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JSON Context

This module allows a server to create a JSON Context - single object that supports querying and contains all data required to render a view/page. When sent to the client it also provides an event stream for syncing with server and data-binding.

The idea is it works in a similar way to your database (well depends on the database), using a update whole object at once (i.e. row/document based) aproach. Any changes that come off our database can be streamed straight in and it will figure out what local objects and fields to update, and notify that the object has been changed.

It's mostly a wrapper around JSON Query adding the ability to build contexts with JSON queries and stream changes.

It is intended to be used in conjunction with Realtime Templates however can be used standalone if that's what you're into.


$ npm install json-context


See Realtime Templates and ContextDB for example usage.



Returns event emitting datasource.


  • data: optional starting object (for params, or when deserializing existing context)
  • dataFilters: passed to the inner instance of JSON Query
  • matchers: route incoming objects to queries, and verify change is allowed - see Matchers

datasource.pushChange(object, changeInfo)

Pushes an object into the datasource using the specified matchers to decide where it should go. If the object already existed in the datasource (as decided by the matcher), it will be updated to match the attributes of the object being pushed in.

Because of this, if you have a reference to the original object in the datasource (say when you are binding to it), the reference will still work after the new version has been pushed in. This allows the complete object to be bounced around the intertubes or wherever and only update existing objects rather than creating new references all the time.

Returns an array detailing the changes.

object: The complete object that has been changed, created, or deleted.

changeInfo options:

  • verifiedChange: optional - set to true if you want to override the validation/permission checking. Good for syncing with trusted sources such as a primary database server.
  • any other metadata that we may want to access further down stream - it will be emitted with the change event.

Browser Changes

If you want to update an object in the browser with a form for example, you must first obtain a copy of the object you wish to change. You can use datasource.obtain(query) to do this, or clone an object using datasource.obtain(element.templateContext.source). Once you have this copy, make the desired changes, then push back in using datasource.pushChange(changedObject, {source: 'user'}). It will check the matcher to ensure the change they have requested is allowed.

To delete an object, obtain in the same way as changing, but add the key _deleted with the value true.

var object = window.context.obtain(['comments[][id=?]', 1]) // get a copy of the object 
object._deleted = true
window.context.pushChange(object, {source: 'user'})

If you want to append a new object, just push it directly. As long as it has attributes corresponding to the matcher, everything should just work.

datasource.query(query, localContext, options)

Queries the context using JSON Query and returns an object representing the result and other useful info (especially when doing databinding)

localContext: (optional) specifiy a target for . queries to get their data. This is used when matcher queries execute (e.g. item) and is set to the new object being pushed and great when using repeaters when template/data binding.

options: additional options to be passed to the inner JSON Query.

Returns an object with the following keys:

  • value: the result of the query
  • parents: a list of parent objects and the keys that lead to the next layer
  • references: an array of objects that if changed would invalidate the result of the query - we can use this to add binding metadata.
  • key: the array index or key of the resulting value

datasource.get(query, localContext, options)

A shortcut for datasource.query(...).value - exactly the same, but only returns the value of the query, not the other info.


Returns a duplex stream of line delimited json encoded text. Can be used to pipe changes between multiple JSON Context datasources, such as those created by ContextDB.

Only changes not originating from this stream will be sent, so no chance of feedback. Multiple streams can be created and piped around the place to allow all kinds of crazy peer based syncing.

defaultChangeInfo: (optional) see datasource.pushChange(object, changeInfo)

datasource.on('change', function(object, changeInfo))

The datasource emits a change event every time a pushed object is matched. This can be used to determine when to update bound elements, etc.


  • action: append, update, or remove
  • changes: the root key/values on the object that were changed
  • original: the state of the object before it was changed
  • collection: the parent object
  • key: the objects key in the collection
  • matcher: The instance of the matcher that allowed this change to come through
  • any other fields set when pushing the data in, cb)

Pass in a JSON Filter to match objects against, and any time an object changes that matches the filter, the callback will be fired.

Returns a function that when called removes watcher.

Helper Methods

Some handy functions that get stuff done, fast.

datasource.obtain(queryOrObject, localContext)

Obtains a deep copy of the result of the query, or if object passed in, deep clones it.


Returns an array of matchers that accept the object specified.


Returns details of the specified objects siblings {previous, next}.

datasource.update(queryOrObject, changes, changeInfo)

Does a datasource.obtain, merges in the specified changes then pushes it back using datasource.pushChange. Easy one line updates - good for console use.


