From 30679250a3c68fe5cd34bb8b1a48681a62e1e308 Mon Sep 17 00:00:00 2001
From: Abhijeet Prasad <aprasad@sentry.io>
Date: Wed, 8 Mar 2023 16:10:42 +0100
Subject: [PATCH 01/11] add hooks types and base implementation

---
 packages/core/src/baseclient.ts | 29 +++++++++++++++++++++++++++++
 packages/types/src/client.ts    | 25 +++++++++++++++++++++++++
 packages/types/src/hooks.ts     | 19 +++++++++++++++++++
 packages/types/src/index.ts     |  1 +
 4 files changed, 74 insertions(+)
 create mode 100644 packages/types/src/hooks.ts

diff --git a/packages/core/src/baseclient.ts b/packages/core/src/baseclient.ts
index b551cfafb8e8..d1534940ae5a 100644
--- a/packages/core/src/baseclient.ts
+++ b/packages/core/src/baseclient.ts
@@ -9,6 +9,9 @@ import type {
   Event,
   EventDropReason,
   EventHint,
+  HookCallback,
+  HookName,
+  HookStore,
   Integration,
   IntegrationClass,
   Outcome,
@@ -97,6 +100,8 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
   /** Holds flushable  */
   private _outcomes: { [key: string]: number } = {};
 
+  private _hooks: HookStore = {};
+
   /**
    * Initializes this client instance.
    *
@@ -351,6 +356,30 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
     }
   }
 
+  /**
+   * @inheritDoc
+   */
+  public on(hook: HookName, callback: HookCallback): void {
+    if (this._hooks[hook]) {
+      // @ts-ignore we cannot enforce the callback to match the hook
+      // while saving bundle size
+      this._hooks[hook].push(callback);
+    } else {
+      this._hooks[hook] = [];
+    }
+  }
+
+  /**
+   * @inheritDoc
+   */
+  public emit(hook: HookName, ...args: Parameters<HookCallback>): void {
+    if (this._hooks[hook]) {
+      // @ts-ignore we cannot enforce the callback to match the hook
+      // while saving bundle size
+      this._hooks[hook].forEach((callback: HookCallback) => callback(...args));
+    }
+  }
+
   /** Updates existing session based on the provided event */
   protected _updateSessionFromEvent(session: Session, event: Event): void {
     let crashed = false;
diff --git a/packages/types/src/client.ts b/packages/types/src/client.ts
index 28d23025ce84..c68d8417c611 100644
--- a/packages/types/src/client.ts
+++ b/packages/types/src/client.ts
@@ -2,6 +2,7 @@ import type { EventDropReason } from './clientreport';
 import type { DataCategory } from './datacategory';
 import type { DsnComponents } from './dsn';
 import type { Event, EventHint } from './event';
+import type { EnvelopeHookCallback, EnvelopeHookName, TransactionHookCallback, TransactionHookName } from './hooks';
 import type { Integration, IntegrationClass } from './integration';
 import type { ClientOptions } from './options';
 import type { Scope } from './scope';
@@ -147,4 +148,28 @@ export interface Client<O extends ClientOptions = ClientOptions> {
    * @param event The dropped event.
    */
   recordDroppedEvent(reason: EventDropReason, dataCategory: DataCategory, event?: Event): void;
+
+  // HOOKS
+
+  /**
+   * Register a callback for transaction start and finish.
+   */
+  on(hook: TransactionHookName, callback: TransactionHookCallback): void;
+
+  /**
+   * Register a callback for envelope creation and sending.
+   */
+  on(hook: EnvelopeHookName, callback: EnvelopeHookCallback): void;
+
+  /**
+   * Fire a hook event for transaction start and finish. Expects to be given a transaction as the
+   * second argument.
+   */
+  emit(hook: TransactionHookName, ...params: Parameters<TransactionHookCallback>): void;
+
+  /*
+   * Fire a hook event for envelope creation and sending. Expects to be given an envelope as the
+   * second argument.
+   */
+  emit(hook: EnvelopeHookName, ...params: Parameters<EnvelopeHookCallback>): void;
 }
diff --git a/packages/types/src/hooks.ts b/packages/types/src/hooks.ts
new file mode 100644
index 000000000000..e478d9cc9b49
--- /dev/null
+++ b/packages/types/src/hooks.ts
@@ -0,0 +1,19 @@
+import type { Envelope } from './envelope';
+import type { Transaction } from './transaction';
+
+export type TransactionHookName = 'startTransaction' | 'transactionFinish';
+export type TransactionHookCallback = (transaction: Transaction) => void;
+
+export type EnvelopeHookName = 'beforeEnvelope';
+export type EnvelopeHookCallback = (envelope: Envelope) => void;
+
+export type HookName = TransactionHookName | EnvelopeHookName;
+export type HookCallback = TransactionHookCallback | EnvelopeHookCallback;
+
+export type HookStoreItem<N extends HookName, C extends HookCallback> = Partial<{ [key in N]: C[] }>;
+
+export type HookStore =
+  // Hooks related to transaction start/finish
+  HookStoreItem<TransactionHookName, TransactionHookCallback> &
+    // Hooks related to envelope create and send
+    HookStoreItem<EnvelopeHookName, EnvelopeHookCallback>;
diff --git a/packages/types/src/index.ts b/packages/types/src/index.ts
index 7f7ed86cfc9a..db9bf0adef78 100644
--- a/packages/types/src/index.ts
+++ b/packages/types/src/index.ts
@@ -96,3 +96,4 @@ export type { WrappedFunction } from './wrappedfunction';
 export type { Instrumenter } from './instrumenter';
 
