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Enketo Core

The engine that powers Enketo Express and various third party tools including this selection.

Enketo's form engine is compatible with tools in the ODK ecosystem and complies with its XForms specification though not all features in that specification have been implemented yet.

This repo is meant to be used as a building block for any Enketo-powered application. See this page for a schematic overview of a real-life full-fledged data collection application and how Enketo Core fits into this.

Project status

As of 2022, Enketo is maintained by the ODK team (primarily Trevor Schmidt). Martijn, its original author, continues to provide advice and continuity. The ODK project sets priorities in collaboration with its Technical Advisory Board.

Broader context is available in the Enketo Express repository.

Browser support

The following browsers are officially supported:

  • latest Android webview on latest Android OS
  • latest WKWebView on latest iOS
  • latest version of Chrome/Chromium on Mac OS, Linux, Windows, Android and iOS
  • latest version of Firefox on Mac OS, Windows, Linux, Android and iOS
  • latest version of Safari on Mac OS, Windows, and on the latest version of iOS
  • latest version of Microsoft Edge

We have to admit we do not test on all of these, but are committed to fixing browser-specific bugs that are reported for these browsers. Naturally, older browsers versions will often work as well - they are just not officially supported.

Performance (live)

See graphs

Usage as a library

  1. Install with npm install enketo-core --save or include as a git submodule.
  2. Develop a way to perform an XSL Transformation on OpenRosa-flavoured XForms inside your app. The transformation will output an XML instance and a HTML form. See enketo-transformer for an available library/app to use or develop your own.
  3. Add themes to your stylesheet build system (2 stylesheets per theme, 1 is for media="print").
  4. Override the files under "browser", e.g. using aliasify with your app-specific versions.
  5. Main methods illustrated in code below:
// assumes the enketo-core package is mapped from the node_modules folder
import { Form } from 'enketo-core';

// The XSL transformation result contains a HTML Form and XML instance.
// These can be obtained dynamically on the client, or at the server/
// In this example we assume the HTML was injected at the server and modelStr
// was injected as a global variable inside a <script> tag.

// required HTML Form DOM element
const formEl = document.querySelector('form.or');

// required object containing data for the form
const data = {
    // required string of the default instance defined in the XForm
    modelStr: globalXMLInstance,
    // optional string of an existing instance to be edited
    instanceStr: null,
    // optional boolean whether this instance has ever been submitted before
    submitted: false,
    // optional array of external data objects containing:
    // {id: 'someInstanceId', xml: XMLDocument}
    external: [],
    // optional object of session properties
    // 'deviceid', 'username', 'email', 'phonenumber', 'simserial', 'subscriberid'
    session: {},

// Form-specific configuration
const options = {};

// Instantiate a form, with 2 parameters
const form = new Form(formEl, data, options);

// Initialize the form and capture any load errors
let loadErrors = form.init();

// If desired, scroll to a specific question with any XPath location expression,
// and aggregate any loadErrors.
loadErrors = loadErrors.concat(form.goTo('//repeat[3]/node'));

// submit button handler for validate button
$('#submit').on('click', function () {
    // clear non-relevant questions and validate
    form.validate().then(function (valid) {
        if (!valid) {
            alert('Form contains errors. Please see fields marked in red.');
        } else {
            // Record is valid!
            const record = form.getDataStr();

            // reset the form view

            // reinstantiate a new form with the default model and no options
            form = new Form(formSelector, { modelStr: modelStr }, {});

            // do what you want with the record

Global Configuration

Global configuration (per app) is done in config.json which is meant to be overridden by a config file in your own application (e.g. by using rollup).


The maps configuration can include an array of Mapbox TileJSON objects (or a subset of these with at least a name, tiles (array) and an attribution property, and optionally maxzoom and minzoom). You can also mix and match Google Maps layers. Below is an example of a mix of two map layers provided by OSM (in TileJSON format) and Google maps.

    "name": "street",
    "tiles": [ "http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png" ],
    "attribution": "Map data © <a href=\"http://openstreetmap.org\">OpenStreetMap</a> contributors"
    "name": "satellite",
    "tiles": "GOOGLE_SATELLITE"

For GMaps layers you have the four options as tiles values: "GOOGLE_SATELLITE", "GOOGLE_ROADMAP", "GOOGLE_HYBRID", "GOOGLE_TERRAIN". You can also add other TileJSON properties, such as minZoom, maxZoom, id to all layers.


The Google API key that is used for geocoding (in the geo widgets' search box). Can be obtained here. Make sure to enable the GeoCoding API service. If you are using Google Maps layers, the same API key is used. Make sure to enable the Google Maps JavaScript API v3 service as well in that case (see next item).


This setting with the default false value determines whether Enketo should validate questions immediately if a related value changes. E.g. if question A has a constraint that depends on question B, this mode would re-validate question A if the value for question B changes. This mode will slow down form traversal. When set to false that type of validation is only done at the end when the Submit button is clicked or in Pages mode when the user clicks Next.


