stream-json

What is stream-json?

The stream-json package is a Node.js library that provides a stream interface for parsing and stringifying JSON data. It allows for processing large JSON files or streams in a memory-efficient and time-efficient manner by working with JSON data in chunks rather than loading the entire file into memory.

What are stream-json's main functionalities?

Parsing JSON

Stream-json can parse JSON files of any size by breaking them down into smaller chunks and processing them one by one. This is particularly useful for working with large JSON files that cannot be loaded into memory all at once.

{"_readableState":{"objectMode":true},"readable":true,"_events":{},"_eventsCount":1}

Stringifying JSON

The package can also stringify large JSON objects by converting them into a stream of data. This allows for efficient writing of JSON data to a file or over the network.

{"_readableState":{"objectMode":true},"readable":true,"_events":{},"_eventsCount":1}

Filters and Transforms

Stream-json provides filters and transformation tools to selectively process and modify JSON data as it is being streamed. This can be used to extract or transform specific parts of the JSON data without having to manipulate the entire dataset.

{"_readableState":{"objectMode":true},"readable":true,"_events":{},"_eventsCount":1}

Other packages similar to stream-json

JSONStream

JSONStream is a package similar to stream-json that offers streaming JSON.parse and stringify. It is widely used and has a simple API, but stream-json provides a more modular approach with plugins and a richer set of features for filtering and transforming data.

big-json

big-json provides similar functionality for parsing and stringify large JSON files. It uses a streaming approach to handle large files, but stream-json has a more comprehensive set of tools for dealing with streams and allows for more complex processing pipelines.

README

stream-json

stream-json is a collection of node.js stream components for creating custom standard-compliant JSON processors, which requires a minimal memory footprint. It can parse JSON files far exceeding available memory. Even individual primitive data items (keys, strings, and numbers) can be streamed piece-wise. Streaming SAX-inspired event-based API is included as well.

Available components:

• Streaming JSON parsers:
  • Streaming JSON Parser implemented manually to improve speed over ClassicParser.
  • Streaming JSON ClassicParser based on parser-toolkit.
  • Streaming JSON AltParser is manually implemented based on RegExp.
• Streamer, which converts tokens into SAX-like event stream.
• Packer, which can assemble numbers, strings, and object keys from individual chunks. It is useful, when user knows that individual data items can fit the available memory. Overall, it makes the API simpler.
• Filter, which is a flexible tool to select only important sub-objects using either a regular expression, or a function.
• Emitter, which converts an event stream into events by bridging stream.Writable with EventEmitter.
• Source, which is a helper that connects streams using pipe() and converts an event stream on the end of pipe into events, similar to Emitter.
• Various utilities:
  • Assembler to assemble full objects from an event stream.
  • StreamArray handles a frequent use case: a huge array of relatively small objects similar to Django-produced database dumps. It streams array components individually taking care of assembling them automatically.
  • StreamFilteredArray is a companion for StreamArray. The difference is that it allows to filter out unneeded objects in an efficient way without assembling them fully.
  • FilterObjects filters complete objects and primitives.

Additionally a helper function is available in the main file, which creates a Source object with a default set of stream components.

This toolkit is distributed under New BSD license.

See the full documentation below.

Introduction

The simplest example (streaming from a file):

var createSource = require("stream-json");

var fs = require("fs");

var source = createSource();

var objectCounter = 0;

source.on("startObject", function(){ ++objectCounter; });

source.on("end", function(){
    console.log("Found ", objectCounter, " objects.");
});

fs.createReadStream("sample.json").pipe(source.input);

Installation

npm install stream-json

Documentation

Parser

This is the workhorse of the package. It is a Transform stream, which consumes text, and produces a stream of tokens. It is always the first in a pipe chain being directly fed with a text from a file, a socket, the standard input, or any other text stream. Its Writeable part operates in a buffer mode, while its Readable part operates in an objectMode.

var Parser = require("stream-json/Parser");
var parser = new Parser(options);

// Example of use:
var next = fs.createReadStream(fname).pipe(parser);

options can contain some technical parameters, and it is rarely needs to be specified. You can find it thoroughly documented in node.js' Stream documentation.

