BFJ


Big-Friendly JSON. Asynchronous streaming functions for large JSON data sets.

* Why would I want those?
* Is it fast?
* What functions does it implement?
* How do I install it?
* How do I read a JSON file?
* How do I parse a stream of JSON?
* How do I selectively parse individual items from a JSON stream?
* How do I write a JSON file?
* How do I create a stream of JSON?
* How do I create a JSON string?
* What other methods are there?
* bfj.walk (stream, options)
* bfj.eventify (data, options)
* What options can I specify?
* Options for parsing functions
* Options for serialisation functions
* Is it possible to pause parsing or serialisation from calling code?
* Can it break long strings into chunks?
* Can it recursively parse JSON nested inside a JSON string?
* Can it handle newline-delimited JSON (NDJSON)?
* Is there a change log?
* How do I set up the dev environment?
* What versions of Node.js does it support?
* What license is it released under?

Why would I want those?

If you need
to parse huge JSON strings
or stringify huge JavaScript data sets,
it monopolises the event loop
and can lead to out-of-memory exceptions.
BFJ implements asynchronous functions
and uses pre-allocated fixed-length arrays
to try and alleviate those issues.

Is it fast?

No.

BFJ yields frequently
to avoid monopolising the event loop,
interrupting its own execution
to let other event handlers run.
The frequency of those yields
can be controlled with the yieldRate option,
but fundamentally it is not designed for speed.

Furthermore,
when serialising data to a stream,
BFJ uses a fixed-length buffer
to avoid exhausting available memory.
Whenever that buffer is full,
serialisation is paused
until the receiving stream processes some more data,
regardless of the value of yieldRate.
You can control the size of the buffer
using the bufferLength option
but really,
if you need quick results,
BFJ is not for you.

What functions does it implement?

Nine functions
are exported.

Five are
concerned with
parsing, or
turning JSON strings
into JavaScript data:

* read
asynchronously parses
a JSON file from disk.

* parse and unpipe
are for asynchronously parsing
streams of JSON.

* match
selectively parses individual items
from a JSON stream.

* walk
asynchronously walks
a stream,
emitting events
as it encounters
JSON tokens.
Analagous to a
[SAX parser][sax].

The other four functions
handle the reverse transformations,
serialising
JavaScript data
to JSON:

* write
asynchronously serialises data
to a JSON file on disk.

* streamify
asynchronously serialises data
to a stream of JSON.

* stringify
asynchronously serialises data
to a JSON string.

* eventify
asynchronously traverses
a data structure
depth-first,
emitting events
as it encounters items.
By default
it coerces
promises, buffers and iterables
to JSON-friendly values.

How do I install it?

If you're using npm:

``
npm i bfj --save
`

Or if you just want
the git repo:

`
git clone git@gitlab.com:philbooth/bfj.git
`

How do I read a JSON file?

`js
const bfj = require('bfj');

bfj.read(path, options)
.then(data => {
// :)
})
.catch(error => {
// :(
});
`

read returns a promise and
asynchronously parses
a JSON file
from disk.

It takes two arguments;
the path to the JSON file
and an options object.

If there are
no syntax errors,
the returned promise is resolved
with the parsed data.
If syntax errors occur,
the promise is rejected
with the first error.

How do I parse a stream of JSON?

`js
const bfj = require('bfj');

// By passing a readable stream to bfj.parse():
bfj.parse(fs.createReadStream(path), options)
.then(data => {
// :)
})
.catch(error => {
// :(
});

// ...or by passing the result from bfj.unpipe() to stream.pipe():
request({ url }).pipe(bfj.unpipe((error, data) => {
if (error) {
// :(
} else {
// :)
}
}))
`

* parse returns a promise
and asynchronously parses
a stream of JSON data.

It takes two arguments;
a [readable stream][readable]
from which
the JSON
will be parsed
and an options object.

If there are
no syntax errors,
the returned promise is resolved
with the parsed data.
If syntax errors occur,
the promise is rejected
with the first error.

* unpipe returns a [writable stream][writable]
that can be passed to [stream.pipe][pipe],
then parses JSON data
read from the stream.

It takes two arguments;
a callback function
that will be called
after parsing is complete
and an options object.

If there are no errors,
the callback is invoked
with the result as the second argument.
If errors occur,
the first error is passed
the callback
as the first argument.

How do I selectively parse individual items from a JSON stream?

`js
const bfj = require('bfj');

// Call match with your stream and a selector predicate/regex/JSONPath/string
const dataStream = bfj.match(jsonStream, selector, options);

// Get data out of the returned stream with event handlers
dataStream.on('data', item => { / ... / });
dataStream.on('end', () => { / ... /);
dataStream.on('error', () => { / ... /);
dataStream.on('dataError', () => { / ... /);

// ...or you can pipe it to another stream
dataStream.pipe(someOtherStream);
`

match returns a readable, object-mode stream
and asynchronously parses individual matching items
from an input JSON stream.

