returning `undefined` rather than `NaN` to comply with d3.mean as @mbostock suggested

d3.js

@@ -171,7 +171,7 @@ d3 = function() {
   };
   d3.variance = function(array, f) {
     var n = array.length, m = 0, a, d, s = 0, i = -1, j = 0;
-    if (n < 2) return NaN;
+    if (n < 2) return undefined;
     if (arguments.length === 1) {
       while (++i < n) {
         if (d3_number(a = array[i])) {
@@ -191,7 +191,7 @@ d3 = function() {
         }
       }
     }
-    return j ? s / (j - 1) : NaN;
+    return j ? s / (j - 1) : undefined;
   };
   d3.deviation = function(array, f) {
     var v = f ? d3.variance(array, f) : d3.variance(array);

src/core/variance.js

@@ -6,7 +6,7 @@ d3.variance = function(array, f) {
 			s = 0,
 			i = -1,
 			j = 0;
-	if(n < 2) return NaN;
+	if(n < 2) return undefined;
 	if (arguments.length === 1) {
 		while (++i < n){
 			if (d3_number(a = array[i])){
@@ -26,5 +26,5 @@ d3.variance = function(array, f) {
 			}
 		}
 	}
-	return j ? (s / (j-1)) : NaN;
+	return j ? (s / (j-1)) : undefined;
 };

test/core/deviation-test.js

@@ -11,7 +11,7 @@ suite.addBatch({
       return d3.deviation;
     },
     "returns the sd value for numbers": function(deviation) {
-      assert.isNaN(deviation([1]));
+      assert.isUndefined(deviation([1]));
       assert.equal(deviation([5, 1, 2, 3, 4]), 1.5811388300841898);
       assert.equal(deviation([20, 3]), 12.020815280171307);
       assert.equal(deviation([3, 20]), 12.020815280171307);
@@ -26,11 +26,11 @@ suite.addBatch({
       assert.equal(deviation([-Number.MAX_VALUE, -Number.MAX_VALUE]), 0);
     },
     "returns undefined for empty array": function(deviation) {
-      assert.isNaN(deviation([]));
-      assert.isNaN(deviation([null]));
-      assert.isNaN(deviation([undefined]));
-      assert.isNaN(deviation([NaN]));
-      assert.isNaN(deviation([NaN, NaN]));
+      assert.isUndefined(deviation([]));
+      assert.isUndefined(deviation([null]));
+      assert.isUndefined(deviation([undefined]));
+      assert.isUndefined(deviation([NaN]));
+      assert.isUndefined(deviation([NaN, NaN]));
     },
     "applies the optional accessor function": function(deviation) {
       assert.equal(d3.deviation([[1, 2, 3, 4, 5], [2, 4, 6, 8, 10]], function(d) { return d3.mean(d); }), 2.1213203435596424);

test/core/variance-test.js

@@ -11,7 +11,7 @@ suite.addBatch({
       return d3.variance;
     },
     "returns the variance value for numbers": function(variance) {
-      assert.isNaN(variance([1]));
+      assert.isUndefined(variance([1]));
       assert.equal(variance([5, 1, 2, 3, 4]), 2.5);
       assert.equal(variance([20, 3]), 144.5);
       assert.equal(variance([3, 20]), 144.5);
@@ -25,12 +25,12 @@ suite.addBatch({
       assert.equal(variance([Number.MAX_VALUE, Number.MAX_VALUE]), 0);
       assert.equal(variance([-Number.MAX_VALUE, -Number.MAX_VALUE]), 0);
     },
-    "returns NaN for empty array": function(variance) {
-      assert.isNaN(variance([]));
-      assert.isNaN(variance([null]));
-      assert.isNaN(variance([undefined]));
-      assert.isNaN(variance([NaN]));
-      assert.isNaN(variance([NaN, NaN]));
+    "returns undefined for empty array": function(variance) {
+      assert.isUndefined(variance([]));
+      assert.isUndefined(variance([null]));
+      assert.isUndefined(variance([undefined]));
+      assert.isUndefined(variance([NaN]));
+      assert.isUndefined(variance([NaN, NaN]));
     },
     "applies the optional accessor function": function(variance) {
       assert.equal(d3.variance([[1, 2, 3, 4, 5], [2, 4, 6, 8, 10]], function(d) { return d3.mean(d); }), 4.5);