npm package: @streamparser/json-whatwg
Description: Streaming JSON parser in Javascript for Node.js, Deno and the browser
Author: Juanjo Diaz
Latest version: 0.0.22

# @streamparser/json-whatwg

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Fast dependency-free library to parse a JSON stream using utf-8 encoding in Node.js, Deno or any modern browser. Fully compliant with the JSON spec and `JSON.parse(...)`.

*tldr;*

```javascript
import { JSONParser } from '@streamparser/json-whatwg';

const inputStream = new ReadableStream({
  async start(controller) {
    controller.enqueue('{ "test": ["a"] }');
    controller.close();
  },
});

const parser = new JSONParser();
const reader = inputStream.pipeThrough(jsonparser).pipeTo(destinationStream)

// Or manually getting the values

const reader = inputStream.pipeThrough(jsonparser).getReader();
while (true) {
  const { done, value } = await reader.read();
  if (done) break;
  processValue(value);
  // There will be 3 value:
  // "a"
  // ["a"]
  // { test: ["a"] }
}
```

## @streamparser/json ecosystem

There are multiple flavours of @streamparser:

* The **[@streamparser/json](https://www.npmjs.com/package/@streamparser/json)** package allows to parse any JSON string or stream using pure Javascript.
* The **[@streamparser/json-whatwg](https://www.npmjs.com/package/@streamparser/json-whatwg)** wraps `@streamparser/json` into a WHATWG TransformStream.
* The **[@streamparser/json-node](https://www.npmjs.com/package/@streamparser/json-node)** wraps `@streamparser/json` into a node Transform stream.

## Dependencies / Polyfilling

@streamparser/json requires a few ES6 classes:

* [Uint8Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array)
* [TextEncoder](https://developer.mozilla.org/en-US/docs/Web/API/TextEncoder)
* [TextDecoder](https://developer.mozilla.org/en-US/docs/Web/API/TextDecoder)
* [TransformStream](https://developer.mozilla.org/en-US/docs/Web/API/TransformStream)

If you are targeting browsers or systems in which these might be missing, you need to polyfil them.

## Components

### Tokenizer

A JSON compliant tokenizer that parses a utf-8 stream into JSON tokens

```javascript
import { Tokenizer } from '@streamparser/json-whatwg';

const tokenizer = new Tokenizer(opts, writableStrategy, readableStrategy);
```

Writable and readable strategy are standard WhatWG Stream settings (see [MDN](https://developer.mozilla.org/en-US/docs/Web/API/TransformStream/TransformStream)).

The available options are:

```javascript
{
  stringBufferSize: <number>, // set to 0 to don't buffer. Min valid value is 4.
  numberBufferSize: <number>, // set to 0 to don't buffer.
  separator: <string>, // separator between object. For example `\n` for nd-js.
  emitPartialTokens: <boolean> // whether to emit tokens mid-parsing.
}
```

If buffer sizes are set to anything else than zero, instead of using a string to apppend the data as it comes in, the data is buffered using a TypedArray. A reasonable size could be `64 * 1024` (64 KB).

#### Buffering

When parsing strings or numbers, the parser needs to gather the data in-memory until the whole value is ready.

Strings are inmutable in Javascript so every string operation creates a new string. The V8 engine, behind Node, Deno and most modern browsers, performs a many different types of optimization. One of this optimizations is to over-allocate memory when it detects many string concatenations. This increases significatly the memory consumption and can easily exhaust your memory when parsing JSON containing very large strings or numbers. For those cases, the parser can buffer the characters using a TypedArray. This requires encoding/decoding from/to the buffer into an actual string once the value is ready. This is done using the `TextEncoder` and `TextDecoder` APIs. Unfortunately, these APIs creates a significant overhead when the strings are small so should be used only when strictly necessary.

### TokenParser

A token parser that processes JSON tokens as emitted by the `Tokenizer` and emits JSON values/objects.

```javascript
import { TokenParser} from '@streamparser/json-whatwg';

const tokenParser = new TokenParser(opts, writableStrategy, readableStrategy);
```

Writable and readable strategy are standard WhatWG Stream settings (see [MDN](https://developer.mozilla.org/en-US/docs/Web/API/TransformStream/TransformStream)).

