chore: use more private

[SimenB]

Summary

Guides consumers away from private APIs. Also makes the type definitions smaller in some cases since we don't expose as much

Built diff:

diff --git i/packages/jest-mock/build/index.d.ts w/packages/jest-mock/build/index.d.ts
index 5dfae37ba..42348609a 100644
--- i/packages/jest-mock/build/index.d.ts
+++ w/packages/jest-mock/build/index.d.ts
@@ -51,7 +51,7 @@ interface Mock<T, Y extends unknown[] = unknown[]> extends Function, MockInstanc
 interface SpyInstance<T, Y extends unknown[]> extends MockInstance<T, Y> {
 }
 interface MockInstance<T, Y extends unknown[]> {
-    _isMockFunction: boolean;
+    _isMockFunction: true;
     _protoImpl: Function;
     getMockName(): string;
     getMockImplementation(): Function | undefined;
diff --git i/packages/jest-util/build/FakeTimers.d.ts w/packages/jest-util/build/FakeTimers.d.ts
index 75850864c..324976b7d 100644
--- i/packages/jest-util/build/FakeTimers.d.ts
+++ w/packages/jest-util/build/FakeTimers.d.ts
@@ -12,52 +12,26 @@ declare type ModuleMocker = typeof mock;
  * we are disabling the flowtype/no-weak-types rule here.
  */
 declare type Callback = (...args: any) => void;
-declare type TimerID = string;
-declare type Tick = {
-    uuid: string;
-    callback: Callback;
-};
-declare type Timer = {
-    type: string;
-    callback: Callback;
-    expiry: number;
-    interval?: number;
-};
-declare type TimerAPI = {
-    clearImmediate: typeof global.clearImmediate;
-    clearInterval: typeof global.clearInterval;
-    clearTimeout: typeof global.clearTimeout;
-    nextTick: typeof process.nextTick;
-    setImmediate: typeof global.setImmediate;
-    setInterval: typeof global.setInterval;
-    setTimeout: typeof global.setTimeout;
-};
 declare type TimerConfig<Ref> = {
     idToRef: (id: number) => Ref;
     refToId: (ref: Ref) => number | void;
 };
 export default class FakeTimers<TimerRef> {
-    _cancelledImmediates: {
-        [key: string]: boolean;
-    };
-    _cancelledTicks: {
-        [key: string]: boolean;
-    };
-    _config: StackTraceConfig;
-    _disposed?: boolean;
-    _fakeTimerAPIs: TimerAPI;
-    _global: NodeJS.Global;
-    _immediates: Array<Tick>;
-    _maxLoops: number;
-    _moduleMocker: ModuleMocker;
-    _now: number;
-    _ticks: Array<Tick>;
-    _timerAPIs: TimerAPI;
-    _timers: {
-        [key: string]: Timer;
-    };
-    _uuidCounter: number;
-    _timerConfig: TimerConfig<TimerRef>;
+    private _cancelledImmediates;
+    private _cancelledTicks;
+    private _config;
+    private _disposed?;
+    private _fakeTimerAPIs;
+    private _global;
+    private _immediates;
+    private _maxLoops;
+    private _moduleMocker;
+    private _now;
+    private _ticks;
+    private _timerAPIs;
+    private _timers;
+    private _uuidCounter;
+    private _timerConfig;
     constructor({ global, moduleMocker, timerConfig, config, maxLoops, }: {
         global: NodeJS.Global;
         moduleMocker: ModuleMocker;
@@ -70,7 +44,7 @@ export default class FakeTimers<TimerRef> {
     reset(): void;
     runAllTicks(): void;
     runAllImmediates(): void;
-    _runImmediate(immediate: Tick): void;
+    private _runImmediate;
     runAllTimers(): void;
     runOnlyPendingTimers(): void;
     advanceTimersByTime(msToRun: number): void;
@@ -78,16 +52,16 @@ export default class FakeTimers<TimerRef> {
     useRealTimers(): void;