 export type { BrowserClientReplayOptions } from './browseroptions';
+export type { HookStore, HookName, HookCallback } from './hooks';

From 712feb2ee6e93cdd755f285cec5bb7cdb6713159 Mon Sep 17 00:00:00 2001
From: Abhijeet Prasad <aprasad@sentry.io>
Date: Wed, 8 Mar 2023 16:22:58 +0100
Subject: [PATCH 02/11] add tests

---
 packages/core/src/baseclient.ts     | 10 +++++-----
 packages/core/test/lib/base.test.ts | 21 ++++++++++++++++++++-
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/packages/core/src/baseclient.ts b/packages/core/src/baseclient.ts
index d1534940ae5a..186f453dd963 100644
--- a/packages/core/src/baseclient.ts
+++ b/packages/core/src/baseclient.ts
@@ -360,13 +360,13 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
    * @inheritDoc
    */
   public on(hook: HookName, callback: HookCallback): void {
-    if (this._hooks[hook]) {
-      // @ts-ignore we cannot enforce the callback to match the hook
-      // while saving bundle size
-      this._hooks[hook].push(callback);
-    } else {
+    if (!this._hooks[hook]) {
       this._hooks[hook] = [];
     }
+
+    // @ts-ignore we cannot enforce the callback to match the hook
+    // while saving bundle size
+    this._hooks[hook].push(callback);
   }
 
   /**
diff --git a/packages/core/test/lib/base.test.ts b/packages/core/test/lib/base.test.ts
index d7382ceeeacb..b857f913d51b 100644
--- a/packages/core/test/lib/base.test.ts
+++ b/packages/core/test/lib/base.test.ts
@@ -1,4 +1,4 @@
-import type { Event, Span } from '@sentry/types';
+import type { Event, Span, Transaction } from '@sentry/types';
 import { dsnToString, logger, SentryError, SyncPromise } from '@sentry/utils';
 
 import { Hub, makeSession, Scope } from '../../src';
@@ -1730,4 +1730,23 @@ describe('BaseClient', () => {
       expect(clearedOutcomes4.length).toEqual(0);
     });
   });
+
+  describe('hooks', () => {
+    it('should call a startTransaction hook', () => {
+      expect.assertions(1);
+
+      const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN });
+      const client = new TestClient(options);
+
+      let mockTransaction = {
+        traceId: '86f39e84263a4de99c326acab3bfe3bd',
+      } as Transaction;
+
+      client.on('startTransaction', (transaction: Transaction) => {
+        expect(transaction).toEqual(mockTransaction);
+      });
+
+      client.emit('startTransaction', mockTransaction);
+    });
+  });
 });

