[immutable-arraybuffer] ArrayBuffer.prototype.sliceToImmutable by gibson042 · Pull Request #4553 · tc39/test262

gibson042 · 2025-08-01T01:26:51Z

New ArrayBuffer.prototype properties

ArrayBuffer.prototype.sliceToImmutable

verifyPrimordialCallableProperty(ArrayBuffer.prototype, "sliceToImmutable", "sliceToImmutable", 2);

brand-checking (throws TypeError unless receiver is an ArrayBuffer and not a SharedArrayBuffer), before resolving bounds

throws TypeError if receiver is detached, before resolving bounds

resolves bounds after passing the preceding checks—first start, clamping negative values to [0, len - 1] and positive values to [0, len], then end, defaulting to len and clamping negative values to [0, len - 1] and positive values to [0, len] (and capturing len before coercing either start or end)

throws TypeError if receiver was detached by bounds resolution, after bounds resolution

throws RangeError if length < specified upper bound, after passing the second detachment check and even if length was ≥ specified upper bound until bounds resolution

returns an immutable ArrayBuffer with the correct length and contents (both unaffected by subsequent changes to the original receiver, including detachment)

ptomato

Minor comments, feel free to merge when resolved

ptomato · 2025-08-25T23:18:39Z

test/built-ins/ArrayBuffer/prototype/sliceToImmutable/this-is-sharedarraybuffer.js

+---*/
+
+var fn = ArrayBuffer.prototype.sliceToImmutable;
+assert.sameValue(typeof fn, "function", "Method must exist.");


Suggested change

assert.sameValue(typeof fn, "function", "Method must exist.");

I think this is sufficiently implied by the immutable-arraybuffer feature flag.

ptomato · 2025-08-25T23:19:00Z

test/built-ins/ArrayBuffer/prototype/sliceToImmutable/this-is-not-object.js

+---*/
+
+var fn = ArrayBuffer.prototype.sliceToImmutable;
+assert.sameValue(typeof fn, "function", "Method must exist.");


ptomato · 2025-08-25T23:20:32Z

test/built-ins/ArrayBuffer/prototype/sliceToImmutable/this-is-not-object.js

+  var label = badReceivers[i][0];
+  var value = badReceivers[i][1];


This will break on implementations without Symbol or BigInt, I think you probably want if (!badReceivers[i]) continue;

ptomato · 2025-08-25T23:20:59Z

test/built-ins/ArrayBuffer/prototype/sliceToImmutable/this-has-no-arraybufferdata-internal.js

+---*/
+
+var fn = ArrayBuffer.prototype.sliceToImmutable;
+assert.sameValue(typeof fn, "function", "Method must exist.");


Already implied by feature flag.

ptomato · 2025-08-25T23:26:11Z

test/built-ins/ArrayBuffer/prototype/sliceToImmutable/argument-coercion.js

+      "sliceToImmutable(" + repr(rawStart) + ", " + repr(rawEnd) + ")");
+    assert.compareArray(new Uint8Array(dest), expectContents,
+      "sliceToImmutable(" + repr(rawStart) + ", " + repr(rawEnd) + ")");
+    assert.sameValue(dest.immutable, true,
+      "sliceToImmutable(" + repr(rawStart) + ", " + repr(rawEnd) + ")");


Minor suggestion, add something to these assertion messages to distinguish which assertion is failing (byte length, contents, or immutability) (Same for below)

iamstolis · 2025-10-06T16:25:19Z

test/built-ins/ArrayBuffer/prototype/sliceToImmutable/modify-source-after-return.js

+var view = new Uint8Array(source);
+for (var i = 0; i < 8; i++) view[i] = i + 1;
+
+var dest = new ArrayBuffer.sliceToImmutable();


new ArrayBuffer.sliceToImmutable() makes no sense. I guess it should be something like source.sliceToImmutable().

iamstolis · 2025-10-06T16:26:12Z

test/built-ins/ArrayBuffer/prototype/sliceToImmutable/argument-coercion.js

+  return String(value);
+}
+
+var make32ByteArrayBuffer() {


Typo? var make32ByteArrayBuffer() -> function make32ByteArrayBuffer()

iamstolis · 2025-10-06T16:29:54Z

test/built-ins/ArrayBuffer/prototype/sliceToImmutable/argument-coercion.js

+  for (var j = 0; j < goodInputs.length; j++) {
+    var rawEnd = goodInputs[j][0];
+    var intEnd = goodInputs[j][1];
+    var intLength = intEnd - intStart;


This is not a correct calculation of the length of the slice. It does not work when intStart or intEnd is negative or when intStart > intEnd. Note that there is the same problem on lines 151 and 170.

iamstolis · 2025-10-06T16:32:12Z

test/built-ins/ArrayBuffer/prototype/sliceToImmutable/argument-coercion.js

+      calls,
+      ["start[Symbol.toPrimitive](number)", "end[Symbol.toPrimitive](number)"],
+      "sliceToImmutable" + reprArgs("[Symbol.toPrimitive]", "[Symbol.toPrimitive]"));
+    );


SyntaxError, duplicate ); (here and on the previous line). There is the same problem on lines 316 and 324.

iamstolis · 2025-10-06T16:34:22Z

test/built-ins/ArrayBuffer/prototype/sliceToImmutable/argument-coercion.js

+  [-1, RangeError],
+  [9007199254740992, RangeError], // Math.pow(2, 53) = 9007199254740992
+  [Infinity, RangeError],
+  [-Infinity, RangeError],


I don't see why would these out of range numbers caused RangeErrors.

gibson042 requested a review from a team as a code owner August 1, 2025 01:26

gibson042 mentioned this pull request Jul 29, 2025

Testing plan for Immutable ArrayBuffers #4509

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[immutable-arraybuffer] ArrayBuffer.prototype.sliceToImmutable

f9ed2c1

gibson042 force-pushed the 2025-07-immutable-arraybuffer-slicetoimmutable branch from 51629c7 to f9ed2c1 Compare August 1, 2025 01:30

ptomato added the needs review label Aug 1, 2025

ptomato approved these changes Aug 25, 2025

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[immutable-arraybuffer] ArrayBuffer.prototype.sliceToImmutable#4553

[immutable-arraybuffer] ArrayBuffer.prototype.sliceToImmutable#4553
gibson042 wants to merge 1 commit intotc39:mainfrom
gibson042:2025-07-immutable-arraybuffer-slicetoimmutable

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		var label = badReceivers[i][0];
		var value = badReceivers[i][1];

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gibson042 commented Aug 1, 2025

New ArrayBuffer.prototype properties

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