Access class functions from d3 event handler

Question

I'm trying to use es6 classes in a d3 app. I have an event handler bound to the "drag" event, but I cannot access the class from within the event handler. this is my class when I manually call the handler, but when the event is triggered, this is the DOM element.

Code:

class Point {

    constructor(croot, cx, cy, ccolor) {
        this.x = cx;
        this.y = cy;
        this.size = 5;
        this.draggable = true;
        this.root = croot;
        this.color = ccolor;
        this.circle = this.root.append('circle')
            .attr('class', "draggable")
            .attr('r', this.size)
            .attr('fill', "white")
            .attr('stroke', this.color)
            .attr('stroke-width', 2)
            .call(d3.drag(this).on('drag', this.onDrag));
        this.circle.attr('transform',
            `translate(${(this.x + 0.5) * scale} ${(this.y + 0.5) * scale})`);
    }


    onDrag() {
        console.log(this);
        this.x = (d3.event.x / scale);
        this.y = (d3.event.y / scale);
        this.circle.attr('transform',
            `translate(${(this.x + 0.5) * scale} ${(this.y + 0.5) * scale})`);
    }

}

Answer 1

You loose the binding to this when you pass a function that way. It passes a reference to the function, which D3 calls, so the calling context changes. You can keep this by explicitly binding it.

call(d3.drag(this).on('drag', this.onDrag.bind(this))