Javascript This Function Context §

Without use scrict §

Consider the following codes:

function f1() {
  return this;
}
 
console.log(f1()); 

Since the following code is not in strict mode, and because the value of this is not set by the call, this will default to the global object

// In a browser:
Window {...} // 'window' global object
 
// In Node:
Object [global] {...} // 'global' global object

see more info in Javascript Global Object

With use strict §

In strict mode, however, if the value of this is called directly (e.g. not set when entering an execution context), it remains as undefined

function f2() {
  'use strict'; // enable strict mode
  return this;
}
 
console.log(f2()); // outputs 'undefined' no matter in what environment 

Called as a method of an object §

However, if f3 was called as a method or property of an object, then this would return the object a

function f3() {
  'use strict'; 
  return this;
}
 
// f3 was called as a method of an object
var a = { b: f3 }; 
console.log(a.b()); // { b: [Function: f2] }

Reference §

this - JavaScript | MDN