Asynchronous Functions in Expressions

Expression questions support custom asynchronous functions that you can use for time-consuming operations, such as requests to a server. In this example, you can select a country from a drop-down list. Two Expression questions use asynchronous functions to load and display the selected country's region and official name.

To implement an asynchronous function for use in expressions, follow the steps below:

1. Declare an asynchronous JavaScript function.
   This function accepts all arguments in one array-like object. You can pass as many arguments as needed when you call the function from an expression. To return the function's result, pass it to the this.returnResult function.

   function asyncFunc(params) {
       const arg1 = params[0];
       const arg2 = params[1];
       // ...
       setTimeout(() => {
           // Return the function result via the callback
           this.returnResult(yourValue);
       }, 100);
   }
   

2. Register your function in FunctionFactory.
   Call a static register() method on the FunctionFactory.Instance object. This method accepts a function name that you want to use in expressions, the function itself, and a true flag that indicates an asynchronous function.

   import { FunctionFactory } from "survey-core";
   
   FunctionFactory.Instance.register("asyncFunc", asyncFunc, true);
   

3. Use your function in an Expression question.
   Specify the question's expression property.

   const surveyJson = {
       "elements": [{
           "type": "expression",
           "name": "asyncExpression",
           // ...
           "expression": "asyncFunc({question1}, {question2})"
       }, {
           // ...
       }]
   };
   

4. (Optional) Hide an Expression question until it has a value.
   You may want to hide your Expression question until its asynchronous function has executed. To do it, set the visibleIf property to a Boolean expression that uses the notempty function. Refer to the following help topic for more information: Conditional Visibility.

   const surveyJson = {
       "elements": [{
           "type": "expression",
           "name": "asyncExpression",
           // ...
           "expression": "asyncFunc({question1}, {question2})",
           "visibleIf": "{asyncExpression} notempty"
       }, {
           // ...
       }]
   };