It takes three arguments:
a [readable stream][readable]
from which the JSON will be parsed;
a selector argument for determining matches,
which may be a string, a regular expression, a JSONPath expression, or a predicate function;
and an options object.

If the selector is a string,
it will be compared to property keys
to determine whether
each item in the data is a match.
If it is a regular expression,
the comparison will be made
by calling the [RegExp test method][regexp-test]
with the property key.
If it is a JSONPath expression,
it must start with $. to identify the root node
and only use child scope expressions for subsequent nodes.
Predicate functions will be called with three arguments:
key, value and depth.
If the result of the predicate is a truthy value
then the item will be deemed a match.

In addition to the regular options
accepted by other parsing functions,
you can also specify minDepth
to only apply the selector
to certain depths.
This can improve performance
and memory usage,
if you know that
you're not interested in
parsing top-level items.

If there are any syntax errors in the JSON,
a dataError event will be emitted.
If any other errors occur,
an error event will be emitted.

How do I write a JSON file?

`js
const bfj = require('bfj');

bfj.write(path, data, options)
.then(() => {
// :)
})
.catch(error => {
// :(
});
`

write returns a promise
and asynchronously serialises a data structure
to a JSON file on disk.
The promise is resolved
when the file has been written,
or rejected with the error
if writing failed.

It takes three arguments;
the path to the JSON file,
the data structure to serialise
and an options object.

How do I create a stream of JSON?

`js
const bfj = require('bfj');

const stream = bfj.streamify(data, options);

// Get data out of the stream with event handlers
stream.on('data', chunk => { / ... / });
stream.on('end', () => { / ... /);
stream.on('error', () => { / ... /);
stream.on('dataError', () => { / ... /);

// ...or you can pipe it to another stream
stream.pipe(someOtherStream);
`

streamify returns a [readable stream][readable]
and asynchronously serialises
a data structure to JSON,
pushing the result
to the returned stream.

It takes two arguments;
the data structure to serialise
and an options object.

If there a circular reference is encountered in the data
and options.circular is not set to 'ignore',
a dataError event will be emitted.
If any other errors occur,
an error event will be emitted.

How do I create a JSON string?

`js
const bfj = require('bfj');

bfj.stringify(data, options)
.then(json => {
// :)
})
.catch(error => {
// :(
});
`

stringify returns a promise and
asynchronously serialises a data structure
to a JSON string.
The promise is resolved
to the JSON string
when serialisation is complete.

It takes two arguments;
the data structure to serialise
and an options object.

What other methods are there?

$3

`js
const bfj = require('bfj');

const emitter = bfj.walk(fs.createReadStream(path), options);

emitter.on(bfj.events.array, () => { / ... / });
emitter.on(bfj.events.object, () => { / ... / });
emitter.on(bfj.events.property, name => { / ... / });
emitter.on(bfj.events.string, value => { / ... / });
emitter.on(bfj.events.number, value => { / ... / });
emitter.on(bfj.events.literal, value => { / ... / });
emitter.on(bfj.events.endArray, () => { / ... / });
emitter.on(bfj.events.endObject, () => { / ... / });
emitter.on(bfj.events.error, error => { / ... / });
emitter.on(bfj.events.dataError, error => { / ... / });
emitter.on(bfj.events.end, () => { / ... / });
`

walk returns an [event emitter][eventemitter]
and asynchronously walks
a stream of JSON data,
emitting events
as it encounters
tokens.

It takes two arguments;
a [readable stream][readable]
from which
the JSON
will be read
and an options object.

The emitted events
are defined
as public properties
of an object,
bfj.events:

* bfj.events.array
indicates that
an array context
has been entered
by encountering
the [ character.

* bfj.events.endArray
indicates that
an array context
has been left
by encountering
the ] character.

* bfj.events.object
indicates that
an object context
has been entered
by encountering
the { character.

* bfj.events.endObject
indicates that
an object context
has been left
by encountering
the } character.

* bfj.events.property
indicates that
a property
has been encountered
in an object.
The listener
will be passed
the name of the property
as its argument
and the next event
to be emitted
will represent
the property's value.

* bfj.events.string
indicates that
a string
has been encountered.
The listener
will be passed
the value
as its argument.

* bfj.events.stringChunk
indicates that
a string chunk
has been encountered
if the stringChunkSize option was set.
The listener
will be passed
the chunk
as its argument.

* bfj.events.number
indicates that
a number
has been encountered.
The listener
will be passed
the value
as its argument.

* bfj.events.literal
indicates that
a JSON literal
(either true, false or null)
has been encountered.
The listener
will be passed
the value
as its argument.

