Have a simple code:

function PC (){
    this.buf = 10;

   
    this.peek = function peek() { 
        var buf = this.buf;
        buf = 20;
        console.log(this.buf);
        console.log(buf);
    };
}

var a = new PC();
a.peek();

The PC object has priviliged property buf. The method peek has private buf variable. When you place caret on the var buf = this.buf in peek function, then is marked buf after this as well. It shouldn't be.