Alguns exemplos de erros que são normalmente omitidos, mas aparecem quando utilizamos o strict mode

01-strict-mode.js

// Sem strict mode
(function () {
  
  x = 1; // Tudo bem
  delete Object.prototype; // false
  function test(a, a, c) { }; // ok
})();

// Com strict mode
(function () {
  'use strict';
  
  x = 1; // ReferenceError: x is not defined
  delete Object.prototype; // TypeError: Cannot delete property 'prototype' of function Object()
  function test(a, a, c) { }; // SyntaxError: Strict mode function may not have duplicate parameter names
})();