chore(docs): note how to get jest object in ESM tests (#10718)

Author: SimenB

docs/ECMAScriptModules.md

@@ -14,3 +14,17 @@ With the warnings out of the way, this is how you activate ESM support in your t
 1. Ensure you either disable [code transforms](./configuration#transform-objectstring-pathtotransformer--pathtotransformer-object) by passing `transform: {}` or otherwise configure your transformer to emit ESM rather than the default CommonJS (CJS).
 1. Execute `node` with `--experimental-vm-modules`, e.g. `node --experimental-vm-modules node_modules/.bin/jest` or `NODE_OPTIONS=--experimental-vm-modules npx jest` etc.. On Windows, you can use [`cross-env`](https://github.com/kentcdodds/cross-env) to be able to set environment variables
 1. Beyond that, we attempt to follow `node`'s logic for activating "ESM mode" (such as looking at `type` in `package.json` or `mjs` files), see [their docs](https://nodejs.org/api/esm.html#esm_enabling) for details
+
+## Differences between ESM and CommonJS
+
+Most of the differences are explained in [Node's documentation](https://nodejs.org/api/esm.html#esm_differences_between_es_modules_and_commonjs), but in addition to the things mentioned there, Jest injects a special variable into all executed files - the [`jest` object](JestObjectAPI.md). To access this object in ESM, you need to import it from the `@jest/globals` module.
+
+```js
+import {jest} from '@jest/globals';
+
+jest.useFakeTimers();
+
+// etc.
+```
+
+Please note that we currently don't support `jest.mock` in a clean way in ESM, but that is something we intend to add proper support for in the future. Follow [this issue](https://github.com/facebook/jest/issues/10025) for updates.

website/versioned_docs/version-26.0/ECMAScriptModules.md

@@ -15,3 +15,17 @@ With the warnings out of the way, this is how you activate ESM support in your t
 1. Ensure you either disable [code transforms](./configuration#transform-objectstring-pathtotransformer--pathtotransformer-object) by passing `transform: {}` or otherwise configure your transformer to emit ESM rather than the default CommonJS (CJS).
 1. Execute `node` with `--experimental-vm-modules`, e.g. `node --experimental-vm-modules node_modules/.bin/jest` or `NODE_OPTIONS=--experimental-vm-modules npx jest` etc.. On Windows, you can use [`cross-env`](https://github.com/kentcdodds/cross-env) to be able to set environment variables
 1. Beyond that, we attempt to follow `node`'s logic for activating "ESM mode" (such as looking at `type` in `package.json` or `mjs` files), see [their docs](https://nodejs.org/api/esm.html#esm_enabling) for details
+
+## Differences between ESM and CommonJS
+
+Most of the differences are explained in [Node's documentation](https://nodejs.org/api/esm.html#esm_differences_between_es_modules_and_commonjs), but in addition to the things mentioned there, Jest injects a special variable into all executed files - the [`jest` object](JestObjectAPI.md). To access this object in ESM, you need to import it from the `@jest/globals` module.
+
+```js
+import {jest} from '@jest/globals';
+
+jest.useFakeTimers();
+
+// etc.
+```
+
+Please note that we currently don't support `jest.mock` in a clean way in ESM, but that is something we intend to add proper support for in the future. Follow [this issue](https://github.com/facebook/jest/issues/10025) for updates.