Update README; restore scan test.

README.md

@@ -132,8 +132,8 @@ Returns the least element of the specified *iterable* according to the specified
 
 ```js
 const array = [{foo: 42}, {foo: 91}];
-d3.scan(array, (a, b) => a.foo - b.foo); // {foo: 42}
-d3.scan(array, (a, b) => b.foo - a.foo); // {foo: 91}
+d3.least(array, (a, b) => a.foo - b.foo); // {foo: 42}
+d3.least(array, (a, b) => b.foo - a.foo); // {foo: 91}
 ```
 
 This function is similar to [min](#min), except it allows the use of a comparator rather than an accessor.
@@ -144,8 +144,8 @@ Returns the index of the least element of the specified *iterable* according to
 
 ```js
 const array = [{foo: 42}, {foo: 91}];
-d3.scan(array, (a, b) => a.foo - b.foo); // 0
-d3.scan(array, (a, b) => b.foo - a.foo); // 1
+d3.leastIndex(array, (a, b) => a.foo - b.foo); // 0
+d3.leastIndex(array, (a, b) => b.foo - a.foo); // 1
 ```
 
 This function is similar to [minIndex](#minIndex), except it allows the use of a comparator rather than an accessor.

test/scan-test.js

@@ -0,0 +1,46 @@
+var tape = require("tape"),
+    arrays = require("../");
+
+tape("scan(array) compares using natural order", function(test) {
+  test.strictEqual(arrays.scan([0, 1]), 0);
+  test.strictEqual(arrays.scan([1, 0]), 1);
+  test.strictEqual(arrays.scan([0, "1"]), 0);
+  test.strictEqual(arrays.scan(["1", 0]), 1);
+  test.strictEqual(arrays.scan(["10", "2"]), 0);
+  test.strictEqual(arrays.scan(["2", "10"]), 1);
+  test.strictEqual(arrays.scan(["10", "2", NaN]), 0);
+  test.strictEqual(arrays.scan([NaN, "10", "2"]), 1);
+  test.strictEqual(arrays.scan(["2", NaN, "10"]), 2);
+  test.strictEqual(arrays.scan([2, NaN, 10]), 0);
+  test.strictEqual(arrays.scan([10, 2, NaN]), 1);
+  test.strictEqual(arrays.scan([NaN, 10, 2]), 2);
+  test.end();
+});
+
+tape("scan(array, compare) compares using the specified compare function", function(test) {
+  var a = {name: "a"}, b = {name: "b"};
+  test.strictEqual(arrays.scan([a, b], function(a, b) { return a.name.localeCompare(b.name); }), 0);
+  test.strictEqual(arrays.scan([1, 0], arrays.descending), 0);
+  test.strictEqual(arrays.scan(["1", 0], arrays.descending), 0);
+  test.strictEqual(arrays.scan(["2", "10"], arrays.descending), 0);
+  test.strictEqual(arrays.scan(["2", NaN, "10"], arrays.descending), 0);
+  test.strictEqual(arrays.scan([2, NaN, 10], arrays.descending), 2);
+  test.end();
+});
+
+tape("scan(array) returns undefined if the array is empty", function(test) {
+  test.strictEqual(arrays.scan([]), undefined);
+  test.end();
+});
+
+tape("scan(array) returns undefined if the array contains only incomparable values", function(test) {
+  test.strictEqual(arrays.scan([NaN, undefined]), undefined);
+  test.strictEqual(arrays.scan([NaN, "foo"], function(a, b) { return a - b; }), undefined);
+  test.end();
+});
+
+tape("scan(array) returns the first of equal values", function(test) {
+  test.strictEqual(arrays.scan([2, 2, 1, 1, 0, 0, 0, 3, 0]), 4);
+  test.strictEqual(arrays.scan([3, 2, 2, 1, 1, 0, 0, 0, 3, 0], arrays.descending), 0);
+  test.end();
+});