d3 js know if an svg shape has a class

Question

I want to have a toggle button in an svg shape, and scale it down when the button is clicked, then scale up when clicked again, firstly I added a class collapse like this. I want to remove the class if it has a class collapse

g.append("circle")
    .classed("collapse", true)
    .attr("cx", 0)
    .attr("cy", 0)
    .attr("r", radius * 0.4 - 10)
    .attr("fill", color(radius * 0.4 - 10))
    .on("click", (d) ->
      if(hasClass("collapse")) # HOW DO I DO THIS
    )

Answer 1

You can do this with the DOM API:

g.append("circle")
    .classed("collapse", true)
    .attr("cx", 0)
    .attr("cy", 0)
    .attr("r", radius * 0.4 - 10)
    .attr("fill", color(radius * 0.4 - 10))
    .on("click", (d) ->
      if(this.classList.contains("collapse")) {
        // ...
        // this.classList.remove('collapse')
      } else {
        // ...
        // this.classList.add('collapse')
      }
    )    

Or jQuery:

g.append("circle")
    .classed("collapse", true)
    .attr("cx", 0)
    .attr("cy", 0)
    .attr("r", radius * 0.4 - 10)
    .attr("fill", color(radius * 0.4 - 10))
    .on("click", (d) ->
      if($(this).hasClass("collapse")) {
        // ...
      }
    )

The this inside the call backs refers to the DOM Element. This is, however, not quite the D3-isque way of doing things. One should save the collapsed status on the data associated with the node and act on it rather than save the status in the class of the DOM elements.