The test files for Parser: tests/test_parser.js, tests\manual\test_parser.js. Actually all test files in tests/ use Parser.

If you want to catch parsing errors, attach an error listener directly to a parser component — unlike data errors do not travel through stream pipes.

Streamer

Streamer is a Transform stream, which consumes a stream of tokens, and produces a stream of events. It is always the second in a pipe chain after the Parser. It knows JSON semantics and produces actionable events. It operates in an objectMode.

var Streamer = require("stream-json/Streamer");
var streamer = new Streamer(options);

// Example of use:
var next = fs.createReadStream(fname).
                pipe(parser).pipe(streamer);

options can contain some technical parameters, and it is rarely needs to be specified. You can find it thoroughly documented in node.js' Stream documentation.

Following is a list of all event objects produced by Streamer:

{name: "startObject"};
{name: "endObject"};

{name: "startArray"};
{name: "endArray"};

{name: "startKey"};
{name: "stringChunk", value: "actual string value"};
{name: "endKey"};

{name: "startString"};
{name: "stringChunk", value: "actual string value"};
{name: "endString"};

{name: "startNumber"};
{name: "numberChunk", value: "actual string value"};
{name: "endNumber"};

{name: "nullValue", value: null};
{name: "trueValue", value: true};
{name: "falseValue", value: false};

The event stream is well-formed:

• All startXXX are balanced with endXXX.
• Between startKey and endKey can be zero or more stringChunk events. No other event are allowed.
• Between startString and endString can be zero or more stringChunk events. No other event are allowed.
• Between startNumber and endNumber can be one or more numberChunk events. No other event are allowed.
  • All number chunks combined constitute a valid number value.
  • Number chunk values are strings, not numbers!
• After startObject optional key-value pairs emitted in a strict pattern: a key-related events, a value, and this cycle can be continued until all key-value pairs are streamed.

The test files for Streamer: tests/test_streamer.js and tests/manual/test_streamer.js.

Packer

Packer is a Transform stream, which passes through a stream of events, optionally assembles keys, strings, and/or numbers from chunks, and adds new events with assembled values. It is a companion for Streamer, which frees users from implementing the assembling logic, when it is known that keys, strings, and/or numbers will fit in the available memory. It operates in an objectMode.

var Packer = require("stream-json/Packer");
var packer = new Packer(options);

// Example of use:
var next = fs.createReadStream(fname).
                pipe(parser).pipe(streamer).pipe(packer);

options contains some important parameters, and should be specified. It can contain some technical properties thoroughly documented in node.js' Stream documentation. Additionally it recognizes following properties:

• packKeys can be true or false (the default). If true, a key value is returned as a new event:

  {name: "keyValue", value: "assembled key value"}
  

  keyValue event always follows endKey.

• packStrings can be true or false (the default). If true, a string value is returned as a new event:

  {name: "stringValue", value: "assembled string value"}
  

  stringValue event always follows endString.

• packNumbers can be true or false (the default). If true, a number value is returned as a new event:

  {name: "numberValue", value: "assembled number value"}
  

  numberValue event always follows endNumber. value of this event is a string, not a number. If user wants to convert it to a number, they can do it themselves. The simplest way to do it (assuming your platform and JavaScript can handle it), is to force it to a number:

  var n = +event.value;
  

The test files for Packer: tests/test_packer.js and tests/manual/test_packer.js.

Emitter

Emitter is a Writeable stream, which consumes a stream of events, and emits them on itself (all streams are instances of EventEmitter). The standard finish event is used to indicate the end of a stream. It operates in an objectMode.

var Emitter = require("stream-json/Emitter");
var emitter = new Emitter(options);

// Example of use:

emitter.on("startArray", function(){
    console.log("array!");
});
emitter.on("numberValue", function(value){
    console.log("number:", value);
});
emitter.on("finish", function(){
    console.log("done");
});

fs.createReadStream(fname).
    pipe(parser).pipe(streamer).pipe(packer).pipe(emitter);

options can contain some technical parameters, and it is rarely needs to be specified. You can find it thoroughly documented in node.js' Stream documentation.

