[jest-transform]: add a new option to `transformOptions` to detect when jest runs in watch mode

[ahnpnl]

🚀 Feature Proposal

A new option for transformOptions to tell transformers if Jest runs in watch mode.

Motivation

I have a scenario in ts-jest related to kulshekhar/ts-jest#943

• First I have 2 files

// foo.ts
export function getWelcomeMessage(username: string): string {
  return `yolo ${username}`;
}

// foo.spec.ts
it(`should return a message for this username`, (): void => {
    const result = getWelcomeMessage(username);

    expect(result).toStrictEqual(`yolo C0ZEN`);
});

• Run test in watch mode, the test passes successfully
• Modify foo.ts to

// foo.ts
export function getWelcomeMessage(username: number): string {
  return `yolo ${username}`;
}

• Now test is rerun, however test still passes because ts-jest doesn't know if Jest runs in watch mode to properly redo type check

Example

A new option for transformOptions can be named something like watchMode and ts-jest can use that.

Currently I'm using a workaround that I have a check process.argv.includes('--watch'), it works but not optimal.

Pitch

Allow transformers to perform certain logic depending on watch mode

cc @SimenB @thymikee

[ahnpnl]

@SimenB do you have any suggestions for this?

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