"this" do NOT refer to dragged element in event-listener of d3.drag.on()

[tarepan]

abstract

"this" do NOT refer to dragged element in event-listener of d3.drag.on(*, event-listener) when I use "arrow function" for event-listener.
If I use normal function (e.g. function(){...}), it seems normal.
I think this problem is related to this-binding in arrow function.
I don't good at "this" binding, so I hope someone fix this bug(bug?) or write it in document.

problem

"this" do NOT refer to dragged element in event-listener of d3.drag.on(*, event-listener) when I use "arrow function" for event-listener.
I wrote and upload sample on bl.ocks.org => sample

In event-listener of d3.drag().on(*, event-listener), "this" should refer to current DOM element.

from_API_reference ... listener will be invoked ... with the this context as the current DOM element.

But, when I use "arrow function" for event-listener, "this" do NOT refer to the current DOM element, instead, refer to "Window object".

// code for explanation. I do NOT check this code work or not. Sample in bl.ocks.org is checked to work.
<div id=div1>div1</div>
<div id=div2>div2</div>
d3.select("#div1").call(d3.drag().on("start", function(){console.log(this);})); // -> "<div>" ◎  
d3.select("#div2").call(d3.drag().on("start", ()=>{console.log(this);})); // -> "Window" !?

condition

• PC: Microsoft sarface Pro (1)
• OS: Windows 10
• browser: google Chrome 55.0.2883.87 m (64-bit)

cause

I am not good at javascript grammar, so I have no confident to my opinion.
I think this problem is related this-binding of arrow function.
I am glad if this opinion is useful for bag-sweepers.

hope

If can, please someone fix this bug (bug?).
Even if that is difficult, please write this phenomenon in document.
I hope this issue will helpful for d3.js users (this is my first issue to github lol).

[mbostock]

See the documentation for arrow functions.

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