This setting with default true value determines whether the Next button should trigger validation of the current page and block the user from moving to the next page if validation fails.


This setting with default true value determines whether to enable support for swiping to the next and previous page for forms that are divided into pages.

Form Configuration

Per-form configuration is done by adding an (optional) options object as 3rd parameter when instantiating a form.

Print only the "relevant" parts of the form

If printRelevantOnly is set to true or not set at all, printing the form only includes what is visible, ie. all the groups and questions that do not have a relevant expression or for which the expression evaluates to true.

new Form(formselector, data, {
  printRelevantOnly: false

Explicitly set the default form language

The language option overrides the default languages rules of the XForm itself. Pass any valid and present-in-the-form IANA subtag string, e.g. ar.

How to develop Enketo Core

  1. install prerequisites:
  • Volta (optional, but recommended)
  • Node.js 16 and npm 6 (Node.js 14 is also supported)
  • grunt-cli
  1. install dependencies with npm install
  2. build with grunt (npx grunt)
  3. start built-in auto-reloading development server with npm start
  4. browse to http://localhost:8005 and load an XForm url with the xform queryparameter or load a local from the /tests/forms folder in this repo
  5. run tests with npm test (headless chrome) and npm run test-browsers (browsers); note: running tests updates the coverage badge in README.md, but these changes should not be committed except when preparing a release
  6. adding the querystring touch=true and reducing the window size allows you to simulate mobile touchscreens

Notes for JavaScript Developers

  • When creating new functions/Classes, make sure to describe them with JSDoc comments.
  • JavaScript style see ESLint config files. The check is added to the grunt test task. You can also manually run grunt eslint:fix to fix style issues.
  • Testing is done with Mocha and Karma (all: grunt karma, headless: grunt karma:headless, browsers: grunt karma:browsers)
  • Tests can be run in watch mode for TDD workflows with npm run test-watch, and support for debugging in VSCode is provided. For instructions see Debugging test watch mode in VSCode below
  • When making a pull request, please add tests where relevant

Debugging test watch mode in VSCode

Basic usage:

  1. Go to VSCode's "Run and Debug" panel
  2. Select "Test (watch + debug)"
  3. Click the play button

Optionally, you can add a keyboard shortcut to select launch tasks:

  1. Open the keyboard shortcuts settings (cmd+k cmd+s on Mac, ctrl+k ctrl+s on other OSes)
  2. Search for workbench.action.debug.selectandstart
  3. Click the + button to add your preferred keybinding keybinding

Notes for CSS Developers

The core can be fairly easily extended with alternative themes. See the plain, the grid, and the formhub themes already included in /src/sass. We would be happy to discuss whether your contribution should be a part of the core, the default theme or be turned into a new theme.

For custom themes that go beyond just changing colors and fonts, keep in mind all the different contexts for a theme:

  1. non-touchscreen vs touchscreen (add ?touch=true during development)
  2. default one-page-mode and multiple-pages-mode
  3. right-to-left form language vs left-to-right form language (!) - also the UI-language may have a different directionality
  4. screen view vs. print view
  5. questions inside a (nested) repeat group have a different background
  6. large screen size --> smaller screen size ---> smallest screen size
  7. question in valid vs. invalid state

Widgets in Enketo Core

Widgets extend the Widget class. This is an example:

(see full functioning example at /src/widget/example/my-widget.js

import Widget from '../../js/widget';

 * Make sure to give the widget a unique widget class name and extend Widget.
class MyWidget extends Widget {
     * The selector that determines on which form control the widget is instantiated.
     * Make sure that any other widgets that target the same from control are not interfering with this widget by disabling
     * the other widget or making them complementary.
     * This function is always required.
    static get selector() {
        return '.or-appearance-my-widget input[type="number"]';

     * Initialize the widget that has been instantiated using the Widget (super) constructor.
     * The _init function is called by that super constructor unless that constructor is overridden.
     * This function is always required.
    _init() {
        // Hide the original input

        // Create the widget's DOM fragment.
        const fragment = document.createRange().createContextualFragment(
            `<div class="widget">
                <input class="ignore" type="range" min="0" max="100" step="1"/>

        // Only when the new DOM has been fully created as a HTML fragment, we append it.

        const widget = this.element.parentElement.querySelector('.widget');
        this.range = widget.querySelector('input');

        // Set the current loaded value into the widget
        this.value = this.originalInputValue;

        // Set event handlers for the widget
        this.range.addEventListener('change', this._change.bind(this));
            .addEventListener('click', this._reset.bind(this));

        // This widget initializes synchronously so we don't return anything.
        // If the widget initializes asynchronously return a promise that resolves to `this`.