     useFakeTimers(): void;
     getTimerCount(): number;
-    _checkFakeTimers(): void;
-    _createMocks(): void;
-    _fakeClearTimer(timerRef: TimerRef): void;
-    _fakeClearImmediate(uuid: TimerID): void;
-    _fakeNextTick(callback: Callback, ...args: Array<any>): void;
-    _fakeSetImmediate(callback: Callback, ...args: Array<any>): number | null;
-    _fakeSetInterval(callback: Callback, intervalDelay?: number, ...args: Array<any>): TimerRef | null;
-    _fakeSetTimeout(callback: Callback, delay?: number, ...args: Array<any>): TimerRef | null;
-    _getNextTimerHandle(): string | null;
-    _runTimerHandle(timerHandle: TimerID): void;
+    private _checkFakeTimers;
+    private _createMocks;
+    private _fakeClearTimer;
+    private _fakeClearImmediate;
+    private _fakeNextTick;
+    private _fakeSetImmediate;
+    private _fakeSetInterval;
+    private _fakeSetTimeout;
+    private _getNextTimerHandle;
+    private _runTimerHandle;
 }
 export {};
 //# sourceMappingURL=FakeTimers.d.ts.map
\ No newline at end of file
diff --git i/packages/jest-watcher/build/BaseWatchPlugin.d.ts w/packages/jest-watcher/build/BaseWatchPlugin.d.ts
index 5a2c698cb..f15b66222 100644
--- i/packages/jest-watcher/build/BaseWatchPlugin.d.ts
+++ w/packages/jest-watcher/build/BaseWatchPlugin.d.ts
@@ -6,8 +6,8 @@
  */
 import { WatchPlugin } from './types';
 declare class BaseWatchPlugin implements WatchPlugin {
-    _stdin: NodeJS.ReadableStream;
-    _stdout: NodeJS.WritableStream;
+    protected _stdin: NodeJS.ReadableStream;
+    protected _stdout: NodeJS.WritableStream;
     constructor({ stdin, stdout, }: {
         stdin: NodeJS.ReadableStream;
         stdout: NodeJS.WritableStream;
diff --git i/packages/jest-watcher/build/JestHooks.d.ts w/packages/jest-watcher/build/JestHooks.d.ts
index e9fd7d2fd..31e82d7d1 100644
--- i/packages/jest-watcher/build/JestHooks.d.ts
+++ w/packages/jest-watcher/build/JestHooks.d.ts
@@ -4,14 +4,10 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  */
-import { JestHookSubscriber, JestHookEmitter, FileChange, ShouldRunTestSuite, TestRunComplete } from './types';
+import { JestHookSubscriber, JestHookEmitter } from './types';
 declare type AvailableHooks = 'onFileChange' | 'onTestRunComplete' | 'shouldRunTestSuite';
 declare class JestHooks {
-    _listeners: {
-        onFileChange: Array<FileChange>;
-        onTestRunComplete: Array<TestRunComplete>;
-        shouldRunTestSuite: Array<ShouldRunTestSuite>;
-    };
+    private _listeners;
     constructor();
     isUsed(hook: AvailableHooks): number;
     getSubscriber(): JestHookSubscriber;
diff --git i/packages/jest-watcher/build/PatternPrompt.d.ts w/packages/jest-watcher/build/PatternPrompt.d.ts
index 6486482d3..3ac674e71 100644
--- i/packages/jest-watcher/build/PatternPrompt.d.ts
+++ w/packages/jest-watcher/build/PatternPrompt.d.ts
@@ -6,10 +6,10 @@
  */
 import Prompt from './lib/Prompt';
 export default class PatternPrompt {
-    _pipe: NodeJS.WritableStream;
-    _prompt: Prompt;
-    _entityName: string;
-    _currentUsageRows: number;
+    protected _pipe: NodeJS.WritableStream;
+    protected _prompt: Prompt;
+    protected _entityName: string;
+    protected _currentUsageRows: number;
     constructor(pipe: NodeJS.WritableStream, prompt: Prompt);
     run(onSuccess: () => void, onCancel: () => void, options?: {
         header: string;
