fix: preserve byteLength when cloning a DataView

copyBuffer routed DataView through the TypedArray path, passing cur.length
(undefined on a DataView) as the constructor's third argument. Since that
argument is a byte length for DataView, a partial view was extended to the
end of the buffer. Clone DataView explicitly using byteLength.

PR.md

@@ -0,0 +1,77 @@
+# Fix DataView clones getting the wrong byteLength
+
+## Problem
+
+Cloning a `DataView` that covers only part of its underlying `ArrayBuffer`
+produces a clone with the wrong `byteLength`: the copy spans from the original
+`byteOffset` to the end of the buffer instead of keeping the original length.
+
+```js
+const clone = require('rfdc')()
+
+const buffer = new ArrayBuffer(16)
+const view = new DataView(buffer, 4, 8) // offset 4, length 8
+
+const copy = clone({ a: view }).a
+copy.byteLength // 12  (expected 8)
+```
+
+The value at the start of the view is copied correctly and the clone is
+independent of the source, but the clone can now read 4 bytes past the end of
+the original view.
+
+## Root cause
+
+`index.js`, in `copyBuffer`:
+
+```js
+return new cur.constructor(cur.buffer.slice(), cur.byteOffset, cur.length)
+```
+
+This path handles every `ArrayBuffer.isView` value, i.e. both TypedArrays and
+`DataView`. It is correct for TypedArrays, whose third constructor argument is an
+element count matching `.length`. A `DataView` is different:
+
+- it has no `.length` property, so `cur.length` is `undefined`;
+- its third constructor argument is a byte length, not an element count.
+
+`new DataView(buffer, byteOffset, undefined)` extends the view to the end of the
+buffer, so a partial view's length is lost. (A DataView over the whole buffer
+happens to work, because "to the end of the buffer" equals its byteLength.)
+
+## Fix
+
+Handle `DataView` explicitly and size it with `byteLength`:
+
+```js
+if (cur instanceof DataView) {
+  return new DataView(cur.buffer.slice(), cur.byteOffset, cur.byteLength)
+}
+```
+
+TypedArrays keep using the existing `.length`-based path.
+
+## Test
+
+Added tests (run for all four `rfdc` variants via the shared `types()` helper)
+that clone a full-buffer DataView and a partial DataView, asserting `byteOffset`,
+`byteLength`, the stored value, and independence from the source.
+
+Counterfactual (revert the `copyBuffer` DataView branch): the partial-DataView
+test fails with
+
+```
+not ok - byteLength preserved
+  is(cloned.byteLength, view.byteLength, 'byteLength preserved')
+```
+
+(expected 8, got 12), while the full-buffer case still passes.
+
+## Gate
+
+- Tests: `node test/index.js` -> `1..144`, 144 ok, 0 failing, exit 0.
+  The repository's `npm test` invokes the `tap` CLI, whose bundled `esm` loader
+  crashes on load under Node 22 (`Function.prototype.apply was called on
+  undefined`) independently of this change; running the test file directly
+  executes the same assertions.
+- Lint: `standard index.js test/index.js` -> exit 0.

index.js

@@ -6,6 +6,12 @@ function copyBuffer (cur) {
     return Buffer.from(cur)
   }
 
+  // A DataView has no `length`; its third constructor argument is a byte
+  // length, so use `byteLength`. TypedArrays take an element count via `length`.
+  if (cur instanceof DataView) {
+    return new DataView(cur.buffer.slice(), cur.byteOffset, cur.byteLength)
+  }
+
   return new cur.constructor(cur.buffer.slice(), cur.byteOffset, cur.length)
 }
 

test/index.js

@@ -283,6 +283,33 @@ function types (clone, label) {
     deepEqual(Array.from(cloned.view2), [input2, input3], 'cloned value content is correct')
     deepEqual(Array.from(cloned.view3), [input1, 0, input2, input3], 'cloned value content is correct')
   })
+  test(`${label} copies a DataView over the full buffer correctly`, async ({ ok, is, isNot }) => {
+    const input = rnd(1000)
+    const buffer = new ArrayBuffer(8)
+    const view = new DataView(buffer)
+    view.setInt32(0, input)
+    const cloned = clone({ a: view }).a
+    ok(cloned instanceof DataView, 'cloned value is instance of DataView')
+    isNot(cloned, view, 'cloned value is not same as input value')
+    is(cloned.byteOffset, view.byteOffset, 'byteOffset preserved')
+    is(cloned.byteLength, view.byteLength, 'byteLength preserved')
+    is(cloned.getInt32(0), input, 'cloned value content is correct')
+  })
+  test(`${label} copies a partial DataView correctly`, async ({ ok, is, isNot }) => {
+    const input = rnd(100)
+    const buffer = new ArrayBuffer(16)
+    const view = new DataView(buffer, 4, 8) // offset 4, length 8 of a 16-byte buffer
+    view.setInt32(0, input)
+    const cloned = clone({ a: view }).a
+    ok(cloned instanceof DataView, 'cloned value is instance of DataView')
+    isNot(cloned, view, 'cloned value is not same as input value')
+    is(cloned.byteOffset, view.byteOffset, 'byteOffset preserved')
+    is(cloned.byteLength, view.byteLength, 'byteLength preserved')
+    is(cloned.getInt32(0), input, 'cloned value content is correct')
+    // Mutating the source must not affect the clone.
+    view.setInt32(0, input + 1)
+    is(cloned.getInt32(0), input, 'clone is independent of the source')
+  })
   test(`${label} - maps`, async ({ same, isNot }) => {
     const map = new Map([['a', 1]])
     same(Array.from(clone(map)), [['a', 1]], 'same value')