accessing vars inside a js function

So I have a psudo "Class" with functions and vars inside of it
for this question I'll just use an example:

function MyClass()
 {
     this.a = 0;
     this.b = function(c)
     {
         this.a += c;
     }
 }
 

then when I go to use it later i'll do this:

var myObject = new MyClass();
 myObject.b(3);
 myObject.b(5);
 

but when I do this:

console.log("A: " + myObject.a);
 

I get:

A: 0
 

What am I doing wrong?

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