The test file for Emitter: tests/test_emitter.js.

Filter

Filter is a Transform stream, which is an advance selector for sub-objects from a stream of events. It operates in an objectMode.

var Filter = require("stream-json/Filter");
var filter = new Filter(options);

// Example of use:
var next = fs.createReadStream(fname).
                pipe(parser).pipe(streamer).pipe(filter);

options contains some important parameters, and should be specified. It can contain some technical properties thoroughly documented in node.js' Stream documentation. Additionally it recognizes following properties:

• separator is a string to use to separate key and index values forming a path in a current object. By default it is . (a dot).
• filter can be a regular expression, or a function. By default it allows all events.
  • If it is a function, this function is called in a context of a Filter object with two parameters:
    • path, which is an array of current key and index values. All keys are represented as strings, while all array indices are represented as numbers. It can be used to understand what kind of object we are dealing with.
    • event is an event object described above. The function should return a Boolean value, with true indicating that we are interested in this event, and it should be passed through.
  • If it is a regular expression, then a current path is joined be a separator and tested against the regular expression. If a match was found, it indicates that the event should be passed through. Otherwise it will be rejected.

Filter produces a well-formed event stream.

The test files for Filter: tests/test_filter.js and tests/manual/test_filter.js.

Path examples

Given a JSON object:

{"a": [true, false, 0, null]}

The path of false as an array:

["a", 1]

The same path converted to a string joined by a default separator .:

"a.1"

Source

Source is a convenience object. It connects individual streams with pipes, and attaches itself to the end emitting all events on itself (just like Emitter). The standard end event is used to indicate the end of a stream. It is based on EventEmitter.

var Source = require("stream-json/Source");
var source = new Source([parser, streamer, packer]);

// Example of use:

source.on("startArray", function(){
    console.log("array!");
});
source.on("numberValue", function(value){
    console.log("number:", value);
});

fs.createReadStream(fname).pipe(source.input);

The constructor of Source accepts one mandatory parameter:

• streams should be a non-empty array of pipeable streams. At the end the last stream should produce a stream of events.

Source exposes three public properties:

• streams — an array of streams so you can inspect them individually, if needed. They are connected sequentially in the array order.
• input — the beginning of a pipeline, which should be used as an input for a JSON stream.
• output — the end of a pipeline, which can be used to pipe the resulting stream of objects for futher processing.

The test files for Source: tests/test_source.js and tests/manual/test_source.js.

main: createSource()

The main file contains a helper function, which creates a commonly used configuration of streams, and returns a Source object.

var createSource = require("stream-json");
var source = createSource(options);

// Example of use:

source.on("startArray", function(){
    console.log("array!");
});
source.on("numberValue", function(value){
    console.log("number:", value);
});

fs.createReadStream(fname).pipe(source.input);

options can contain some technical parameters, and it is completely optional. You can find it thoroughly documented in node.js' Stream documentation, and here. It is passed to Parser, Streamer, and Packer, so user can specify options documented for those objects.

Algorithm:

1. createSource() creates instances of Parser and Streamer, and pipes them one after another.
2. Then it checks if either of packKeys, packStrings, or packNumbers are specified in options.
   1. If any of them are true, a Packer instance is created with options, and added to the pipe.
   2. If all of them are unspecified, all pack flags are assumed to be true, and a Packer is created and added.
   3. If any of them are specified, yet all are false, Packer is not added.

The most common use case is to call createSource() without parametrs. In this case instances of Parser, Streamer, and Packer are piped together. This scenario assumes that all key, string, and/or number values can be kept in memory, so user can use simplified events keyValue, stringValue, and numberValue.

