source: node_modules/d3-selection/src/selection/on.js@ e4c61dd

function contextListener(listener) {
  return function(event) {
    listener.call(this, event, this.__data__);
  };
}

function parseTypenames(typenames) {
  return typenames.trim().split(/^|\s+/).map(function(t) {
    var name = "", i = t.indexOf(".");
    if (i >= 0) name = t.slice(i + 1), t = t.slice(0, i);
    return {type: t, name: name};
  });
}

function onRemove(typename) {
  return function() {
    var on = this.__on;
    if (!on) return;
    for (var j = 0, i = -1, m = on.length, o; j < m; ++j) {
      if (o = on[j], (!typename.type || o.type === typename.type) && o.name === typename.name) {
        this.removeEventListener(o.type, o.listener, o.options);
      } else {
        on[++i] = o;
      }
    }
    if (++i) on.length = i;
    else delete this.__on;
  };
}

function onAdd(typename, value, options) {
  return function() {
    var on = this.__on, o, listener = contextListener(value);
    if (on) for (var j = 0, m = on.length; j < m; ++j) {
      if ((o = on[j]).type === typename.type && o.name === typename.name) {
        this.removeEventListener(o.type, o.listener, o.options);
        this.addEventListener(o.type, o.listener = listener, o.options = options);
        o.value = value;
        return;
      }
    }
    this.addEventListener(typename.type, listener, options);
    o = {type: typename.type, name: typename.name, value: value, listener: listener, options: options};
    if (!on) this.__on = [o];
    else on.push(o);
  };
}

export default function(typename, value, options) {
  var typenames = parseTypenames(typename + ""), i, n = typenames.length, t;

  if (arguments.length < 2) {
    var on = this.node().__on;
    if (on) for (var j = 0, m = on.length, o; j < m; ++j) {
      for (i = 0, o = on[j]; i < n; ++i) {
        if ((t = typenames[i]).type === o.type && t.name === o.name) {
          return o.value;
        }
      }
    }
    return;
  }

  on = value ? onAdd : onRemove;
  for (i = 0; i < n; ++i) this.each(on(typenames[i], value, options));
  return this;
}