A custom async function used in `defaultValueExpression` results in an infinite loop

[JaneSjs]

T26288 - defaultValueExpression async custom functions cause infinite loop since v2.5.0
https://surveyjs.answerdesk.io/internal/ticket/details/T26288

[JaneSjs]

We researched this issue and found that it is caused by the following commit, which added support for Promises in custom functions in SurveyJS v2.5.0: dfbea95.

The infinite loop occurs because the following custom function is marked as async, but it does not return a Promise:

import { FunctionFactory } from "survey-core";

async function randomDefaultValue(params) {
  console.log("randomDefaultValue called", params, this);
  setTimeout(() => {
    const random = Math.random();
    console;
    this.returnResult(random);
  }, 1000);
}
FunctionFactory.Instance.register(
  "randomDefaultValue",
  randomDefaultValue,
  true
);

As a result, the Promise is implicitly resolved with undefined, and the Form Library cannot set the default value expression result.

To resolve the issue, you can do one of the following:

• Remove the async modifier from your custom function:

import { FunctionFactory } from "survey-core";

function randomDefaultValue(params) {
  console.log("randomDefaultValue called", params, this);
  setTimeout(() => {
    const random = Math.random();
    console;
    this.returnResult(random);
  }, 1000);
}
FunctionFactory.Instance.register(
  "randomDefaultValue",
  randomDefaultValue,
  true
);

• Keep the async modifier and explicitly return a Promise. This approach is suitable if you need to wait for a long-running operation to complete:

import { FunctionFactory } from "survey-core";

async function someVeryLongAndComplexOperation(params) {
  return new Promise((resolve) => {
    setTimeout(() => {
      const random = Math.random();
      resolve(random);
    }, 1000);
  });
}

async function randomDefaultValue(params) {
  const result = await someVeryLongAndComplexOperation(params);
  return result;
}

FunctionFactory.Instance.register(
  "randomDefaultValue",
  randomDefaultValue,
  true
);