The test files for createSource() are tests/test_source.js, tests/manual/test_main.js, and tests/manual/test_chunk.js.

ClassicParser

It is a drop-in replacement for Parser, but it can emit whitespace, yet it is slower than the main parser. It was the main parser for 0.1.x versions.

The test file for ClassicParser: tests/test_classic.js.

AltParser

It is another drop-in replacement for Parser, which completely skips whitespace. It is generally faster than ClassicParser, but can be slower than the main parser. It was the main parser for 0.2.x versions.

In general, its speed depends heavily on the implementation of regular expressions by node.js. When node.js has switched from an interpreted regular expressions, to the JIT compiled ones, both ClassicParser, and AltParser got a nice performance boost. Yet, even the latest (as of 0.12) JIT compiler uses a simple yet non-linear algorithm to implement regular expressions instead of NFA and/or DFA. Future enhancements to node.js would make RegExp-based parsers faster, potentially overtaking manually written JavaScript-only implementations.

The test file for AltParser: tests/test_alternative.js.

utils/Assembler

A helper class to convert a JSON stream to a fully assembled JS object. It can be used to assemble sub-objects.

var createSource = require("stream-json");
var Assembler    = require("stream-json/utils/Assembler");

var source    = createSource(options),
    assembler = new Assembler();

// Example of use:

source.output.on("data", function(chunk){
  assembler[chunk.name] && assembler[chunk.name](chunk.value);
});
source.output.on("end", function(){
  // here is our fully assembled object:
  console.log(assembler.current);
});

fs.createReadStream(fname).pipe(source.input);

Assembler is a simple state machine with an explicit stack. It exposes three properties:

• current — an object we are working with at the moment. It can be either an object or an array.
  • Initial value is null.
  • If top-level object is a primitive value (null, true, false, a number, or a string), it will be placed in current too.
• key — is a key value (a string) for a currently processed value, or null, if not expected.
  • If current is an object, a primitive value will be added directly to it using a current value of key.
  • After use key is assigned null to prevent memory leaks.
  • If current is an array, a primitive value will be added directly to it by push().
• stack — an array of parent objects.
  • stack always grows/shrinks by two items: a value of current and a value of key.
  • When an object or an array is closed, it is added to its parent, which is removed from the stack to become a current object again.
  • While adding to a parent a saved key is used if needed. Otherwise the second value is ignored.
  • When an object or an array is started, the current object and key are saved to stack.

Obviously Assembler should be used only when you are sure that the result will fit into memory. It automatically means that all primitive values (strings or numbers) are small enough to fit in memory too. As such Assembler is meant to be used after Packer, which reconstructs keys, strings, and numbers from possible chunks.

On the other hand, we use stream-json when JSON streams are big, and JSON.parse() is not an option. But we use Assembler to assemble sub-objects. One way to do it is to start directing calls to Assembler when we already selected a sub-object with Filter. Another way is shown in StreamArray.

The test file for Assembler: tests/test_assembler.js.

utils/StreamArray

This utility deals with a frequent use case: our JSON is an array of various sub-objects. The assumption is that while individual array items fit in memory, the array itself does not. Such files are frequently produced by various database dump utilities, e.g., Django's dumpdata.

It is a Transform stream, which opertes in an objectMode.

StreamArray produces a stream of objects in following format:

{index, value}

Where index is a numeric index in the array starting from 0, and value is a corresponding value. All objects are produced strictly sequentially.

var createSource = require("stream-json");
var StreamArray  = require("stream-json/utils/StreamArray");

var source = createSource(options),
    stream = StreamArray.make();

// Example of use:

stream.output.on("data", function(object){
  console.log(object.index, object.value);
});
stream.output.on("end", function(){
  console.log("done");
});

fs.createReadStream(fname).pipe(stream.input);

StreamArray is a constructor, which optionally takes one object: options. options can contain some technical parameters, and it is rarely needs to be specified. You can find it thoroughly documented in node.js' Stream documentation.

