Custom Extension


jsreport is built with extensibility in mind since the very start. The whole platform is build from many independent pieces called extensions which can be plugged or unplugged as needed. There is for example an extension allowing to embed static assets or an extension scheduling reports. The extensions are written the same as the whole platform in javascript. This article describes how you can write and plug in your custom extension. Such an extension can be then easily distributed through npm and help the community with solving common tasks.

Starter kit for extensions development

How to write a custom engine

How to write a custom recipe


A jsreport extension can be stored anywhere in the application directory tree but needs to contain the specific file jsreport.config.js in its root directory. This file defines the basic extension attributes like its name or main entry points.

module.exports = {
  name: 'custom',
  main: './lib/main.js', 
  requires: {
    core: '3.x.x',
    studio: '3.x.x'

An extension needs to contain the server-side code identified with the main attribute as the relative path to the javascript file. The client-side code doesn't require any configuration in jsreport.config.js.


Extension's server-side main entry point needs to export a function which is called during the initialization process. This function can accept two parameters. The first is an instance of Reporter which is a kind of a facade covering the whole jsreport. A second parameter is a plain object containing attributes from jsreport.config.js merged with options provided to the jsreport in the attribute with the same name as the extension.

module.exports = (reporter, definition) => {
  console.log("My custom extension is initialized.").

Server-side worker

jsreport is designed for the safe processing of users' code which may be potentially dangerous. This code is processed in the extra worker threads and you need to define worker entry point when you want to hook into the pipeline running in the worker thread.

module.exports = {
  name: 'custom',
  main: './lib/main.js', 
  worker: './lib/worker.js',
  requires: {
    core: '3.x.x',
    studio: '3.x.x'
module.exports = (reporter, definition) => {
  console.log("The worker part of extension is initialized.").

Listeners and promises

jsreport core provides hooks into most of the operations through event listeners. You just need to choose the desired listener and add the hook function. Such a function can run in a synchronous way or return a promise when running in an asynchronous way.

reporter.validateRenderListeners.add("myCustomListener", (req, res) => {
    console.log("Processing template " +;
    throw new Error("Invalid rendering requests!");

Reporter listeners

  • initializeListeners()- called when all extensions are initialized
  • beforeRenderListeners(req, res) - very first in the rendering pipeline, used to load templates and parse input data
  • renderErrorListeners(req, res, err) - called when there is an error when trying to render a report
  • validateRenderListeners(req, res) - possible to reject rendering before it starts, just return failed promise or exception
  • afterTemplatingEnginesExecutedListeners(req, res) - engine like handlebars or jsrender extracted the content, the res.content contains Buffer with extracted content
  • afterRenderListeners(req, res) - recipes are executed, res.content contains final buffer which will be returned as a stream back, the last change to modify the output or send it elsewhere
  • closeListeners() called when jsreport is about to be closed, you will usually put here some code that cleans up some resource that your extension is using.

Rendering pipeline

The most common scenario for the custom extensions is to hook into the rendering process and adapt the inputs before the html is actually printed into pdf and so on. This can be achieved by adding the function into beforeRenderListeners.

Quite often you want to execute your code just after the template is loaded but before the child templates are extracted. For this purpose, you can use the insert method on the listeners' collection instead of add. This method accepts a condition as the first parameter which specifies more precisely when the hook should be invoked. Note this makes sense only in the worker part because there is most of the rendering pipeline running.

    after: "templates",
    before: "childTemplates"
}, "myCustomExtension", this, (req, res) => {
    req.template.content = "<p>" + req.template.content + "</p>";

The listeners can be added to the main entry point as well as in the worker entry point. It depends on the situation to which part you want to hook in. The initializeListeners in the main process will be invoked just once when jsreport starts. But the same listener inside the worker entry point will be invoked for every worker thread allocated based on the config workers.numberOfWorkers.

Document store

Access to the stored jsreport data is provided through abstraction template stores.

const templates = reporter.documentStore.collection("templates").find({ "shortid": "foo"})

DocumentStore provides a similar API as the mongo client. First, you need to get the desired document collection and then call obvious methods find, update, insert, and remove. Note that every method returns a promise as everything else in jsreport.

Every custom extension can define new entity/document types as well as modify an existing one. This is provided again through DocumentStore abstraction. The definitions of the types follow the OData terminology because everything is then exposed through external OData API.

The type is defined and exposed in the main entry point the following way:

//define the type properties first
reporter.documentStore.registerEntityType("CustomType", {
    "_id": { type: "Edm.String", key: true },
    "test": {type: "Edm.String"}   

//expose entity set through internal API as well as external OData
reporter.documentStore.registerEntitySet("custom", {
    entityType: "jsreport.CustomType"

To modify the existing type you just need to know its name:

reporter.documentStore.model.entityTypes["TemplateType"].foo = {
    type: "Edm.String"

DocumentStore listeners

The document store listeners can be attached to the particular collection in the main entry point.

reporter.documentStore.collection('templates').beforeInsertListeners('my extension', (doc, req) => {
    doc.extraCalc = ...
  • beforeFindListeners(query)
  • beforeUpdateListeners(query, update)
  • beforeInsertListeners(doc)
  • beforeRemoveListeners(query)


Extensions typically include also adaptations for the browser-based jsreport studio. This topic is fully covered in the dedicated article here.

Distributing extension

The preferred way of distributing a custom extension is through public npm. This is very convenient because jsreport searches for extensions in the whole application directory including node_modules directory where npm modules are dropped.

When the custom extension is published in npm user just needs to type the single command or specify a dependency in the package.json.

npm install jsreport-custom-extension

Further study

This is article is far away from being a full reference to creating a custom extension. It's rather a quick start to the basic architecture. If you are interested in creating advanced extensions check out the source code of the existing ones. There are plenty of them available on github and could be used as examples you need.

How to write a custom engine

How to write a custom recipe

jsreport version