In an attempt to simplify my question I originally wrote out a question that did in fact work. So let's suppose I have this code which uses D3 inside an ES6 class:

```
export default class MyClass{
    constructor(){
        this.radius = 1;
    }
    myFunc(){
        this.tooltip //defined elsewhere in the class don't worry about it
            .on('mouseover', function(){
                d3.select(this).transition()
                    .ease('elastic')
                    .duration('250')
                    .attr('r', this.radius*1.5);
                    //keyword this has now been overridden
            });
    }
}

```

But how can I achieve the above functionality, or should I be taking a different approach?