Used for providing custom scripts for execution in CHT apps

Introduced in v4.2.0


Extension libraries are blocks of code that are cached with the CHT web application giving app developers a powerful tool to extend the CHT. This is an advanced feature and requires an app developer with some software development experience.

An example of a use for this feature is to provide a function to calculate a risk score based on a machine learning model. The function can then be called passing in values from app forms and return the result to be stored with the report.


The first step is to create the js file to return the function that will be called by the web application. Create a new file using this template:

module.exports = function(/* parameters */) {
  return result;

Now populate the function as needed. For complex functions, it is recommended to use third party libraries (such as momentjs, lodash, etc) and use a bundler (eg: webpack) to make it easy to build a single file. It’s recommended to use development best practices such as linting, unit tests, and minification to ensure quality and small download size.

xpath functions

To call the function from within a form the parameters and return value will need to have a very specific structure to work with Enketo xforms.

  "t": <type>,
  "v": <value>

Where the type is one of “bool”, “num”, “str”, “date”, or “arr”. For example, to calculate the average of two inputs, you would use:

const getValue = function(obj) {
  let val;
  if (obj.t === 'arr') {
    val = obj.v && obj.v.length && obj.v[0];
  } else {
    val = obj.v;
  if (!val) {
    return 0;
  const parsed = parseInt(val.textContent);
  return isNaN(parsed) ? 0 : parsed;

module.exports = function(first, second) {
  const average = (getValue(first) + getValue(second)) / 2;
  return {
    t: 'num',
    v: average

Uploading to the CHT

Create a folder within the project configuration to contain the libraries, for example:


Now run the command: cht upload-extension-libs

This will create or update a document to CouchDB with an ID of extension-libs with each of the configured scripts attached. Once this is created the webapp service worker will be updated so the libraries are cached on the phone ready for use offline.

Invoking the function


The function will now be available via the CHT API for tasks, targets, and contact summary configurations.

CHT xPath functions

To execute the function from within an xform use the cht:extension-lib xpath function.