From bec0354560f0c07feabd912cee518d3433cc5b80 Mon Sep 17 00:00:00 2001
From: Mike Bostock <mbostock@gmail.com>
Date: Sat, 3 Dec 2022 09:51:48 -0800
Subject: [PATCH] =?UTF-8?q?fix=20#263;=20don=E2=80=99t=20mutate=20user=20t?=
 =?UTF-8?q?hresholds?=
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---
 src/bin.js       | 10 ++++++----
 test/bin-test.js | 16 ++++++++++++++++
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/src/bin.js b/src/bin.js
index 28d64aad..6f179ad4 100644
--- a/src/bin.js
+++ b/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;
diff --git a/test/bin-test.js b/test/bin-test.js
index 389e505d..e6453b2d 100644
--- a/test/bin-test.js
+++ b/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;