How does 'this' work in constructor function for Object creation in Javascript

I am really struggling to understand how does this works in constructor function for object creation in Javascript environment.

below is the code snippet:

function Employee() {
    basicSalary = 12000;
    overtime = 10;
    rate = 200;
    this.getWage = function () {
        return basicSalary+(overtime*rate);
    }
}
let emp = new Employee();
console.log(emp.basicSalary);//undefined
console.log(emp.getWage());//14000

in above code I am getting undefined for basicSalary variable. So in order to get basicSalary value I prefix it with this inside function like:

this.basicSalary = 12000;

But, in doing so I am getting error in accessing getWage() method. It says:

Uncaught ReferenceError: basicSalary is not defined
    at Employee.getWage (index.js:6)
    at index.js:11

However, if I prefix this to the basicSalary in returning statement inside getWage() function, I am able to access both basicSalary variable and getWage() method.

Also If I replace all my code to factory function i.e. Object literal type like here:

function employee(){
    return {
        basicSalary : 12000,
        overtime : 10,
        rate : 200,
        getWage:function(){
            return this.basicSalary+(this.overtime*this.rate); // accessing values through `this`
        }
    };
}

const e2 = employee();
console.log(e2.basicSalary);
console.log(e2.getWage());

Now I am easily able to get both basicSalary and getWage() method. Only thing that I needed to do was put this in return statement inside getWage() method.

in both these approaches this works differently. I am confused. Please share some knowledge on how to use this in Javascript.