Where is Blob binary data stored?

Question

Given

var data = new Array(1000000);
for (var i = 0; i < data.length; i++) {
  data[i] = 1;
}
var blob = new Blob([data]);

where is binary data representation of array stored?

Answer 1

All variables that are not explicitly represented in any other storage are stored in memory (RAM) and lives there till end of your program or while you unset it (clear it from memory).

TLDR; In RAM

Answer 2

Blobs represent a bunch of data that could live anywhere. The File API specification intentionally does not offer any synchronous way of reading a Blob's contents.

Here are some concrete possibilities.

1. When you create a Blob via the constructor and pass it in-memory data, like an Uint8Array, the Blob's contents lives in memory, at least for a while.
2. When you get a Blob from <input type="file">, the Blob's contents lives on disk, in the file selected by the user. The spec mentions snapshotting, but no implementation does it, because it'd add a lot of lag to user operations.
3. When you get a Blob from another client-side storage API like IndexedDB or the Cache Storage API, the Blob's contents lives in the API's backing store on disk.
4. Some APIs may return a Blob whose data streams from the network. The XMLHttpRequest spec makes this impossible, and I think the fetch spec also requires retrieving the entire response before creating the Blob. However, there could be a future spec that streams an HTTP response.
5. Blobs created via the Blob constructor via an array of pieces may have their contents scattered across all the places mentioned above.

In Chrome, we use a multi-process architecture where the browser process has a central registry of all live Blobs, and serves as the source of truth for blob contents. When a Blob is created in a renderer (by JavaScript), its contents is moved to the browser process via IPC, shared memory, or temporary files, depending on the size of the Blob. The browser process may also evict in-memory Blob contents to temporary files. The 500mb limit mentioned in a previous answer was lifted around 2016. More implementation details are in the README for Chrome's Blobs subsystem.