source: node_modules/d3-transition/src/transition/styleTween.js

function styleInterpolate(name, i, priority) {
  return function(t) {
    this.style.setProperty(name, i.call(this, t), priority);
  };
}

function styleTween(name, value, priority) {
  var t, i0;
  function tween() {
    var i = value.apply(this, arguments);
    if (i !== i0) t = (i0 = i) && styleInterpolate(name, i, priority);
    return t;
  }
  tween._value = value;
  return tween;
}

export default function(name, value, priority) {
  var key = "style." + (name += "");
  if (arguments.length < 2) return (key = this.tween(key)) && key._value;
  if (value == null) return this.tween(key, null);
  if (typeof value !== "function") throw new Error;
  return this.tween(key, styleTween(name, value, priority == null ? "" : priority));
}