    _reset() {
        this.value = '';
        this.originalInputValue = '';

    _change(ev) {
        // propagate value changes to original input and make sure a change event is fired
        this.originalInputValue = ev.target.value;

     * Disallow user input into widget by making it readonly.
    disable() {
        this.range.disabled = true;

     * Performs opposite action of disable() function.
    enable() {
        this.range.disabled = false;

     * Update the language, list of options and value of the widget.
    update() {
        this.value = this.originalInputValue;

     * Obtain the current value from the widget. Usually required.
    get value() {
        return this.element.classList.contains('empty') ? '' : this.range.value;

     * Set a value in the widget. Usually required.
    set value(value) {
        this.range.value = value;

export default MyWidget;

Some of the tests are common to all widgets, and can be run with a few lines:

(see full functioning example at /test/spec/widget.example.spec.js)

import ExampleWidget from '../../src/widget/example/my-widget';
import { runAllCommonWidgetTests } from '../helpers/testWidget';

const FORM = `<label class="question or-appearance-my-widget">
        <input type="number" name="/data/node">
const VALUE = '2';

runAllCommonWidgetTests(ExampleWidget, FORM, VALUE);


  • use the rank widget as a more complex example that uses the best practices (some other widgets use an older style)
  • add an _init function to your widget that either returns nothing or a Promise (if it initializes asynchronously)
  • include a widget.my-widget.spec.js file in the /test folder
  • run at least the standardized common widget tests by doing: TBD
  • make the widget responsive up to a minimum window width of 320px
  • ensure the widget's scss and js file is/are loaded in widgets.js and _widgets.scss respectively
  • if hiding the original input element, it needs to load the default value this.originalInputValue into the widget
  • if hiding the original input element, keep its value syncronized using this.originalInputValue = ...
  • if hiding the original input element, it needs to listen for the applyfocus event on the original input and focus the widget
  • if hiding the original input element, the widget value needs to update when the original input updates due to a calculation or becoming non-relevant (update)
  • apply the widget css class to the top level elements it adds to the DOM (but not to their children)
  • new input/select/textarea elements inside widgets should have the ignore class to isolate them from the Enketo form engine
  • include enable(), disable() and update() method overrides. See the Widget class.
  • if the widget needs tweaks or needs to be disabled for mobile use, use support.js to detect this and override the static condition() function in Widget.js.
  • allow clearing of the original input (i.e. setting value to '')
  • if the widget does not get automatic (built-in HTML) focus, trigger a fakefocus event to the original input when the widget gets focus (rarely required, but see rank widget)


  • do not include jQuery, React, Vue or any other general purpose libraries or frameworks

Events in Enketo Core


Fired on a form control when it is programmatically updated and when this results in a change in value


Fired on a form control when it is updated directly by the user and when this results in a change in value


Fired on a form control when it has failed constraint, datatype, or required validation.


Fired on model.$events, when a single model value has changed its value, a repeat is added, or a node is removed. It passes an "update object". This event is propagated for external use by firing it on the form.or element as well.


Fired on model.events when a new record (instance) is loaded for the first time. It's described here: odk-instance-first-load.


Fired on a newly added repeat. It's described here: odk-instance-first-load.


Fired on the repeat or repeat element immediately following a removed repeat.


Fired on model.events, when a node is removed. It passes an "update object". This event is propagated for external use by firing it on the form.or element as well.


Fired on form control when an attempt is made to 'go to' this field but it is hidden from view because it is non-relevant.


Fired on form control when an attempt is made to 'go to' this field but it is hidden from view because it is has no form control.


Fired when user flips to a new page, on the page element itself.


Fired on form.or element when user makes first edit in form. Fires only once.


Fired on form.or element when validation completes.


Fired when the user moves to a different question in the form.


  1. Create release PR
  2. Update CHANGELOG.md
  3. Update version in package.json
    • Bump to major version if consumers have to make changes.
  4. Check Dependabot for alerts
  5. Run npm update
    • Check if node-forge has been updated and if so, verify encrypted submissions end-to-end
    • If enketo-transformer has been updated, change Form.requiredTransformerVersion
  6. Run npm audit
    • Run npm audit fix --production to apply most important fixes
  7. Run npm i
  8. Run npm test
  9. Merge PR with all changes
  10. Create GitHub release
  11. Tag and publish the release
    • GitHub Action will publish it to npm


The development of this library is now led by ODK and funded by customers of the ODK Cloud hosted service.

Past sponsors include:


See license document and additional clause below:

Any product that uses enketo-core is required to have a "Powered by Enketo" footer, according to the specifications below, on all screens in which enketo-core or parts thereof, are used, unless explicity exempted from this requirement by Enketo LLC in writing. Partners and sponsors of the Enketo Project, listed on https://enketo.org/about/sponsors/ and on this page are exempted from this requirements and so are contributors listed in package.json.

The aim of this requirement is to force adopters to give something back to the Enketo project, by at least spreading the word and thereby encouraging further adoption.


  1. The word "Enketo" is displayed using Enketo's logo.
  2. The minimum font-size of "Powered by" is 12 points.
  3. The minimum height of the Enketo logo matches the font-size used.
  4. The Enketo logo is hyperlinked to https://enketo.org


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