From 2350bdb3cbd933090878ce363a9be905dceef7d8 Mon Sep 17 00:00:00 2001
From: Abhijeet Prasad <aprasad@sentry.io>
Date: Wed, 8 Mar 2023 16:30:42 +0100
Subject: [PATCH 03/11] make hook methods optional

---
 packages/core/test/lib/base.test.ts | 4 ++--
 packages/types/src/client.ts        | 9 +++++----
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/packages/core/test/lib/base.test.ts b/packages/core/test/lib/base.test.ts
index b857f913d51b..d50c6d02eaf2 100644
--- a/packages/core/test/lib/base.test.ts
+++ b/packages/core/test/lib/base.test.ts
@@ -1742,11 +1742,11 @@ describe('BaseClient', () => {
         traceId: '86f39e84263a4de99c326acab3bfe3bd',
       } as Transaction;
 
-      client.on('startTransaction', (transaction: Transaction) => {
+      client?.on('startTransaction', (transaction: Transaction) => {
         expect(transaction).toEqual(mockTransaction);
       });
 
-      client.emit('startTransaction', mockTransaction);
+      client?.emit('startTransaction', mockTransaction);
     });
   });
 });
diff --git a/packages/types/src/client.ts b/packages/types/src/client.ts
index c68d8417c611..fd0b58d89f3e 100644
--- a/packages/types/src/client.ts
+++ b/packages/types/src/client.ts
@@ -150,26 +150,27 @@ export interface Client<O extends ClientOptions = ClientOptions> {
   recordDroppedEvent(reason: EventDropReason, dataCategory: DataCategory, event?: Event): void;
 
   // HOOKS
+  // TODO(v8): Make the hooks non-optional.
 
   /**
    * Register a callback for transaction start and finish.
    */
-  on(hook: TransactionHookName, callback: TransactionHookCallback): void;
+  on?(hook: TransactionHookName, callback: TransactionHookCallback): void;
 
   /**
    * Register a callback for envelope creation and sending.
    */
-  on(hook: EnvelopeHookName, callback: EnvelopeHookCallback): void;
+  on?(hook: EnvelopeHookName, callback: EnvelopeHookCallback): void;
 
   /**
    * Fire a hook event for transaction start and finish. Expects to be given a transaction as the
    * second argument.
    */
-  emit(hook: TransactionHookName, ...params: Parameters<TransactionHookCallback>): void;
+  emit?(hook: TransactionHookName, ...params: Parameters<TransactionHookCallback>): void;
 
   /*
    * Fire a hook event for envelope creation and sending. Expects to be given an envelope as the
    * second argument.
    */
-  emit(hook: EnvelopeHookName, ...params: Parameters<EnvelopeHookCallback>): void;
+  emit?(hook: EnvelopeHookName, ...params: Parameters<EnvelopeHookCallback>): void;
 }

From 0e337f65198839dcd4a8a00b234b0b98dd86c32f Mon Sep 17 00:00:00 2001
From: Abhijeet Prasad <aprasad@sentry.io>
Date: Wed, 8 Mar 2023 16:42:33 +0100
Subject: [PATCH 04/11] fix lint

---
 packages/core/test/lib/base.test.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/core/test/lib/base.test.ts b/packages/core/test/lib/base.test.ts
index d50c6d02eaf2..6dec03e1b918 100644
--- a/packages/core/test/lib/base.test.ts
+++ b/packages/core/test/lib/base.test.ts
@@ -1738,7 +1738,7 @@ describe('BaseClient', () => {
       const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN });
       const client = new TestClient(options);
 
-      let mockTransaction = {
+      const mockTransaction = {
         traceId: '86f39e84263a4de99c326acab3bfe3bd',
       } as Transaction;
 

From 79e1f592bf60d3a2b5ed5b058ccf0155d41cff2b Mon Sep 17 00:00:00 2001
From: Francesco Novy <francesco.novy@sentry.io>
Date: Wed, 8 Mar 2023 17:14:31 +0100
Subject: [PATCH 05/11] ref: Make baseclient hooks typesafe when using generics