Matchers are a collection of filters and queries that explain what to do with incoming objects.

  • ref: give the matcher a unique name so it can be refered to (optional)
  • item: a query specifying how to find existing object and where to put the object if no collection was specified.
  • collection: a query specifying the collection the object will be added to (optional)
  • collectionKey: Specify a query to use to generate the objects key if the collection is not an array, but rather an object - good for building lookups (optional)
  • match: This filter is checked using JSON Filter to see if resposible for the object or not.
  • allow: (all optional) - queries to check using datasource.get with changeInfo as the input. These will be bypassed if verifiedChange is set to true
    • change: All queries must return true for all types of changes.
    • append: If the object is being appended, only allow if the specified queries return true.
    • update: If the object is being updated (an original was found), only allow if the specified queries return true.
    • remove: If the object is being updated (has _deleted: true), only allow if the specified queries return true.

Here's a simple matcher that will save any incoming object with the ID of 'abc123' and the type 'post' into the key 'current_post'.

  match: {
    id: 'abc123',
    type: 'post'
  item: 'current_post'

This one stores a collection of all users. If a new user is pushed in, will be stored, if an existing user is passed in, the original will be updated to match the attributes of the object.

  match: {
    type: 'user'
  item: 'users[id={.id}]',
  collection: 'users'

To make things a little more interesting, this example groups tasks by heading_id, and allows tasks to be added, updated, removed, or moved to another heading, but only allows the user to specify certain fields and requires them to assign their own user_id (we'll use our own custom allow queries to do this).

var filters = {
  allowChange: function(input, params){
    if (input.action === 'remove'){
      return true
    } else {
      return check(input, {
        equal: {user_id:},
        changes: ['optional_field'],
        required: ['heading_id', 'description'],
        appendChanges: ['new_item_field']
var data = {
  current_user_id: 123
var matchers = [
  { match: {
      type: 'task'
    allow: {
      change: ':allowChange'
    item: 'tasks_by_heading[][id={.id}]',
    collection: 'tasks_by_heading[{.heading_id}]'
var context = jsonContext(data, {dataFilters: filters, matchers: matchers})
function check(changeInfo, options){
  var changeKeys = [].concat(options.changes).concat(options.required)
  if (options.required && !checkRequired(options.required, changeInfo)){
    return false
  if (options.changes && !checkChanges(changeKeys, changeInfo, options.appendChanges)){
    return false
  if (options.equal && !checkEqual(options.equal, changeInfo)){
    return false
  return true
function checkRequired(required, changeInfo){
  return required.every(function(key){
    return !!changeInfo.object[key]
function checkChanges(allowed, changeInfo, appendExceptions){
  return Object.keys(changeInfo.changes).every(function(key){
    return !!~allowed.indexOf(key) || (appendExceptions && !!~appendExceptions.indexOf(key))
function checkEqual(equal, changeInfo){
  return Object.keys(equal).every(function(key){
    return changeInfo.object[key] == equal[key]


The best way to handle data binding with JSON Context is using $meta attributes on the objects, then linking back to the object from the dom-node.

When objects are updated, JSON Context ignores any attribute that starts with $ and will leave in place. What this means is you can use them for storing metadata about an object. Even after the object is updated by pushChange the meta data will still be there.

The only way a '$' key can get lost is if the item is removed. Makes them great for storing binding info.

Basic example (don't try this at home variety)

(instead use Realtime Templates)

var datasource = require('json-context')({
  comments: [
    {id: 1, type: 'comment', name: 'Matt', body: 'Hello test 123'}
}, {
  matchers: [
      match: {type: 'comment'},
      allow: {
        update: true,
        append: true
      item: 'comments[id={.id}]',
      collection: 'comments'
  dataFilters: {}
datasource.on('change', function(object, changeInfo){
  if (changeInfo.action === 'update'){
    object.$boundElements && object.$boundElements.forEach(function(element){
      var queryResult = datasource.query(element.getAttribute('data-bind'))
      element.innerHTML = escapeHtml(queryResult.value)
  } else if (changeInfo.action === 'remove'){
  } else if (changeInfo.action === 'update'){
var elementsToBind = document.querySelectorAll('[data-bind]')
  var queryResult = element.getAttribute('data-bind')
  element.innerHTML = escapeHtml(queryResult.value)
    reference.$boundElements = reference.$boundElements || []