buffer: introduce Blob

[jasnell]

The Blob object is an immutable data buffer. This is a first step
towards alignment with the Blob Web API.

Signed-off-by: James M Snell [email protected]

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/35315/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/35317/

[targos]

I pushed a commit to add the WPT for Blob. There's one failure that looks worth fixing:

const blob = new Blob(); should create an empty blob, but it throws an error with this PR.

[mcollina]

I think we should add some support for Blob around before calling it stable. It's not of much use if it can't be used in fs, streams, crypto, etc..

[benjamingr]

I'm happy to review this - but can you explain the motivation of having this in core?

For EventTarget/AbortController the motivation was quite clear to me - does Blob actually enable any new use cases we want to support?

[jasnell]

@mcollina:

I think we should add some support for Blob around before calling it stable. It's not of much use if it can't be used in fs, streams, crypto, etc..

Agreed.

@benjamingr:

I'm happy to review this - but can you explain the motivation of having this in core? ... For EventTarget/AbortController the motivation was quite clear to me - does Blob actually enable any new use cases we want to support?

Partly because it's a step towards supporting APIs like fetch and File (both of which require Blob). But also (in the future) to enable cases like:

const blob1 = await fs.promises.readAsBlob('path/to/some/file1')
const blob2 = await fs.promises.readAsBlob('path/to/some/file2')
const blobs = {
  foo: URL.createObjectURL(blob1),
  bar: URL.createObjectURL(blob2),
}

// blobs.foo is a blob:// URL that identifies a persistent memory
// location where blob is stored. The URL would work process wide
// and for multiple APIs...

const stream = ... // set up QUIC or HTTP stream somehow...
stream.send(blobs.foo) // references blob URL

// ... eventually...
const in = fs.createReadStream(new URL(blobs.foo));

// ... eventually...
const cipher = await subtle.crypto(alg, key, blob1);

Also see the conversation @mikeal started here: #27080 with regards to immutable buffers and the conversation from @bmeck on support for createObjectURL #16167 ...

[jimmywarting]

When i test if the input i get is a blob or not then i check the name with Symbol.toStringTag if it matches blob

That is currently missing.
you should add that

class Blob {
  [Symbol.toStringTag] = 'Blob'
}

it currently says it's a [object Object] when i call console.log(Object.prototype.toString.call(object))

[jimmywarting]

something else that fails

> new buffer.Blob(['2']).slice(0, 500)
Uncaught:
RangeError [ERR_OUT_OF_RANGE]: The value of "end" is out of range. It must be less than or equal to length. Received 500

This should not happen.
slicing behaves much like string.slice and buffer.slice with negative values and such

currently had to add a own check with Math.min(x, y) to support your buffer.Blob version

just a tip: This is a good references if you want to support slicing over multiple async blob parts.
a slice is not as easy as to just slice one single internal arraybuffer container or updating a offset. Since it can have multiple async blob containers
it can be good to know how blob really works under the hood in some cases here is some of my research of how to build a good Blob class

• use a byte sequence instead of a buffer node-fetch/fetch-blob#40 (comment)
• https://chromium.googlesource.com/chromium/src/+/master/storage/browser/blob/README.md
  • read upon different "Blob system terminology"