Invoke accessor once per value in d3.extent.

This allows the use of nondeterministic accessors. Also add a test.

d3.js

@@ -105,8 +105,9 @@ d3.max = function(array, f) {
   }
   return a;
 };
-d3.extent = function() {
-  return [d3.min.apply(d3, arguments), d3.max.apply(d3, arguments)];
+d3.extent = function(array, f) {
+  if (arguments.length > 1) array = array.map(f);
+  return [d3.min(array), d3.max(array)];
 };
 d3.random = {
   normal: function(mean, deviation) {

src/core/extent.js

@@ -1,3 +1,4 @@
-d3.extent = function() {
-  return [d3.min.apply(d3, arguments), d3.max.apply(d3, arguments)];
+d3.extent = function(array, f) {
+  if (arguments.length > 1) array = array.map(f);
+  return [d3.min(array), d3.max(array)];
 };

test/core/extent-test.js

@@ -0,0 +1,50 @@
+require("../env");
+require("../../d3");
+
+var vows = require("vows"),
+    assert = require("assert");
+
+var suite = vows.describe("d3.extent");
+
+suite.addBatch({
+  "extent": {
+    topic: function() {
+      return d3.extent;
+    },
+    "returns the numeric extent for numbers": function(extent) {
+      assert.deepEqual(extent([1]), [1, 1]);
+      assert.deepEqual(extent([5, 1, 2, 3, 4]), [1, 5]);
+      assert.deepEqual(extent([20, 3]), [3, 20]);
+      assert.deepEqual(extent([3, 20]), [3, 20]);
+    },
+    "returns the lexicographic extent for strings": function(extent) {
+      assert.deepEqual(extent(["c", "a", "b"]), ["a", "c"]);
+      assert.deepEqual(extent(["20", "3"]), ["20", "3"]);
+      assert.deepEqual(extent(["3", "20"]), ["20", "3"]);
+    },
+    "ignores null, undefined and NaN": function(extent) {
+      assert.deepEqual(extent([NaN, 1, 2, 3, 4, 5]), [1, 5]);
+      assert.deepEqual(extent([1, 2, 3, 4, 5, NaN]), [1, 5]);
+      assert.deepEqual(extent([10, null, 3, undefined, 5, NaN]), [3, 10]);
+    },
+    "compares heterogenous types as numbers": function(extent) {
+      assert.deepEqual(extent([20, "3"]), ["3", 20]);
+      assert.deepEqual(extent(["20", 3]), [3, "20"]);
+      assert.deepEqual(extent([3, "20"]), [3, "20"]);
+      assert.deepEqual(extent(["3", 20]), ["3", 20]);
+    },
+    "returns undefined for empty array": function(extent) {
+      assert.deepEqual(extent([]), [undefined, undefined]);
+      assert.deepEqual(extent([null]), [undefined, undefined]);
+      assert.deepEqual(extent([undefined]), [undefined, undefined]);
+      assert.deepEqual(extent([NaN]), [undefined, undefined]);
+      assert.deepEqual(extent([NaN, NaN]), [undefined, undefined]);
+    },
+    "applies the optional accessor function exactly once": function(extent) {
+      var i = 10;
+      assert.deepEqual(d3.extent([0,1,2,3], function() { return ++i; }), [11, 14]);
+    }
+  }
+});
+
+suite.export(module);