through-object-compare

Transform stream that compare each object in the stream with objects in an array of objects to find differences. The intention is to find differences and flag objects we compare as "added", "changed" or "deleted".

Installation

$ npm install through-object-compare

Example

Compare an object stream based on a created and updated timestamp in each object.

const throughObjectCompare = require('through-object-compare'),
	  JSONStream = require('JSONStream')
      fs = require('fs');


// Compare function which compare timestamp fields in each object

const compareFunction = (objOld, objNew) => {
    return objOld.updated === objNew.updated && objOld.created === objNew.created;
};


// The array of objects we compare the stream against

const arrayOfObjects = [{
    uuid : '001',
    created : 1,
    updated : 1
}, {
    uuid : '002',
    created : 2,
    updated : 20
}, {
    uuid : '003',
    created : 3,
    updated : 3
}];


fs.createReadStream('objects.json')
  .pipe(JSONStream.parse('*'))
  .pipe(throughObjectCompare(arrayOfObjects, compareFunction))
  .pipe(JSONStream.stringify())
  .pipe(fs.createWriteStream('comparedObjects.json'));

Description

The intention of this module is to compare two sets of objects and find what is different between them. This is done by holding an array of objects and then check each object in a stream against this array.

The output of the stream will then emit objects which each are flagged as "added", "changed" or "deleted".

The only requirement is that each object we compare has an unique identifier key.

API

This module have the following API:

throughObjectCompare(compareArray, compareFunction)

Creates an instance for comparing.

const throughObjectCompare = require('through-object-compare');
let compare = throughObjectCompare(compareArray, compareFunction);

compareArray (required)

An array of objects to compare the objects in the stream against.

compareFunction

A compare function used to compare two matching objects based on a unique identifier key in the objects. Must return true or false.

The first argument parameter on the function is an object from the compareArray. The second argument parameter to the function is the current object in the stream.

let compare = throughObjectCompare(compareArray, (objOld, objNew) => {
    return objOld.updated === objNew.updated && objOld.created === objNew.created;
});

Events

through-object-compare expose expose the following events.

append

Emitted when a object occur in the stream which are not in the array of objects compared against.

compare.on('append', function (obj) {
    console.log('appended', obj);
});

change

Emitted when a object occur in the stream which also are in the array of objects compared against but they differ from each other.

compare.on('change', function (streamObj, compareObj) {
    console.log('changed', streamObj, compareObj);
});

delete

Emitted on objects which are in the array of objects compared against but not in the stream.

compare.on('delete', function (obj) {
    console.log('deleted', obj);
});

equal

Emitted for objects in the stream which are equal in the array of objects compared against.

compare.on('equal', function (streamObj, compareObj) {
    console.log('equal', streamObj, compareObj);
});

node.js compabillity

This module use some native ES6 functions only found in node.js 4.x and newer. This module will not function with older than 4.x versions of node.js.

error handling

This module does not handle errors for you, so you must handle errors on whatever streams you pipe into this module. This is a general rule when programming with node.js streams: always handle errors on each and every stream.

We recommend using end-of-stream or pump for writing error tolerant stream code.

License

The MIT License (MIT)

Copyright (c) 2015 - Trygve Lie - [email protected]

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.