diff --git i/packages/jest-watcher/build/lib/Prompt.d.ts w/packages/jest-watcher/build/lib/Prompt.d.ts
index 5f94140e6..259353aa6 100644
--- i/packages/jest-watcher/build/lib/Prompt.d.ts
+++ w/packages/jest-watcher/build/lib/Prompt.d.ts
@@ -6,14 +6,14 @@
  */
 import { ScrollOptions } from '../types';
 export default class Prompt {
-    _entering: boolean;
-    _value: string;
-    _onChange: () => void;
-    _onSuccess: (value?: string) => void;
-    _onCancel: (value?: string) => void;
-    _offset: number;
-    _promptLength: number;
-    _selection: string | null;
+    private _entering;
+    private _value;
+    private _onChange;
+    private _onSuccess;
+    private _onCancel;
+    private _offset;
+    private _promptLength;
+    private _selection;
     constructor();
     private _onResize;
     enter(onChange: (pattern: string, options: ScrollOptions) => void, onSuccess: () => void, onCancel: () => void): void;
diff --git i/packages/jest-worker/build/Farm.d.ts w/packages/jest-worker/build/Farm.d.ts
index 327738cb8..69da4106d 100644
--- i/packages/jest-worker/build/Farm.d.ts
+++ w/packages/jest-worker/build/Farm.d.ts
@@ -4,24 +4,22 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  */
-import { FarmOptions, QueueChildMessage, WorkerInterface } from './types';
+import { FarmOptions } from './types';
 export default class Farm {
-    _computeWorkerKey: FarmOptions['computeWorkerKey'];
-    _cacheKeys: {
-        [key: string]: WorkerInterface;
-    };
-    _callback: Function;
-    _last: Array<QueueChildMessage>;
-    _locks: Array<boolean>;
-    _numOfWorkers: number;
-    _offset: number;
-    _queue: Array<QueueChildMessage | null>;
+    private _computeWorkerKey;
+    private _cacheKeys;
+    private _callback;
+    private _last;
+    private _locks;
+    private _numOfWorkers;
+    private _offset;
+    private _queue;
     constructor(numOfWorkers: number, callback: Function, computeWorkerKey?: FarmOptions['computeWorkerKey']);
     doWork(method: string, ...args: Array<any>): Promise<unknown>;
-    _getNextJob(workerId: number): QueueChildMessage | null;
-    _process(workerId: number): Farm;
-    _enqueue(task: QueueChildMessage, workerId: number): Farm;
-    _push(task: QueueChildMessage): Farm;
+    private _getNextJob;
+    private _process;
+    private _enqueue;
+    private _push;
     lock(workerId: number): void;
     unlock(workerId: number): void;
     isLocked(workerId: number): boolean;
diff --git i/packages/jest-worker/build/base/BaseWorkerPool.d.ts w/packages/jest-worker/build/base/BaseWorkerPool.d.ts
index 21856415b..2ec7b7c8a 100644
--- i/packages/jest-worker/build/base/BaseWorkerPool.d.ts
+++ w/packages/jest-worker/build/base/BaseWorkerPool.d.ts
@@ -6,10 +6,10 @@
  */
 import { WorkerPoolOptions, WorkerOptions, WorkerInterface } from '../types';
 export default class BaseWorkerPool {
-    _stderr: NodeJS.ReadableStream;
-    _stdout: NodeJS.ReadableStream;
-    _options: WorkerPoolOptions;
-    _workers: Array<WorkerInterface>;
+    private readonly _stderr;
+    private readonly _stdout;
+    protected readonly _options: WorkerPoolOptions;
+    private readonly _workers;
     constructor(workerPath: string, options: WorkerPoolOptions);
     getStderr(): NodeJS.ReadableStream;
     getStdout(): NodeJS.ReadableStream;
diff --git i/packages/jest-worker/build/index.d.ts w/packages/jest-worker/build/index.d.ts
index 65c27f3a5..f0537c7e6 100644
