // 0

jsfe-1-2.js

/*
 * However, there's a trick to confuse everyone. If we assign the `eval`
 * function to a variable, the function inside that variable "detaches"
 * from the current context: `this` becomes the global object.
 */
var x = 0;

function bar() {
  var x = 5,
      evil = eval;
  evil('x = 100');
  return x;
}

x; // 0
bar(); // 5
x; // 100

jsfe-1-3.js

/*
 * How you could write such a function (in reality, you shouldn't) to make
 * everyone superconfused.
 */
var x = 0;

function bar() {
  var x = 5,
      evil = eval.call(this, 'eval');
  evil('x = 100');
  return x;
}

x; // 0
bar(); // 100
x; // 0