Async Test Environment API

[suchipi]

Do you want to request a feature or report a bug?

Feature request

What is the current behavior?

The API for test environments is written with the constraint that the test environment must be able to get its global ready synchronously (see the code here)

What is the desired behavior?

It'd be nice if the API was added to in order to allow the test environment to be constructed asynchronously.

Here's my use case: I'm trying to use NW.js to create an environment which opens an isolated browser window for each test. In NW.js, if you use nw.Window.open to open a new browser window, it is opened asynchronously. So it's not possible to open a window and set up its global for jest synchronously.

Here's the code I tried to use within the environment constructor:

nw.Window.open(config.testURL, (win) => {
  const global = this.global = win.window;
  installCommonGlobals(global, config.globals);
  this.moduleMocker = new mock(global);
  this.fakeTimers = new FakeTimers(global, this.moduleMocker, config);
});

This doesn't work because the callback argument to nw.Window.open is called asynchronously, so once jest gets to this line, it tries to install the console onto the environment's global, which isn't set up yet.

[cpojer]

Yeah, we should do this. Make an async "setup" function in both TestEnvironment's and wait for it in the code. If you can send a PR tomorrow, we can include it into Jest 20.

[suchipi]

Not promising I can have a PR in by tomorrow, but...

Do you want to use an async setup function instead of the current synchronous constructor, or in addition to? The former would be a breaking change. Since you are bumping the major version to 20 anyway (and this api is not commonly used), that would not be a very big deal, but I wanted to confirm.

[thymikee]

Ideally we would change it to async, in v20 or v21. If we can't get it into 20, we may release a minor version with 2 possible ways, along with proper depreciation warning.

[cpojer]

I would keep the constructor, you can't get rid of a constructor anyway and add the setup function as an optional hook. If the function is present, call it and wait for the promise to resolve. If it isn't there, just do what we do now.

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