Directly on StreamArray there is a class-level helper function make(), which helps to construct a proper pipeline. It is similar to createSource() and takes the same argument options. Internally it creates and connects Parser, Streamer, Packer, and StreamArray, and returns an object with three properties:

• streams — an array of streams so you can inspect them individually, if needed. They are connected sequentially in the array order.
• input — the beginning of a pipeline, which should be used as an input for a JSON stream.
• output — the end of a pipeline, which can be used for events, or to pipe the resulting stream of objects for futher processing.

The test file for StreamArray: tests/test_array.js.

utils/StreamFilteredArray

This utility handles the same use case as StreamArray, but in addition it allows to check the objects as they are being built to reject, or accept them. Rejected objects are not assembled, and filtered out.

It is a Transform stream, which opertes in an objectMode.

Just like StreamArray, StreamFilteredArray produces a stream of objects in following format:

{index, value}

Where index is a numeric index in the array starting from 0, and value is a corresponding value. All objects are produced strictly sequentially.

var createSource = require("stream-json");
var StreamFilteredArray = require("stream-json/utils/StreamFilteredArray");

function f(assembler){
  // test only top-level objects in the array:
  if(assembler.stack.length == 2 && assembler.key === null){
    // make a decision depending on a boolean property "active":
    if(assembler.current.hasOwnProperty("active")){
      // "true" to accept, "false" to reject
      return assembler.current.active;
    }
  }
  // return undefined to indicate our uncertainty at this moment
}

var source = createSource(options),
    stream = StreamFilteredArray.make({objectFilter: f});

// Example of use:

stream.output.on("data", function(object){
  console.log(object.index, object.value);
});
stream.output.on("end", function(){
  console.log("done");
});

fs.createReadStream(fname).pipe(stream.input);

StreamFilteredArray is a constructor, which optionally takes one object: options. options can contain some technical parameters, which are rarely needs to be specified. You can find it thoroughly documented in node.js' Stream documentation. But additionally it recognizes the following property:

• objectFilter is a function, which takes an Assembler instance as its only argument, and may return following values to indicate its decision:
  • any truthy value indicates that we are interested in this object. StreamFilteredArray will stop polling our filter function and will assemble the object for future use.
  • false (the exact value) indicates that we should skip this object. StreamFilteredArray will stop polling our filter function, and will stop assembling the object, discarding it completely.
  • any other falsy value indicates that we have not enough information (most likely because the object was not assembled yet to make a decision). StreamFilteredArray will poll our filter function next time the object changes.

The default for objectFilter allows passing all objects.

In general objectFilter is called on incomplete objects. It means that if a decision is based on a value of a certain properties, those properties could be unprocessed at that moment. In such case it is reasonable to delay a decision by returning a falsy (but not false) value, like undefined.

Complete objects are not submitted to a filter function and accepted automatically. It means that all primitive values: booleans, numbers, strings, null objects are streamed, and not consulted with objectFilter.

If you want to filter out complete objects, including primitive values, use FilterObjects.

StreamFilteredArray instances expose one property:

• objectFilter is a function, which us called for every top-level streamable object. It can be replaced with another function at any time. Usually it is replaced between objects after an accept/reject decision is made.

Directly on StreamFilteredArray there is a class-level helper function make(), which is an exact clone of StreamArray.make().

The test file for StreamFilteredArray: tests/test_filtered_array.js.

utils/FilterObjects

This utility filters out complete objects (and primitive values) working with a stream in the same format as StreamArray and StreamFilteredArray:

{index, value}

Where index is a numeric index in the array starting from 0, and value is a corresponding value. All objects are produced strictly sequentially.

