fix #263; don’t mutate user thresholds

src/bin.js

@@ -66,9 +66,11 @@ export default function bin() {
     }
 
     // Remove any thresholds outside the domain.
-    var m = tz.length;
-    while (tz[0] <= x0) tz.shift(), --m;
-    while (tz[m - 1] > x1) tz.pop(), --m;
+    // Be careful not to mutate an array owned by the user!
+    var m = tz.length, a = 0, b = m;
+    while (tz[a] <= x0) ++a;
+    while (tz[b - 1] > x1) --b;
+    if (a || (b < m)) tz = tz.slice(a, b), m = b - a;
 
     var bins = new Array(m + 1),
         bin;
@@ -116,7 +118,7 @@ export default function bin() {
   };
 
   histogram.thresholds = function(_) {
-    return arguments.length ? (threshold = typeof _ === "function" ? _ : Array.isArray(_) ? constant(slice.call(_)) : constant(_), histogram) : threshold;
+    return arguments.length ? (threshold = typeof _ === "function" ? _ : constant(Array.isArray(_) ? slice.call(_) : _), histogram) : threshold;
   };
 
   return histogram;

test/bin-test.js

@@ -241,6 +241,22 @@ it("bin(data) assigns integer values to the correct bins", () => {
   assert.deepStrictEqual(bin().domain([3, 8])(eights), [box([], 3, 4), box([], 4, 5), box([], 5, 6), box([], 6, 7), box(eights, 7, 8)]);
 });
 
+it("bin(data) does not mutate user-supplied thresholds as an array", () => {
+  const thresholds = [3, 4, 5, 6];
+  const b = bin().domain([4, 5]).thresholds(thresholds);
+  assert.deepStrictEqual(b([5]), [box([], 4, 5), box([5], 5, 5)]);
+  assert.deepStrictEqual(thresholds, [3, 4, 5, 6]);
+  assert.deepStrictEqual(b.thresholds()(), [3, 4, 5, 6]);
+});
+
+it("bin(data) does not mutate user-supplied thresholds as a function", () => {
+  const thresholds = [3, 4, 5, 6];
+  const b = bin().domain([4, 5]).thresholds(() => thresholds);
+  assert.deepStrictEqual(b([5]), [box([], 4, 5), box([5], 5, 5)]);
+  assert.deepStrictEqual(thresholds, [3, 4, 5, 6]);
+  assert.deepStrictEqual(b.thresholds()(), [3, 4, 5, 6]);
+});
+
 function box(bin, x0, x1)  {
   bin.x0 = x0;
   bin.x1 = x1;