JavaScript: Variable inside function is not accessible. Please explain behavior [duplicate]

I have following code snippet to be executed,

function OuterFn(){
        var slices = 0; // variable inside Outer function
        this.getSlices = function(){
            return slices;
        }
        this.slice = function(){
            slices++;
        }
    }
    var outerFn = new OuterFn();
    console.log(outerFn.getSlices());
    console.log(outerFn.slices); // print outer function variable

Why output of this code is:
0
undefined