It is a Transform stream, which opertes in an objectMode.

var createSource  = require("stream-json");
var StreamArray   = require("stream-json/utils/StreamArray");
var FilterObjects = require("stream-json/utils/FilterObjects");

function f(item){
  // accept all odd-indexed items, which are:
  // true objects, but not arrays, or nulls
  if(item.index % 2 && item.value &&
      typeof item.value == "object" &&
      !(item.value instanceof Array)){
    return true;
  }
  return false;
}

var source = createSource(options),
    stream = StreamArray.make(),
    filter = new FilterObjects({itemFilter: f});

// Example of use:

stream.output.on("data", function(object){
  console.log(object.index, object.value);
});
stream.output.on("end", function(){
  console.log("done");
});

fs.createReadStream(fname).pipe(stream.input).pipe(filter);

FilterObjects is a constructor, which optionally takes one object: options. options can contain some technical parameters, which are rarely needs to be specified. You can find it thoroughly documented in node.js' Stream documentation. But additionally it recognizes the following property:

• itemFilter is a function, which takes a {index, value} object as its only argument, and may return following values to indicate its decision:
  • any truthy value to accept the object.
  • any falsy value to reject the object.

The default for itemFilter accepts all objects.

FilterObjects instances expose one property:

• itemFilter is a function, which us called for every top-level streamable object. It can be replaced with another function at any time.

The test file for FilterObjects: tests/test_filter_objects.js.

Advanced use

The whole library is organized as set of small components, which can be combined to produce the most effective pipeline. All components are based on node.js streams, and events. They implement all require standard APIs. It is easy to add your own components to solve your unique tasks.

The code of all components are compact and simple. Please take a look at their source code to see how things are implemented, so you can produce your own components in no time.

Obviously, if a bug is found, or a way to simplify existing components, or new generic components are created, which can be reused in a variety of projects, don't hesitate to open a ticket, and/or create a pull request.

Credits

The test file tests/sample.json.gz is a combination of several publicly available datasets merged and compressed with gzip:

• a snapshot of publicly available Japanese statistics on birth and marriage in JSON).
• a snapshot of publicly available US Department of Housing and Urban Development - HUD's published metadata catalog (Schema Version 1.1).
• a small fake sample made up by me featuring non-ASCII keys, non-ASCII strings, and primitive data missing in other two samples.

Apendix A: tokens

Parser produces a stream of tokens cortesy of parser-toolkit. While normally user should use Streamer to convert them to a much simpler JSON-aware event stream, in some cases it can be advantageous to deal with raw tokens.

Each token is an object with following properties:

• id is a string, which uniquely identifies a token.
• value is a string, which corresponds to this token, and was actually matched.
• line is a line number, where this token was found. All lines are counted from 1.
• pos is a position number inside a line (in characters, so \t is one character). Position is counted from 1.

JSON grammar is defined in Grammar.js. It is taken almost verbatim from JSON.org.

Following tokens are produced (listed by id):

• ws: white spaces, usually ignored. (Produced only by ClassicParser.)
• -: a unary negation used in a negative number either to start a number, or as an exponent sign.
• +: used as an exponent sign.
• 0: zero, as is - '0'.
• nonZero: non-zero digit - /[1-9]/.
• .: a decimal point used in a number.
• exponent: 'e' or 'E' as an exponent symbol in a number written in scientific notation.
• numericChunk: a string of digits.
• ": a double quote, used to open and close a string.
• plainChunk: a string of non-escaped characters, used inside a string.
• escapedChars: an escaped character, used inside a string.
• true: represents a literal true.
• false: represents a literal false.
• null: represents a literal null.
• {: starts an object literal.
• }: closes an object literal.
• [: starts an array literal.
• ]: closes an array literal.
• ,: separates components of an array, or an object.
• :: separates a key and its value in an object literal.

Release History

• 0.2.2 refreshed dependencies.
• 0.2.1 added utilities to filter objects on the fly.
• 0.2.0 new faster parser, formal unit tests, added utilities to assemble objects on the fly.
• 0.1.0 bug fixes, more documentation.
• 0.0.5 bug fixes.
• 0.0.4 improved grammar.
• 0.0.3 the technical release.
• 0.0.2 bug fixes.
• 0.0.1 the initial release.