Add "async" keyword to asynchronous functions that return a promise

Description

In order to make it easier for developers to identify asynchronous methods, the following conventions need to be introduced:

1. Use of “async” keyword in the function definitions.
2. Use of “Async” added to the end of the function name.

E.g.

Before changes:

fetchClassroomData(classroomUrlFragment: string): Promise<ClassroomData> {
  return new Promise((resolve, reject) => {
    // some asynchronous operation.
  });
}

After changes:

async fetchClassroomDataAsync(classroomUrlFragment: string): Promise<ClassroomData> {
  return new Promise((resolve, reject) => {
    // some asynchronous operation.
  });
}

Note that (1) “async” keyword was added before function name and (2) fetchClassroomData has changed to fetchClassroomDataAsync (“Async” is added to the name).

For anonymous function definitions, the “async” keyword needs to be added before the “function” keyword. Example:

Before:

const getThreads = function() {
  return new Promise((resolve, reject) => {
    // some asynchronous operation.
  });
}

After:

const getThreadsAsync = async function() {
  return new Promise((resolve, reject) => {
    // some asynchronous operation.
  });
}

Tips

How to find out which functions need this change?

1. Search for methods that are then-able. You can do a simple search for .then( to reveal asynchronous then-able methods.
2. Go to the function definition of a function found in step 1 and check if the return type for the function from step 1 is a Promise object. For example, in the fetchClassroomDataAsync function mentioned above, the return type is Promise<ClassroomData, which means that the function returns a Promise object that resolves with a value of type ClassroomData. As long as the return type is of the form Promise<..>, the function can be changed to follow the convention mentioned in the description.
3. If the function does not have the return type defined, the function definition needs to be updated with the appropriate return type before it can follow the convention mentioned in the description. For information on how to define types, please see “Guide on Defining Types”.

Here are some examples from the code base

https://github.com/oppia/oppia/blob/e4cd1e8c58232f04e642a6c61133e6716d59bf45/core/templates/services/state-top-answers-stats-backend-api.service.ts#L40-L49

https://github.com/oppia/oppia/blob/e4cd1e8c58232f04e642a6c61133e6716d59bf45/core/templates/services/state-top-answers-stats.service.ts#L68-L87

https://github.com/oppia/oppia/blob/e4cd1e8c58232f04e642a6c61133e6716d59bf45/core/templates/services/state-top-answers-stats.service.ts#L112-L117

https://github.com/oppia/oppia/blob/e4cd1e8c58232f04e642a6c61133e6716d59bf45/core/templates/services/exploration-improvements.service.ts#L143-L146

https://github.com/oppia/oppia/blob/e4cd1e8c58232f04e642a6c61133e6716d59bf45/core/templates/domain/classroom/classroom-backend-api.service.ts#L99-L111

Steps to follow

1. On your local setup, add the eslint check @typescript-eslint/promise-function-async (see this).
2. Run the lint check on the CI on your fork of Oppia or on your local machine. The linter will report all the files that require this change.
3. Follow the instructions mentioned in the How to find out which functions need this change? section to introduce the new keyword and pattern.

Important Notes

1. All the files listed by the eslint check needs to be handled and the PR that fully fixes this issue must also introduce the lint check.
2. This task can be assigned and worked on only after the following PRs have been merged https://github.com/oppia/oppia/pull/12302, https://github.com/oppia/oppia/pull/12291, https://github.com/oppia/oppia/pull/12194, https://github.com/oppia/oppia/pull/12188, https://github.com/oppia/oppia/pull/12164, https://github.com/oppia/oppia/pull/12151, https://github.com/oppia/oppia/pull/12020.
3. The assignee for this task must ensure that the async / …Async pattern is introduced across the codebase in their PR.

Note: For a guide on how to access Oppia’s webpages, see this.

[nlok5923]

@vojtechjelinek please assign me

 core/templates/domain/topic/topic-creation-backend-api.service.ts
 core/templates/domain/story_viewer/story-viewer-backend-api.service.ts

[aasiffaizal]

@kevintab95 #12302, #12291 and #12151 are merged. Can you please update the description? Thanks 😃