function execute() {  

   'use strict'; // activate the strict mode

​

   function concat(str1, str2) {

     // the strict mode is enabled too

     console.log(this === undefined); // => true

     return str1 + str2;

   }

   // concat() is invoked as a function in strict mode

   // this in concat() is undefined

   console.log(concat('Hello', ' World!'));

}

​

execute();