---
 packages/core/src/baseclient.ts | 16 ++++++----------
 packages/types/src/client.ts    | 10 +++++-----
 packages/types/src/hooks.ts     | 25 +++++++++++++------------
 packages/types/src/index.ts     |  2 +-
 4 files changed, 25 insertions(+), 28 deletions(-)

diff --git a/packages/core/src/baseclient.ts b/packages/core/src/baseclient.ts
index 186f453dd963..b0c638d9ea32 100644
--- a/packages/core/src/baseclient.ts
+++ b/packages/core/src/baseclient.ts
@@ -9,8 +9,7 @@ import type {
   Event,
   EventDropReason,
   EventHint,
-  HookCallback,
-  HookName,
+  Hook,
   HookStore,
   Integration,
   IntegrationClass,
@@ -359,24 +358,21 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
   /**
    * @inheritDoc
    */
-  public on(hook: HookName, callback: HookCallback): void {
+  public on<HookType extends Hook>(hook: HookType['name'], callback: HookType['callback']): void {
     if (!this._hooks[hook]) {
       this._hooks[hook] = [];
     }
 
-    // @ts-ignore we cannot enforce the callback to match the hook
-    // while saving bundle size
-    this._hooks[hook].push(callback);
+    (this._hooks[hook] as HookType['callback'][]).push(callback);
   }
 
   /**
    * @inheritDoc
    */
-  public emit(hook: HookName, ...args: Parameters<HookCallback>): void {
+  public emit<HookType extends Hook>(hook: HookType['name'], ...args: Parameters<HookType['callback']>): void {
     if (this._hooks[hook]) {
-      // @ts-ignore we cannot enforce the callback to match the hook
-      // while saving bundle size
-      this._hooks[hook].forEach((callback: HookCallback) => callback(...args));
+      // @ts-ignore it does not like ...args, but we know this is correct
+      (this._hooks[hook] as HookType['callback'][]).forEach(callback => callback(...args));
     }
   }
 
diff --git a/packages/types/src/client.ts b/packages/types/src/client.ts
index fd0b58d89f3e..1d5eff7584e6 100644
--- a/packages/types/src/client.ts
+++ b/packages/types/src/client.ts
@@ -2,7 +2,7 @@ import type { EventDropReason } from './clientreport';
 import type { DataCategory } from './datacategory';
 import type { DsnComponents } from './dsn';
 import type { Event, EventHint } from './event';
-import type { EnvelopeHookCallback, EnvelopeHookName, TransactionHookCallback, TransactionHookName } from './hooks';
+import type { EnvelopeHook, TransactionHook } from './hooks';
 import type { Integration, IntegrationClass } from './integration';
 import type { ClientOptions } from './options';
 import type { Scope } from './scope';
@@ -155,22 +155,22 @@ export interface Client<O extends ClientOptions = ClientOptions> {
   /**
    * Register a callback for transaction start and finish.
    */
-  on?(hook: TransactionHookName, callback: TransactionHookCallback): void;
+  on?(hook: TransactionHook['name'], callback: TransactionHook['callback']): void;
 
   /**
    * Register a callback for envelope creation and sending.
    */
-  on?(hook: EnvelopeHookName, callback: EnvelopeHookCallback): void;
+  on?(hook: EnvelopeHook['name'], callback: EnvelopeHook['callback']): void;
 
   /**
    * Fire a hook event for transaction start and finish. Expects to be given a transaction as the
    * second argument.
    */
-  emit?(hook: TransactionHookName, ...params: Parameters<TransactionHookCallback>): void;
+  emit?(hook: TransactionHook['name'], ...params: Parameters<TransactionHook['callback']>): void;
 
   /*
    * Fire a hook event for envelope creation and sending. Expects to be given an envelope as the
    * second argument.
    */
-  emit?(hook: EnvelopeHookName, ...params: Parameters<EnvelopeHookCallback>): void;
+  emit?(hook: EnvelopeHook['name'], ...params: Parameters<EnvelopeHook['callback']>): void;
 }
diff --git a/packages/types/src/hooks.ts b/packages/types/src/hooks.ts
index e478d9cc9b49..6f4988d92811 100644
--- a/packages/types/src/hooks.ts
+++ b/packages/types/src/hooks.ts
@@ -1,19 +1,20 @@
 import type { Envelope } from './envelope';
 import type { Transaction } from './transaction';
 