* bfj.events.error
indicates that
an error was caught
from one of the event handlers
in user code.
The listener
will be passed
the Error instance
as its argument.

* bfj.events.dataError
indicates that
a syntax error was encountered
in the incoming JSON stream.
The listener
will be passed
an Error instance
decorated with actual, expected, lineNumber and columnNumber properties
as its argument.

* bfj.events.end
indicates that
the end of the input
has been reached
and the stream is closed.

* bfj.events.endLine
indicates that a root-level newline character
has been encountered in an NDJSON stream.
Only emitted if the ndjson option is set.

If you are using bfj.walk
to sequentially parse items in an array,
you might also be interested in
the [bfj-collections] module.

$3

`js
const bfj = require('bfj');

const emitter = bfj.eventify(data, options);

emitter.on(bfj.events.array, () => { / ... / });
emitter.on(bfj.events.object, () => { / ... / });
emitter.on(bfj.events.property, name => { / ... / });
emitter.on(bfj.events.string, value => { / ... / });
emitter.on(bfj.events.number, value => { / ... / });
emitter.on(bfj.events.literal, value => { / ... / });
emitter.on(bfj.events.endArray, () => { / ... / });
emitter.on(bfj.events.endObject, () => { / ... / });
emitter.on(bfj.events.error, error => { / ... / });
emitter.on(bfj.events.dataError, error => { / ... / });
emitter.on(bfj.events.end, () => { / ... / });
`

eventify returns an [event emitter][eventemitter]
and asynchronously traverses
a data structure depth-first,
emitting events as it
encounters items.
By default it coerces
promises, buffers and iterables
to JSON-friendly values.

It takes two arguments;
the data structure to traverse
and an options object.

The emitted events
are defined
as public properties
of an object,
bfj.events:

* bfj.events.array
indicates that
an array
has been encountered.

* bfj.events.endArray
indicates that
the end of an array
has been encountered.

* bfj.events.object
indicates that
an object
has been encountered.

* bfj.events.endObject
indicates that
the end of an object
has been encountered.

* bfj.events.property
indicates that
a property
has been encountered
in an object.
The listener
will be passed
the name of the property
as its argument
and the next event
to be emitted
will represent
the property's value.

* bfj.events.string
indicates that
a string
has been encountered.
The listener
will be passed
the value
as its argument.

* bfj.events.number
indicates that
a number
has been encountered.
The listener
will be passed
the value
as its argument.

* bfj.events.literal
indicates that
a JSON literal
(either true, false or null)
has been encountered.
The listener
will be passed
the value
as its argument.

* bfj.events.error
indicates that
an error was caught
from one of the event handlers
in user code.
The listener
will be passed
the Error instance
as its argument.

* bfj.events.dataError
indicates that
a circular reference was encountered in the data
and the circular option was not set to 'ignore'.
The listener
will be passed
an Error instance
as its argument.

* bfj.events.end
indicates that
the end of the data
has been reached and
no further events
will be emitted.

What options can I specify?

$3

* options.reviver:
Transformation function,
invoked depth-first
against the parsed
data structure.
This option
is analagous to the
[reviver parameter for JSON.parse][reviver].

* options.yieldRate:
The number of data items to process
before yielding to the event loop.
Smaller values yield to the event loop more frequently,
meaning less time will be consumed by bfj per tick
but the overall parsing time will be slower.
Larger values yield to the event loop less often,
meaning slower tick times but faster overall parsing time.
The default value is 1024.

* options.ndjson:
If set to true,
newline characters at the root level
will be treated as delimiters between
discrete chunks of JSON.
See NDJSON for more information.

* options.stringChunkSize:
For bfj.walk only,
set this to the character count
at which you wish to chunk strings.
Each chunk will be emitted as a bfj.events.stringChunk event,
followed by the regular bfj.events.string event
after all chunks are emitted.

* options.recursive:
For bfj.match only,
set this to true
if you wish to match against recursively JSON-parsed strings.

* options.numbers:
For bfj.match only,
set this to true
if you wish to match against numbers
with a string or regular expression
selector argument.

* options.bufferLength:
For bfj.match only,
the length of the match buffer.
Smaller values use less memory
but may result in a slower parse time.
The default value is 256.

* options.highWaterMark:
For bfj.match only,
set this if you would like to
pass a value for the highWaterMark option
to the readable stream constructor.

$3

* options.space:
Indentation string
or the number of spaces
to indent
each nested level by.
This option
is analagous to the
[space parameter for JSON.stringify][space].

* options.promises:
By default,
promises are coerced
to their resolved value.
Set this property
to 'ignore'
for improved performance
if you don't need
to coerce promises.

* options.buffers:
By default,
buffers are coerced
using their toString method.
Set this property
to 'ignore'
for improved performance
if you don't need
to coerce buffers.

