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

function textInterpolate(i) {
  return function(t) {
    this.textContent = i.call(this, t);
  };
}

function textTween(value) {
  var t0, i0;
  function tween() {
    var i = value.apply(this, arguments);
    if (i !== i0) t0 = (i0 = i) && textInterpolate(i);
    return t0;
  }
  tween._value = value;
  return tween;
}

export default function(value) {
  var key = "text";
  if (arguments.length < 1) 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, textTween(value));
}