-export type TransactionHookName = 'startTransaction' | 'transactionFinish';
-export type TransactionHookCallback = (transaction: Transaction) => void;
+// Hooks related to transaction start/finish
+export type TransactionHook = {
+  name: 'startTransaction' | 'transactionFinish';
+  callback: (transaction: Transaction) => void;
+};
 
-export type EnvelopeHookName = 'beforeEnvelope';
-export type EnvelopeHookCallback = (envelope: Envelope) => void;
+// Hooks related to envelope create and send
+export type EnvelopeHook = {
+  name: 'beforeEnvelope';
+  callback: (envelope: Envelope) => void;
+};
 
-export type HookName = TransactionHookName | EnvelopeHookName;
-export type HookCallback = TransactionHookCallback | EnvelopeHookCallback;
+export type Hook = TransactionHook | EnvelopeHook;
 
-export type HookStoreItem<N extends HookName, C extends HookCallback> = Partial<{ [key in N]: C[] }>;
+export type HookStoreItem<T extends Hook> = Partial<Record<T['name'], T['callback'][]>>;
 
-export type HookStore =
-  // Hooks related to transaction start/finish
-  HookStoreItem<TransactionHookName, TransactionHookCallback> &
-    // Hooks related to envelope create and send
-    HookStoreItem<EnvelopeHookName, EnvelopeHookCallback>;
+export type HookStore = HookStoreItem<EnvelopeHook> & HookStoreItem<TransactionHook>;
diff --git a/packages/types/src/index.ts b/packages/types/src/index.ts
index db9bf0adef78..961d910adf47 100644
--- a/packages/types/src/index.ts
+++ b/packages/types/src/index.ts
@@ -96,4 +96,4 @@ export type { WrappedFunction } from './wrappedfunction';
 export type { Instrumenter } from './instrumenter';
 
 export type { BrowserClientReplayOptions } from './browseroptions';
-export type { HookStore, HookName, HookCallback } from './hooks';
+export type { HookStore, Hook, TransactionHook, EnvelopeHook } from './hooks';

From 99f67e23b2bdc0a806838aa3a5c633ef7947211b Mon Sep 17 00:00:00 2001
From: Francesco Novy <francesco.novy@sentry.io>
Date: Wed, 8 Mar 2023 17:17:26 +0100
Subject: [PATCH 06/11] use generic for hook

---
 packages/core/test/lib/base.test.ts | 26 +++++++++++++++++++++++---
 packages/types/src/client.ts        | 17 +++--------------
 2 files changed, 26 insertions(+), 17 deletions(-)

diff --git a/packages/core/test/lib/base.test.ts b/packages/core/test/lib/base.test.ts
index 6dec03e1b918..85f12e3f3353 100644
--- a/packages/core/test/lib/base.test.ts
+++ b/packages/core/test/lib/base.test.ts
@@ -1,4 +1,4 @@
-import type { Event, Span, Transaction } from '@sentry/types';
+import { Event, Span, Transaction, TransactionHook, EnvelopeHook, Envelope } from '@sentry/types';
 import { dsnToString, logger, SentryError, SyncPromise } from '@sentry/utils';
 
 import { Hub, makeSession, Scope } from '../../src';
@@ -1742,11 +1742,31 @@ describe('BaseClient', () => {
         traceId: '86f39e84263a4de99c326acab3bfe3bd',
       } as Transaction;
 
-      client?.on('startTransaction', (transaction: Transaction) => {
+      client?.on<TransactionHook>('startTransaction', (transaction: Transaction) => {
         expect(transaction).toEqual(mockTransaction);
       });
 
