Why does Jest needs Babel to test async code?

Question

The Jest "An Async Example" guide starts with:

First, enable Babel support in Jest...

But I miss to see why and where does Jest needs Babel for.

Answer 1

Node.js has supported async functions by default since version 7.6.0, so (as you suspected) Babel is not needed for Jest to run tests using async functions.

I just confirmed this by installing only Jest v24.6.0 and ran this test with Node.js v10.15.1:

test('hi', async () => {
  const val = await Promise.resolve('hello');
  expect(val).toBe('hello');
});

...and it passed just fine.

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On the other hand, Babel is required to use ES6 module syntax.

Many of the examples in the "An Async Example" doc use ES6 module syntax (export default ..., import * as ..., etc.) so Babel is required for any of those examples to work.