How "this" keyword is different in class and module syntax?

Question

I used to think that i have decent understanding of Javascript core. But I came across this little tweak in my code and I realised i do not know much about core. Code is like that -

// FIle:   test.js

let testOuter = (a,b) => {
    return a+b;
}

module.exports = {
  testInner: (a,b) => {
    return a+b;
  },

  test: (a,b) => {
      return  module.exports.testInner(a,b);  // Case 1:  Works Fine
      return  testOuter(a,b);                 // Case 2:  Works Fine
      return  this.testInner(a,b);            // Case 3:  Does'nt work
  }) 
}

// File:  CheckTest.js

let x = require('./test.js')
let a = x.test(2,5);
console.log(a);

After lot of search I found many things to learn and to know about scope and scope chain. But still I don't get satisfaction why case 3 is not working. I just need a deep understanding of this CASE 3 also I will really appreciate if you can provide good content to understand core JS.