[Feature]: Allow option to detect if jest is currently running in watch mode

[billyvg]

🚀 Feature Proposal

Add a way to detect (in a jest environment and possibly elsewhere) for when jest is running in "watch" mode.

Motivation

We have a custom jest environment where we are instrumenting our tests and we'd like to be able to disable the feature in watch mode. Currently you can check process.argv for --watch, but this doesn't work if you call jest.run(<argv>) outside of CLI.

Example

I would expect it to be available in the environment's constructor:

class CustomEnvironment extends JsdomEnvironment {
    constructor(...args) {
      super(...args);
      const [config, context] = args;
      if (context.watchMode) { ... }
    }
}

Pitch

This feature belongs in core because it's already an option in core and just needs to be exposed.

[SimenB]

Right, we only pass you projectConfig, not globalConfig (which has watch: true). Sounds like something we could change for the next major (i.e. constructor({projectConfig, globalConfig}) rather than constructor(projectConfig))

[SimenB]

https://github.com/facebook/jest/releases/tag/v28.0.0-alpha.5

[github-actions]

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