--- i/packages/jest-worker/build/index.d.ts
+++ w/packages/jest-worker/build/index.d.ts
@@ -4,8 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  */
-import Farm from './Farm';
-import { WorkerPoolInterface, FarmOptions } from './types';
+import { FarmOptions } from './types';
 /**
  * The Jest farm (publicly called "Worker") is a class that allows you to queue
  * methods across multiple child processes, in order to parallelize work. This
@@ -32,13 +31,13 @@ import { WorkerPoolInterface, FarmOptions } from './types';
  *   caching results.
  */
 export default class JestWorker {
-    _ending: boolean;
-    _farm: Farm;
-    _options: FarmOptions;
-    _workerPool: WorkerPoolInterface;
+    private _ending;
+    private _farm;
+    private _options;
+    private _workerPool;
     constructor(workerPath: string, options?: FarmOptions);
-    _bindExposedWorkerMethods(workerPath: string, options: FarmOptions): void;
-    _callFunctionWithArgs(method: string, ...args: Array<any>): Promise<any>;
+    private _bindExposedWorkerMethods;
+    private _callFunctionWithArgs;
     getStderr(): NodeJS.ReadableStream;
     getStdout(): NodeJS.ReadableStream;
     end(): void;
diff --git i/packages/jest-worker/build/workers/ChildProcessWorker.d.ts w/packages/jest-worker/build/workers/ChildProcessWorker.d.ts
index 07d42cff6..fbc01f0f9 100644
--- i/packages/jest-worker/build/workers/ChildProcessWorker.d.ts
+++ w/packages/jest-worker/build/workers/ChildProcessWorker.d.ts
@@ -4,7 +4,6 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  */
-import { ChildProcess } from 'child_process';
 import { WorkerInterface, ChildMessage, OnEnd, OnStart, WorkerOptions, ParentMessage } from '../types';
 /**
  * This class wraps the child process and provides a nice interface to
@@ -25,10 +24,10 @@ import { WorkerInterface, ChildMessage, OnEnd, OnStart, WorkerOptions, ParentMes
  * same call skip it.
  */
 export default class ChildProcessWorker implements WorkerInterface {
-    _child: ChildProcess;
-    _options: WorkerOptions;
-    _onProcessEnd: OnEnd;
-    _retries: number;
+    private _child;
+    private _options;
+    private _onProcessEnd;
+    private _retries;
     constructor(options: WorkerOptions);
     initialize(): void;
     onMessage(response: ParentMessage): void;
diff --git i/packages/jest-worker/build/workers/NodeThreadsWorker.d.ts w/packages/jest-worker/build/workers/NodeThreadsWorker.d.ts
index d6a7631c8..71c6f158b 100644
--- i/packages/jest-worker/build/workers/NodeThreadsWorker.d.ts
+++ w/packages/jest-worker/build/workers/NodeThreadsWorker.d.ts
@@ -4,13 +4,12 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  */
-import { Worker } from 'worker_threads';
 import { ChildMessage, OnEnd, OnStart, WorkerOptions, WorkerInterface, ParentMessage } from '../types';
 export default class ExperimentalWorker implements WorkerInterface {
-    _worker: Worker;
-    _options: WorkerOptions;
-    _onProcessEnd: OnEnd;
-    _retries: number;
+    private _worker;
+    private _options;
+    private _onProcessEnd;
+    private _retries;
     constructor(options: WorkerOptions);
     initialize(): void;
     onMessage(response: ParentMessage): void;

Test plan

Nothing really, except looking at the diff above. Build should still be green since we don't use anything now hidden

[mattphillips]

LGTM

[github-actions]

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