* options.maps:
By default,
maps are coerced
to plain objects.
Set this property
to 'ignore'
for improved performance
if you don't need
to coerce maps.

* options.iterables:
By default,
other iterables
(i.e. not arrays, strings or maps)
are coerced
to arrays.
Set this property
to 'ignore'
for improved performance
if you don't need
to coerce iterables.

* options.circular:
By default,
circular references
will cause the write
to fail.
Set this property
to 'ignore'
if you'd prefer
to silently skip past
circular references
in the data.

* options.bufferLength:
The length of the write buffer.
Smaller values use less memory
but may result in a slower serialisation time.
The default value is 256.

* options.highWaterMark:
Set this if you would like to
pass a value for the highWaterMark option
to the readable stream constructor.

* options.yieldRate:
The number of data items to process
before yielding to the event loop.
Smaller values yield to the event loop more frequently,
meaning less time will be consumed by bfj per tick
but the overall serialisation time will be slower.
Larger values yield to the event loop less often,
meaning slower tick times but faster overall serialisation time.
The default value is 1024.

Is it possible to pause parsing or serialisation from calling code?

Yes it is!
Both walk
and eventify
decorate their returned event emitters
with a pause method
that will prevent any further events being emitted.
The pause method itself
returns a resume function
that you can call to indicate
that processing should continue.

For example:

`js
const bfj = require('bfj');
const emitter = bfj.walk(fs.createReadStream(path), options);

// Later, when you want to pause parsing:

const resume = emitter.pause();

// Then when you want to resume:

resume();
`

Can it break long strings into chunks?

Yes.
If you pass the stringChunkSize option
to bfj.walk,
it will emit a bfj.events.stringChunk event
for each chunk of the string.
The regular bfj.events.string event
will still be emitted
after all the chunks.

Can it recursively parse JSON nested inside a JSON string?

Yes.
If you pass the recursive option
to bfj.match,
it will recursively parse any string values
that satisfy the selector argument.
Note the same selector is applied
to every level of recursion,
so this works best in combination
with selectors that are predicate functions.

Can it handle newline-delimited JSON (NDJSON)?

Yes.
If you pass the ndjson option
to bfj.walk, bfj.match or bfj.parse,
newline characters at the root level
will act as delimiters between
discrete JSON values:

* bfj.walk will emit a bfj.events.endLine event
each time it encounters a newline character.

* bfj.match will just ignore the newlines
while it continues looking for matching items.

* bfj.parse will resolve with the first value
and pause the underlying stream.
If it's called again with the same stream,
it will resume processing
and resolve with the second value.
To parse the entire stream,
calls should be made sequentially one-at-a-time
until the returned promise
resolves to undefined
(undefined is not a valid JSON token).

bfj.unpipe and bfj.read will not parse NDJSON.

Can I specify a different promise implementation?

Yes.
Just pass the Promise option
to any method.
You might want to try this
if you get any out-of-memory errors
when parsing huge files.

Is there a change log?

[Yes][history].

How do I set up the dev environment?

The development environment
relies on [Node.js][node],
[ESLint],
[Mocha],
[Chai],
[Proxyquire] and
[Spooks].
Assuming that
you already have
node and NPM
set up,
you just need
to run
npm install
to install
all of the dependencies
as listed in package.json.

You can
lint the code
with the command
npm run lint.

You can
run the tests
with the command
npm test.

What versions of Node.js does it support?

As of version 8.0.0,
only Node.js versions 18 or greater
are supported.

Between versions 3.0.0
and 6.1.2,
only Node.js versions 6 or greater
were supported.

Until version 2.1.2`,
only Node.js versions 4 or greater
were supported.

What license is it released under?

[MIT][license].

[ci-image]: https://secure.travis-ci.org/philbooth/bfj.png?branch=master
[ci-status]: http://travis-ci.org/#!/philbooth/bfj
[sax]: http://en.wikipedia.org/wiki/Simple_API_for_XML
[bfj-collections]: https://github.com/hash-bang/bfj-collections
[eventemitter]: https://nodejs.org/api/events.html#events_class_eventemitter
[readable]: https://nodejs.org/api/stream.html#stream_readable_streams
[writable]: https://nodejs.org/api/stream.html#stream_writable_streams
[pipe]: https://nodejs.org/api/stream.html#stream_readable_pipe_destination_options
[regexp-test]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/test
[reviver]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/JSON/parse#Using_the_reviver_parameter
[space]: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#The_space_argument
[history]: HISTORY.md
[node]: https://nodejs.org/en/
[eslint]: http://eslint.org/
[mocha]: https://mochajs.org/
[chai]: http://chaijs.com/
[proxyquire]: https://github.com/thlorenz/proxyquire
[spooks]: https://gitlab.com/philbooth/spooks.js
[license]: COPYING