Fix transitioning styles to null (removal).

d3.js

@@ -1856,9 +1856,18 @@ function d3_transition(groups, id) {
   return groups;
 }
 
+function d3_transitionTweenNull() {
+  return d3_transitionTweenNullNull;
+}
+
+function d3_transitionTweenNullNull() {
+  return null;
+}
+
 function d3_transitionTween(b) {
   return typeof b === "function"
       ? function(d, i, a) { var v = b.call(this, d, i) + ""; return a != v && d3.interpolate(a, v); }
+      : b == null ? d3_transitionTweenNull
       : (b = b + "", function(d, i, a) { return a != b && d3.interpolate(a, b); });
 }
 
@@ -1951,7 +1960,11 @@ d3_transitionPrototype.styleTween = function(name, tween, priority) {
   return this.tween("style." + name, function(d, i) {
     var f = tween.call(this, d, i, window.getComputedStyle(this, null).getPropertyValue(name));
     return f && function(t) {
-      this.style.setProperty(name, f(t), priority);
+      if ((t = f(t))) {
+        this.style.setProperty(name, t, priority);
+      } else {
+        this.style.removeProperty(name);
+      }
     };
   });
 };

src/core/transition-style.js

@@ -8,7 +8,11 @@ d3_transitionPrototype.styleTween = function(name, tween, priority) {
   return this.tween("style." + name, function(d, i) {
     var f = tween.call(this, d, i, window.getComputedStyle(this, null).getPropertyValue(name));
     return f && function(t) {
-      this.style.setProperty(name, f(t), priority);
+      if ((t = f(t))) {
+        this.style.setProperty(name, t, priority);
+      } else {
+        this.style.removeProperty(name);
+      }
     };
   });
 };

src/core/transition.js

@@ -85,9 +85,18 @@ function d3_transition(groups, id) {
   return groups;
 }
 
+function d3_transitionTweenNull() {
+  return d3_transitionTweenNullNull;
+}
+
+function d3_transitionTweenNullNull() {
+  return null;
+}
+
 function d3_transitionTween(b) {
   return typeof b === "function"
       ? function(d, i, a) { var v = b.call(this, d, i) + ""; return a != v && d3.interpolate(a, v); }
+      : b == null ? d3_transitionTweenNull
       : (b = b + "", function(d, i, a) { return a != b && d3.interpolate(a, b); });
 }
 

test/core/transition-test-style.js

@@ -9,9 +9,11 @@ module.exports = {
 
     var s = d3.select("body").append("div")
         .style("background-color", "white")
-        .style("color", "red");
+        .style("color", "red")
+        .style("display", "none");
 
     var t = s.transition()
+        .style("display", null)
         .style("background-color", "green")
         .style("background-color", "red")
         .style("color", function() { return "green"; }, "important")
@@ -34,5 +36,8 @@ module.exports = {
     var style = result.selection.node().style;
     assert.equal(style.getPropertyPriority("background-color"), "");
     assert.equal(style.getPropertyPriority("color"), "important");
+  },
+  "removes a style": function(result) {
+    assert.equal(result.selection.style("display"), "");
   }
 };