Only coalesce exact string matches.

If there are a lot of matching numbers, it’s faster to do direct string equality
comparisons than it is to coerce to a number and compare numerically.

d3.js

@@ -5624,8 +5624,8 @@
         bs = b.substring(bi, bs);
         if (s[i]) s[i] += bs; else s[++i] = bs;
       }
-      if ((am = +am[0]) === (bm = +(bs = bm[0]))) {
-        if (s[i]) s[i] += bs; else s[++i] = bs;
+      if ((am = am[0]) === (bm = bm[0])) {
+        if (s[i]) s[i] += bm; else s[++i] = bm;
       } else {
         s[++i] = null;
         q.push({

src/interpolate/string.js

@@ -22,9 +22,9 @@ function d3_interpolateString(a, b) {
       if (s[i]) s[i] += bs; // coalesce with previous string
       else s[++i] = bs;
     }
-    if ((am = +am[0]) === (bm = +(bs = bm[0]))) { // coalesce matching numbers
-      if (s[i]) s[i] += bs; // coalesce with previous string
-      else s[++i] = bs;
+    if ((am = am[0]) === (bm = bm[0])) { // numbers in a & b match
+      if (s[i]) s[i] += bm; // coalesce with previous string
+      else s[++i] = bm;
     } else { // interpolate non-matching numbers
       s[++i] = null;
       q.push({i: i, x: d3_interpolateNumber(am, bm)});

test/interpolate/string-test.js

@@ -43,8 +43,9 @@ suite.addBatch({
       assert.strictEqual(interpolate("", "bar")(.5), "bar");
       assert.strictEqual(interpolate("", "")(.5), "");
     },
-    "with two numerically-equivalent numbers, returns the target format": function(interpolate) {
-      assert.strictEqual(interpolate("top: 1000px;", "top: 1e3px;")(.5), "top: 1e3px;");
+    "with two numerically-equivalent numbers, returns the default format": function(interpolate) {
+      assert.strictEqual(interpolate("top: 1000px;", "top: 1e3px;")(.5), "top: 1000px;");
+      assert.strictEqual(interpolate("top: 1e3px;", "top: 1000px;")(.5), "top: 1000px;");
     }
   }
 });