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To process a file larger than 2 GB in a browser with WebAssembly, do not try to hold the whole file in memory. Read it in bounded byte ranges with Blob.slice() or Blob.stream(), pass each chunk through your Wasm module while keeping parser or codec state inside Wasm between calls, and write results out incrementally where the browser allows it. The WebAssembly memory limit of 4 GiB for 32-bit memory is an address-space ceiling, not a file-size limit, and it does not tell you how much memory a particular browser on a particular device will actually grant.
Why file size and memory use are separate problems
A 6 GB log, video, or database export only needs to occupy as much memory as one chunk plus your algorithm’s working state at any given moment. The file itself can stay on disk and be read on demand. Most failures on large files come from designs that quietly undo this, for example by calling arrayBuffer() on the whole File, copying each chunk into a growing JavaScript array, or queuing reads faster than the processor consumes them.
Two units are easy to confuse. The WebAssembly limit of 4 GiB is 4,294,967,296 bytes, which is slightly more than 4 billion bytes. A “2 GB” file in marketing terms may be 2,000,000,000 bytes, and a “2 GiB” file is 2,147,483,648 bytes. Either way, the file is larger than what you should load at once, so the design question is the same.
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The limits that matter
| Item | Value | Notes |
|---|---|---|
| WebAssembly page size | 65,536 bytes (64 KiB) | Stated in the WebAssembly.org JavaScript API guide. Memory grows in whole pages. |
| Maximum 32-bit (wasm32) linear memory | 65,536 pages, or 4 GiB | Formal maximum in the WebAssembly JavaScript Interface. The specification says implementations may run out of resources below this maximum. |
| Maximum 64-bit (memory64) linear memory | 262,144 pages, or 16 GiB | Formal maximum in the same JavaScript Interface document. Actual allocation depends on the browser and device. |
| Blob range | Byte range returned by Blob.slice(start, end) |
MDN describes the result as a new Blob covering only the selected bytes. |
| Blob stream | ReadableStream of raw bytes from Blob.stream() |
Chunk sizes are chosen by the browser, not by your code. |
The specification pages above do not state publication dates. Confirm the current text before relying on exact figures in production documentation.
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Read the input in bounded ranges
A user-selected File is a subtype of Blob, so both reading approaches work on it. Choose one based on how your parser wants to consume data.
Option A: explicit byte ranges with slice()
Use this when you want deterministic chunk boundaries, for example to resume from a known offset or to align records. slice() does not read the file; you read only the range you request.
const CHUNK_SIZE = 8 * 1024 * 1024; // illustrative value; tune by measurement
for (let offset = 0; offset < file.size; offset += CHUNK_SIZE) {
const end = Math.min(offset + CHUNK_SIZE, file.size);
const bytes = new Uint8Array(await file.slice(offset, end).arrayBuffer());
processChunk(bytes, end === file.size);
}
The await inside the loop is deliberate. It ensures that only one chunk is in flight at a time. The sources do not establish an optimal chunk size for any algorithm, so start with a value you can measure on your target devices and adjust it.
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Option B: a byte stream with stream()
Use this when the parser can accept whatever sizes arrive. The browser decides how large each chunk is, so your consumer must handle any length.
const reader = file.stream().getReader();
for (;;) {
const { done, value } = await reader.read();
if (done) break;
processChunk(value, false); // value is a Uint8Array of browser-chosen length
}
processChunk(new Uint8Array(0), true); // signal end of input
Calling reader.read() only when the previous chunk has been processed gives you backpressure for free. Do not collect chunks into an array and process them later.
Process each chunk inside WebAssembly
The Wasm side should follow three rules: allocate working buffers once, keep parser state inside Wasm across calls, and never keep more than one input chunk alive at a time.
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- Reserve a fixed input buffer in linear memory at startup and reuse it for every chunk. Do not allocate a new buffer per chunk.
- Keep state in Wasm, such as the position inside a record, a partial token, or a codec’s dictionary. Your JavaScript should not need to see that state to continue parsing.
- Avoid second full-size copies in JavaScript or Wasm. A chunk-sized copy is fine; a file-sized one defeats the design.
Copy input into the buffer and call into Wasm like this. The view is created on each call so it always points at the current buffer:
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const capacity = exports.input_capacity();
for (let i = 0; i < bytes.length; i += capacity) {
const piece = bytes.subarray(i, Math.min(i + capacity, bytes.length));
const ptr = exports.input_ptr();
new Uint8Array(exports.memory.buffer, ptr, piece.length).set(piece);
exports.process(piece.length, isLast && i + capacity >= bytes.length ? 1 : 0);
}
}
The splitting loop matters for stream input, where a chunk can be larger than your fixed buffer. The exports names are placeholders for your module’s own exports.