The available options are:

```javascript
{
  paths: <string[]>,
  keepStack: <boolean>, // whether to keep all the properties in the stack
  separator: <string>, // separator between object. For example `\n` for nd-js. If left empty or set to undefined, the token parser will end after parsing the first object. To parse multiple object without any delimiter just set it to the empty string `''`.
  emitPartialValues: <boolean>, // whether to emit values mid-parsing.
}
```

* paths: Array of paths to emit. Defaults to `undefined` which emits everything. The paths are intended to suppot jsonpath although at the time being it only supports the root object selector (`$`) and subproperties selectors including wildcards (`$.a`, `$.*`, `$.a.b`, , `$.*.b`, etc). 
* keepStack: Whether to keep full objects on the stack even if they won't be emitted. Defaults to `true`. When set to `false` the it does preserve properties in the parent object some ancestor will be emitted. This means that the parent object passed to the `onValue` function will be empty, which doesn't reflect the truth, but it's more memory-efficient.

### JSONParser

The full blown JSON parser. It basically chains a `Tokenizer` and a `TokenParser`.

```javascript
import { JSONParser } from '@streamparser/json-whatwg';

const parser = new JSONParser();
```

## Usage

You can use both components independently as

```javascript
const tokenizer = new Tokenizer(opts);
const tokenParser = new TokenParser();
const jsonParser = tokenizer.pipeTrough(tokenParser);
```

You can subscribe to the resulting data using the 

```javascript
import { JSONParser } from '@streamparser/json-whatwg';

const inputStream = new ReadableStream({
  async start(controller) {
    controller.enqueue(parser.write('"Hello world!"'));  // will log "Hello world!"
    // Or passing the stream in several chunks
    parser.write('"');
    parser.write('Hello');
    parser.write(' ');
    parser.write('world!');
    parser.write('"');// will log "Hello world!"
    controller.close();
  },
});

const parser = new JSONParser({ stringBufferSize: undefined, paths: ['$'] });
const reader = inputStream.pipeThrough(jsonparser).getReader();
while (true) {
  const { done, value } = await reader.read();
  if (done) break;
  console.log(value);
}
```

Write is always a synchronous operation so any error during the parsing of the stream will be thrown during the write operation. After an error, the parser can't continue parsing.

```javascript
import { JSONParser } from '@streamparser/json-whatwg';

const inputStream = new ReadableStream({
  async start(controller) {
    controller.enqueue(parser.write('"""'));  // will log "Hello world!"
    controller.close();
  },
});
const parser = new JSONParser({ stringBufferSize: undefined });

try {
  const reader = inputStream.pipeThrough(parser).getReader();
  while (true) {
    const { done, value } = await reader.read();
    if (done) break;
    console.log(value);
  }
} catch (err) {
  console.log(err); // logs 
}
```

## Examples

### Stream-parsing a fetch request returning a JSONstream

Imagine an endpoint that send a large amount of JSON objects one after the other (`{"id":1}{"id":2}{"id":3}...`).

```js
  import { JSONParser} from '@streamparser/json-whatwg';

  const parser = new JSONParser();

  const response = await fetch('http://example.com/');
  const reader = response.body.pipeThrough(parser).getReader();
  while(true) {
    const { done, value } = await reader.read();
    if (done) break;
    // TODO process element
  }
```

### Stream-parsing a fetch request returning a JSON array

Imagine an endpoint that send a large amount of JSON objects one after the other (`[{"id":1},{"id":2},{"id":3},...]`).

```js
  import { JSONParser } from '@streamparser/json-whatwg';

  const parser = new JSONParser({ stringBufferSize: undefined, paths: ['$.*'], keepStack: false });

  const response = await fetch('http://example.com/');

  const reader = response.body.pipeThrough(parser).getReader();
  while(true) {
    const { done, value: parsedElementInfo } = await reader.read();
    if (done) break;

    const { value, key, parent, stack } = parsedElementInfo;
    // TODO process element
  }
```

### Stream-parsing a fetch request returning a very long string getting previews of the string

Imagine an endpoint that send a large amount of JSON objects one after the other (`"Once upon a midnight <...>"`).

```js
  import { JSONParser } from '@streamparser/json-whatwg';

  const parser = new JSONParser({ stringBufferSize: undefined, paths: ['$.*'], keepStack: false });

  const response = await fetch('http://example.com/');

  const reader = response.body.pipeThrough(parser).getReader();
  while(true) {
    const { done, value: parsedElementInfo } = await reader.read();
    if (done) break;

    const { value, key, parent, stack, partial } = parsedElementInfo;
    if (partial) {
      console.log(`Parsing value: ${value}... (still parsing)`);
    } else {
      console.log(`Value parsed: ${value}`);
    }
  }
```

## License

See [LICENSE.md](../../LICENSE).

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