feat: support config files exporting (async) functions

[hisco]

Summary

Original #8357 was too old, created this PR to start fresh.
At that time jest cli required the dynamic config module synchronously, that PR enabled loading jest config module async.
It appears that the code in master already have these or similar changes that allows async loading.
All that is missing is documentation

Test plan

No tests needed as all of the changes are already in master, only changed docs

[SimenB]

We wait for the import, we don't invoke the function provided and wait for it to complete.

This is the implementation for the feature:

diff --git i/packages/jest-config/src/index.ts w/packages/jest-config/src/index.ts
index 68d6b5ddf5..de88d4423c 100644
--- i/packages/jest-config/src/index.ts
+++ w/packages/jest-config/src/index.ts
@@ -41,7 +41,9 @@ export async function readConfig(
   parentConfigPath?: Config.Path | null,
   projectIndex: number = Infinity,
 ): Promise<ReadConfig> {
-  let rawOptions;
+  let rawOptions:
+    | Config.InitialOptions
+    | (() => Config.InitialOptions | Promise<Config.InitialOptions>);
   let configPath = null;
 
   if (typeof packageRootOrConfig !== 'string') {
@@ -82,6 +84,10 @@ export async function readConfig(
     rawOptions = await readConfigFileAndSetRootDir(configPath);
   }
 
+  if (typeof rawOptions === 'function') {
+    rawOptions = await rawOptions();
+  }
+
   const {options, hasDeprecationWarnings} = normalize(
     rawOptions,
     argv,

In addition to that we'll need a test. Wanna give it a go? 🙂

[codecov-io]

Codecov Report

Merging #10001 into master will increase coverage by 0.22%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master   #10001      +/-   ##
==========================================
+ Coverage   63.92%   64.15%   +0.22%     
==========================================
  Files         293      293              
  Lines       12496    12498       +2     
  Branches     3083     3084       +1     
==========================================
+ Hits         7988     8018      +30     
+ Misses       3863     3830      -33     
- Partials      645      650       +5     

Impacted Files	Coverage Δ
packages/jest-config/src/index.ts	53.52% <50.00%> (+41.92%)	⬆️

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[SimenB]

Thanks! Can you update the changelog as well?

[SimenB]

Thanks!

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