Custom Expressions and Data Sources

The Form Engine is designed to cover generic functionality and as such, functionality that is specific to particular implementer should reside in the implementer’s ESM but can be registered in the form engine and consumed whenever required. These should be registered in the index.ts file in the startupApp function.

Registering a custom expression:

To register a custom expression, we invoke the registerExpressionHelper function. This is a void function and takes 2 parameters:

Function Name
function

Below is a sample code snippet for registering custom Expression within your custom frontend (esm)

// Sample Test Function
function AgeCalculator(birthYear, currentYear) {
  return currentYear - birthYear;
}
 
// Registering the function (index.ts)
export function startupApp() {
  registerExpressionHelper('customAgeCalc', AgeCalculator);
}

Referencing Patient Attributes in custom expressions

In scenarios where you want to dynamically pass patient data to an expression, the form engine also supports referencing patient attributes directly in your form json. For example you can reference the patient uuid using `patient.id`.

"calculate": {
  "calculateExpression": "customPatientExpression(patient.id, 'sample-second-parameter-or-uuid').then(value => value)"
},

Registering a custom datasource:

To register a custom expression, we invoke the registerCustomDataSource function. This is a void function and takes 2 parameters:

Datasource Name
A promise that returns a datasource

Below is a sample code snippet for registering a custom Datasource:

//Sample Datasource
export class TestDatasource implements DataSource<OpenmrsResource> {
fetchData(searchTerm: string, config?: Record<string, any>): Promise<any[]> {
  return new Promise((resolve, reject) => {
    const data = [
      {
        display: 'Kajambiya',
        uuid: 'U001',
      },
      {
        display: 'Joel',
        uuid: 'U002',
      },
      {
        display: 'Winslet',
        uuid: 'U003',
      },
    ];
 
    setTimeout(() => {
      resolve(data); // Resolve the Promise with the data array
    }, 1000); // Simulated delay of 1 second
  });
}
 
toUuidAndDisplay(data: OpenmrsResource): OpenmrsResource {
  if (typeof data.uuid === 'undefined' || typeof data.display === 'undefined') {
    throw new Error("'uuid' or 'display' not found in the OpenMRS object.");
  }
    return data;
  }
}