# json-ext
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A set of utilities that extend the use of JSON. Designed to be fast and memory efficient
Features:
- [x] `parseChunked()` – Parse JSON that comes by chunks (e.g. FS readable stream or fetch response stream)
- [x] `stringifyStream()` – Stringify stream (Node.js)
- [x] `stringifyInfo()` – Get estimated size and other facts of JSON.stringify() without converting a value to string
- [ ] **TBD** Support for circular references
- [ ] **TBD** Binary representation [branch](https://github.com/discoveryjs/json-ext/tree/binary)
- [ ] **TBD** WHATWG [Streams](https://streams.spec.whatwg.org/) support
## Install
```bash
npm install @discoveryjs/json-ext
```
## API
- [parseChunked(chunkEmitter)](#parsechunkedchunkemitter)
- [stringifyStream(value[, replacer[, space]])](#stringifystreamvalue-replacer-space)
- [stringifyInfo(value[, replacer[, space[, options]]])](#stringifyinfovalue-replacer-space-options)
- [Options](#options)
- [async](#async)
- [continueOnCircular](#continueoncircular)
- [version](#version)
### parseChunked(chunkEmitter)
Works the same as [`JSON.parse()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/parse) but takes `chunkEmitter` instead of string and returns [Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise).
> NOTE: `reviver` parameter is not supported yet, but will be added in next releases.
> NOTE: WHATWG streams aren't supported yet
When to use:
- It's required to avoid freezing the main thread during big JSON parsing, since this process can be distributed in time
- Huge JSON needs to be parsed (e.g. >500MB on Node.js)
- Needed to reduce memory pressure. `JSON.parse()` needs to receive the entire JSON before parsing it. With `parseChunked()` you may parse JSON as first bytes of it comes. This approach helps to avoid storing a huge string in the memory at a single time point and following GC.
[Benchmark](https://github.com/discoveryjs/json-ext/tree/master/benchmarks#parse-chunked)
Usage:
```js
const { parseChunked } = require('@discoveryjs/json-ext');
// as a regular Promise
parseChunked(chunkEmitter)
.then(data => {
/* data is parsed JSON */
});
// using await (keep in mind that not every runtime has a support for top level await)
const data = await parseChunked(chunkEmitter);
```
Parameter `chunkEmitter` can be:
- [`ReadableStream`](https://nodejs.org/dist/latest-v14.x/docs/api/stream.html#stream_readable_streams) (Node.js only)
```js
const fs = require('fs');
const { parseChunked } = require('@discoveryjs/json-ext');
parseChunked(fs.createReadStream('path/to/file.json'))
```
- Generator, async generator or function that returns iterable (chunks). Chunk might be a `string`, `Uint8Array` or `Buffer` (Node.js only):
```js
const { parseChunked } = require('@discoveryjs/json-ext');
const encoder = new TextEncoder();
// generator
parseChunked(function*() {
yield '{ "hello":';
yield Buffer.from(' "wor'); // Node.js only
yield encoder.encode('ld" }'); // returns Uint8Array(5) [ 108, 100, 34, 32, 125 ]
});
// async generator
parseChunked(async function*() {
for await (const chunk of someAsyncSource) {
yield chunk;
}
});
// function that returns iterable
parseChunked(() => ['{ "hello":', ' "world"}'])
```
Using with [fetch()](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API):
```js
async function loadData(url) {
const response = await fetch(url);
const reader = response.body.getReader();
return parseChunked(async function*() {
while (true) {
const { done, value } = await reader.read();
if (done) {
break;
}
yield value;
}
});
}
loadData('https://example.com/data.json')
.then(data => {
/* data is parsed JSON */
})
```
### stringifyStream(value[, replacer[, space]])
Works the same as [`JSON.stringify()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify), but returns an instance of [`ReadableStream`](https://nodejs.org/dist/latest-v14.x/docs/api/stream.html#stream_readable_streams) instead of string.
> NOTE: WHATWG Streams aren't supported yet, so function available for Node.js only for now
Departs from JSON.stringify():
- Outputs `null` when `JSON.stringify()` returns `undefined` (since streams may not emit `undefined`)
- A promise is resolving and the resulting value is stringifying as a regular one
- A stream in non-object mode is piping to output as is
- A stream in object mode is piping to output as an array of objects
When to use:
- Huge JSON needs to be generated (e.g. >500MB on Node.js)
- Needed to reduce memory pressure. `JSON.stringify()` needs to generate the entire JSON before send or write it to somewhere. With `stringifyStream()` you may send a result to somewhere as first bytes of the result appears. This approach helps to avoid storing a huge string in the memory at a single time point.
- The object being serialized contains Promises or Streams (see Usage for examples)
[Benchmark](https://github.com/discoveryjs/json-ext/tree/master/benchmarks#stream-stringifying)
Usage:
```js
const { stringifyStream } = require('@discoveryjs/json-ext');
// handle events
stringifyStream(data)
.on('data', chunk => console.log(chunk))
.on('error', error => consold.error(error))
.on('finish', () => console.log('DONE!'));
// pipe into a stream
stringifyStream(data)
.pipe(writableStream);
```
Using Promise or ReadableStream in serializing object:
```js
const fs = require('fs');
const { stringifyStream } = require('@discoveryjs/json-ext');
// output will be
// {"name":"example","willSerializeResolvedValue":42,"fromFile":[1, 2, 3],"at":{"any":{"level":"promise!"}}}
stringifyStream({
name: 'example',
willSerializeResolvedValue: Promise.resolve(42),
fromFile: fs.createReadStream('path/to/file.json'), // support file content is "[1, 2, 3]", it'll be inserted as it
at: {
any: {
level: new Promise(resolve => setTimeout(() => resolve('promise!'), 100))
}
}
})
// in case several async requests are used in object, it's prefered
// to put fastest requests first, because in this case
stringifyStream({
foo: fetch('http://example.com/request_takes_2s').then(req => req.json()),
bar: fetch('http://example.com/request_takes_5s').then(req => req.json())
});
```
Using with [`WritableStream`](https://nodejs.org/dist/latest-v14.x/docs/api/stream.html#stream_writable_streams) (Node.js only):
```js
const fs = require('fs');
const { stringifyStream } = require('@discoveryjs/json-ext');
// pipe into a console
stringifyStream(data)
.pipe(process.stdout);
// pipe into a file
stringifyStream(data)
.pipe(fs.createWriteStream('path/to/file.json'));
// wrapping into a Promise
new Promise((resolve, reject) => {
stringifyStream(data)
.on('error', reject)
.pipe(stream)
.on('error', reject)
.on('finish', resolve);
});
```
### stringifyInfo(value[, replacer[, space[, options]]])
`value`, `replacer` and `space` arguments are the same as for `JSON.stringify()`.
Result is an object:
```js
{
minLength: Number, // minimal bytes when values is stringified
circular: [...], // list of circular references
duplicate: [...], // list of objects that occur more than once
async: [...] // list of async values, i.e. promises and streams
}
```
Example:
```js
const { stringifyInfo } = require('@discoveryjs/json-ext');
console.log(
stringifyInfo({ test: true }).minLength
);
// > 13
// that equals '{"test":true}'.length
```
#### Options
##### async
Type: `Boolean`
Default: `false`
Collect async values (promises and streams) or not.
##### continueOnCircular
Type: `Boolean`
Default: `false`
Stop collecting info for a value or not whenever circular reference is found. Setting option to `true` allows to find all circular references.
### version
The version of library, e.g. `"0.3.1"`.
## License
MIT
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