D3.js tween & interpolate

HTML

<div id="bar"></div>

CSS

#bar {
    height: 20px;
    width: 20px;
    background: #2394F5;
    margin: 15px;
}

JavaScript

d3.select("#bar").transition()
    .duration(1000)
    .tween("width", function() {
      var i = d3.interpolate(20, 400);
      var ci = d3.interpolate('#2394F5', '#BDF436');
      return function(t) {
      	console.log(t);
          this.style.width = i(t) + 'px';
          this.style.background = ci(t);
      };
    });