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stream: do not chunk strings and Buffer in Readable.from. #30912

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stream: do not chunk strings and Buffer in Readable.from. #30912

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4 changes: 4 additions & 0 deletions doc/api/stream.md
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Expand Up @@ -1642,6 +1642,10 @@ readable.on('data', (chunk) => {
});
```

Calling `Readable.from(string)` or `Readable.from(buffer)` will not have
the strings or buffers be iterated to match the other streams semantics
for performance reasons.

## API for Stream Implementers

<!--type=misc-->
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12 changes: 12 additions & 0 deletions lib/internal/streams/from.js
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Expand Up @@ -4,13 +4,25 @@ const {
Symbol,
SymbolIterator
} = primordials;
const { Buffer } = require('buffer');

const {
ERR_INVALID_ARG_TYPE
} = require('internal/errors').codes;

function from(Readable, iterable, opts) {
let iterator;
if (typeof iterable === 'string' || iterable instanceof Buffer) {
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Should we check for Stream._isUint8Array as well?

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That would only be okay if we turn off object mode, I think

return new Readable({
objectMode: true,
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@ronag ronag Dec 12, 2019 •
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nit, Not sure of the purpose here, but wouldn't objectMode: false make more sense here as a default? Or is that breaking?

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I would keep it consistent with the rest. Also, it would change the encoding, so possibly it's not a good idea.

...opts,
read() {
this.push(iterable);
this.push(null);
}
});
}

if (iterable && iterable[Symbol.asyncIterator])
iterator = iterable[Symbol.asyncIterator]();
else if (iterable && iterable[SymbolIterator])
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13 changes: 12 additions & 1 deletion test/parallel/test-readable-from.js
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Expand Up @@ -56,13 +56,23 @@ async function toReadablePromises() {
async function toReadableString() {
const stream = Readable.from('abc');

const expected = ['a', 'b', 'c'];
const expected = ['abc'];

for await (const chunk of stream) {
strictEqual(chunk, expected.shift());
}
}

async function toReadableBuffer() {
const stream = Readable.from(Buffer.from('abc'));

const expected = ['abc'];

for await (const chunk of stream) {
strictEqual(chunk.toString(), expected.shift());
}
}

async function toReadableOnData() {
async function* generate() {
yield 'a';
Expand Down Expand Up @@ -154,6 +164,7 @@ Promise.all([
toReadableSyncIterator(),
toReadablePromises(),
toReadableString(),
toReadableBuffer(),
toReadableOnData(),
toReadableOnDataNonObject(),
destroysTheStreamWhenThrowing(),
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