Binary data: ArrayBuffer, SharedArrayBuffer, TypedArrays, DataView, Atomics

Normative notes for the binary-data built-ins (ECMA-262 17th edition, clauses 23.2, 23.3, 25.1-25.4). Where the specification leaves room, this file says what the engine does.

Representation

• A buffer is one byte[] holding exactly the current byte length. Resizing (resize, grow) replaces the array and copies; views refer to the buffer object, so this is unobservable. Detaching replaces it with an empty array and sets a flag.
• Element access is BinaryPrimitives over a span of that array: little-endian for typed arrays (the host byte order is not observable), explicit endianness for DataView. Numbers never box; a BigInt element allocates only the BigInteger.
• A typed array stores the buffer, the byte offset and a fixed length, or "auto" for a length-tracking view of a resizable buffer. Length and out-of-bounds status are recomputed on every access, so a resize is seen at once.
• Integer-indexed keys never reach property storage and never consult the prototype chain (10.4.5). A Number key (ta[i]) takes a fast path in the interpreter; string keys go through CanonicalNumericIndexString.
• Float16Array, DataView.prototype.getFloat16/setFloat16 and Math.f16round use System.Half, which rounds to nearest even from a double.

Limits

• The largest buffer is 2,147,483,591 bytes (the .NET array limit). A larger request, or a maxByteLength above it, is RangeError: Array buffer allocation failed. A length that fails ToIndex is RangeError: Invalid array buffer length (Invalid typed array length: N for typed arrays).
• Every allocation (buffer creation, resize, grow, transfer, slice, toSorted, toReversed, with, Uint8Array.from*) is charged to the engine's allocation budget before it happens. Over budget ends the run with a memory limit, which script cannot catch. A resizable buffer is charged for its current length, not its maxByteLength.
• Loops proportional to length charge fuel: one step per eight elements for native loops (fill, indexOf, reverse, set, copies, base64 and hex), one per element for loops that call script or read from array-likes (map, forEach, from, constructor from an array-like, set from an array-like). Sorting with a comparator ticks per comparison.
• Result strings (join, toBase64, toHex) above the engine's string limit are RangeError: Invalid string length.

Host-defined behaviour

• IJsHost.CanBlock (default true) is the agent's [[CanBlock]]. With false, Atomics.wait throws TypeError.
• The engine has one agent. A permitted Atomics.wait on a matching value can only time out; it returns "timed-out" at once and never sleeps. With an infinite timeout nothing could ever wake it, so it throws TypeError.
• Atomics.waitAsync waiters live in the agent. Atomics.notify resolves them with "ok" in FIFO order through a queued job. A waiter with a finite timeout times out on virtual time: when the job queue drains, the waiter with the earliest deadline (ties in creation order) resolves "timed-out" and the loop continues. A waiter with no timeout stays pending. A run therefore never waits on the wall clock and replays identically.
• Atomics.isLockFree(n) is true for 1, 2, 4 and 8.
• The conformance host's $262.detachArrayBuffer detaches an ArrayBuffer and returns null. $262.agent does not exist; test262 cases that start further agents are not run.
• Atomics.pause is part of the next edition and is not provided.

Deviations and choices

• DataView.prototype.byteLength/byteOffset on a detached or out-of-bounds view throw TypeError, as the 17th edition and Node do. The pinned test262 has tests written for a later text that returns 0; they are listed as documented failures.
• Object.freeze, Object.preventExtensions and Reflect.preventExtensions fail for views that can change size (length-tracking, or over a resizable ArrayBuffer), as 10.4.5.1 requires.
• %TypedArray%.prototype.toLocaleString passes its first two arguments on to each element's toLocaleString, as ECMA-402 specifies.
• Messages for fromBase64, fromHex, setFromBase64 and setFromHex are the engine's own (Node 24 does not ship these methods): SyntaxError with "The input is not valid base64", "The base64 input ends with an incomplete chunk", "The base64 input has non-zero padding bits", "The hex input has an odd length" or "The hex input contains a character that is not a hexadecimal digit".

V8 message wording

Recorded by running Node v24.22.1 (full ICU) on each script; the engine produces the same err.message.