Memory growth replaces the buffer
When Wasm memory grows, memory.buffer is replaced by a new ArrayBuffer, and views created on the old buffer are detached and have zero length. The WebAssembly.org JavaScript API guide documents this behavior. Any typed array or DataView you cached before a grow must be recreated afterwards. Creating views per call, as above, avoids the problem. If you cache views for speed, refresh them after every operation that may grow memory.
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Write output incrementally
When the result is too large to build in memory, write it to a file the user chooses. The File System Access API makes this possible in supporting browsers. The steps are:
- Confirm the page is a secure context (
window.isSecureContextistrue) and that the API exists ('showSaveFilePicker' in window). - In a click handler, call
window.showSaveFilePicker()to obtain aFileSystemFileHandle. - Call
handle.createWritable()to get aFileSystemWritableFileStream. The user must grant write permission; if the user declines, handle the error as a cancellation. - Call
write()for each output chunk as it is produced, and let each write finish before sending the next. A write can throwQuotaExceededErrorwhen browser storage quota is exhausted. - Call
close()to commit the file. If processing is cancelled or fails, callabort()instead so the partial output is discarded rather than committed.
If any step is unavailable, fall back to a server-side pipeline or to a smaller set of output files. A Blob-based download holds the full output in memory and is only appropriate when the output itself fits. The MDN documentation for the File System API describes the secure-context and permission model, and the MDN page for createWritable() describes the writable stream and commit behavior. Neither states a universal maximum output size, so test with outputs as large as your product requires.
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Memory64: when a larger address space helps
The memory64 proposal extends linear memory beyond 232 bytes and uses 64-bit memory indexes. It raises the formal ceiling, but it does not make a large working set cheap. The two approaches are different tools:
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| Approach | Memory behavior | Portability and constraints | Good fit |
|---|---|---|---|
| Chunked processing in wasm32 | Holds one input chunk, a fixed output buffer, and algorithm state. The whole file is never resident. | Uses Blob range and stream APIs and standard 32-bit Wasm. Requires careful backpressure, resumable parser state, and bounded copies. | The default for files above 2 GB when the algorithm can run incrementally. |
| Memory64 with a larger in-memory working set | Allows a formal linear address space up to 16 GiB in the JavaScript Interface. Actual allocation remains limited by the browser and device. | Requires memory64 support in the target browser and version. Glue code and toolchain support differ from wasm32. | Only when the algorithm genuinely needs a larger addressable working set and you have verified the browsers you support. |
Chunking is an algorithm and memory-management strategy. Memory64 changes the address-space model. Choosing one does not replace the other: a memory64 build still needs bounded input and output if the file is large.
Implementation status should be checked before relying on memory64. The proposal’s status table lists Firefox and V8/Chrome as done and Safari as unknown. That table is a proposal repository status page, not a per-version compatibility guarantee, and it does not state a publication date. Feature-detect at runtime and confirm the current status for each browser version you target. The WebAssembly.org portability documentation describes wasm64 as allowing linear memory larger than 4 GiB with 64-bit pointers or indexes.
Which algorithms fit this model
- Good fits: streaming parsers for line-oriented or record-oriented data, hashing and checksums, and streaming compression or decompression codecs that can resume across calls.
- Poor fits without redesign: algorithms that need random access to the whole file, global sorting, or whole-file indexing. These may need disk-backed intermediate storage, multiple passes over smaller partitions, or a server-side path.
Failure modes and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Allocation fails during a memory grow or instantiation, even though the file is valid | The browser or device refused the allocation below the specification maximum. | Reduce the working set, shrink the input buffer, and handle the failure with a clear message or fallback path. |
| Output is empty or a write throws after memory grew | A cached typed array still points at the detached old buffer. | Recreate views after every possible grow, or create them per call. |
| Memory climbs until the tab crashes | Chunks are accumulated in an array, or reads are queued faster than processing. | Await each chunk before reading the next, and keep only the current chunk alive. |
| Parsed records are corrupted at chunk boundaries | Parser state was kept only in JavaScript locals and lost between calls. | Move state into Wasm, and test with chunk sizes that split records at every position, including one-byte chunks. |
Save fails with QuotaExceededError |
Browser storage quota has been reached. | Call abort(), tell the user how much space is needed, and offer a server-side or split-output alternative. |
| Save picker does not open | The page is not a secure context, the API is not supported, or the call was not tied to a user action. | Check window.isSecureContext and feature-detect before offering the feature. |
| User cancels the picker or denies permission | Expected user choice. | Treat it as a cancellation, abort any partial stream, and release buffers. |
Test on the actual browsers and device classes you support. Results for large inputs vary with available memory, operating system, and browser version, so a configuration that works on a development laptop may fail on a lower-memory device.
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