-      client?.emit('startTransaction', mockTransaction);
+      client?.emit<TransactionHook>('startTransaction', mockTransaction);
+    });
+
+    it('should call a beforeEnvelope hook', () => {
+      expect.assertions(1);
+
+      const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN });
+      const client = new TestClient(options);
+
+      const mockEnvelope = [
+        {
+          event_id: '12345',
+        },
+        {},
+      ] as Envelope;
+
+      client?.on<EnvelopeHook>('beforeEnvelope', (envelope: Envelope) => {
+        expect(envelope).toEqual(mockEnvelope);
+      });
+
+      client?.emit<EnvelopeHook>('beforeEnvelope', mockEnvelope);
     });
   });
 });
diff --git a/packages/types/src/client.ts b/packages/types/src/client.ts
index 1d5eff7584e6..4481b7ddfea5 100644
--- a/packages/types/src/client.ts
+++ b/packages/types/src/client.ts
@@ -2,7 +2,7 @@ import type { EventDropReason } from './clientreport';
 import type { DataCategory } from './datacategory';
 import type { DsnComponents } from './dsn';
 import type { Event, EventHint } from './event';
-import type { EnvelopeHook, TransactionHook } from './hooks';
+import type { Hook } from './hooks';
 import type { Integration, IntegrationClass } from './integration';
 import type { ClientOptions } from './options';
 import type { Scope } from './scope';
@@ -155,22 +155,11 @@ export interface Client<O extends ClientOptions = ClientOptions> {
   /**
    * Register a callback for transaction start and finish.
    */
-  on?(hook: TransactionHook['name'], callback: TransactionHook['callback']): void;
-
-  /**
-   * Register a callback for envelope creation and sending.
-   */
-  on?(hook: EnvelopeHook['name'], callback: EnvelopeHook['callback']): void;
-
-  /**
-   * Fire a hook event for transaction start and finish. Expects to be given a transaction as the
-   * second argument.
-   */
-  emit?(hook: TransactionHook['name'], ...params: Parameters<TransactionHook['callback']>): void;
+  on?<HookType extends Hook>(hook: HookType['name'], callback: HookType['callback']): void;
 
   /*
    * Fire a hook event for envelope creation and sending. Expects to be given an envelope as the
    * second argument.
    */
-  emit?(hook: EnvelopeHook['name'], ...params: Parameters<EnvelopeHook['callback']>): void;
+  emit?<HookType extends Hook>(hook: HookType['name'], ...args: Parameters<HookType['callback']>): void;
 }

From 729dedb98dda8f8e66ae1c04eb6d1960e5ce6690 Mon Sep 17 00:00:00 2001
From: Francesco Novy <francesco.novy@sentry.io>
Date: Wed, 8 Mar 2023 17:28:21 +0100
Subject: [PATCH 07/11] un-export HookStoreItem

---
 packages/types/src/hooks.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/types/src/hooks.ts b/packages/types/src/hooks.ts
index 6f4988d92811..f3e4ca389bed 100644
--- a/packages/types/src/hooks.ts
+++ b/packages/types/src/hooks.ts
@@ -15,6 +15,6 @@ export type EnvelopeHook = {
 
 export type Hook = TransactionHook | EnvelopeHook;
 
-export type HookStoreItem<T extends Hook> = Partial<Record<T['name'], T['callback'][]>>;
+type HookStoreItem<T extends Hook> = Partial<Record<T['name'], T['callback'][]>>;
 
 export type HookStore = HookStoreItem<EnvelopeHook> & HookStoreItem<TransactionHook>;

From 52cd479264c91e02552c5b3b40fecfac2c0eeff3 Mon Sep 17 00:00:00 2001
From: Francesco Novy <francesco.novy@sentry.io>
Date: Wed, 8 Mar 2023 18:16:45 +0100
Subject: [PATCH 08/11] ref: Make types explicit

---
 packages/core/src/baseclient.ts     | 39 ++++++++++++-------
 packages/core/test/lib/base.test.ts | 58 +++++++++++++++--------------
 packages/types/src/client.ts        | 18 +++++++--
 packages/types/src/hooks.ts         | 20 ----------
 packages/types/src/index.ts         |  1 -
 5 files changed, 71 insertions(+), 65 deletions(-)
 delete mode 100644 packages/types/src/hooks.ts

diff --git a/packages/core/src/baseclient.ts b/packages/core/src/baseclient.ts
index b0c638d9ea32..bf1ae616c4d6 100644
--- a/packages/core/src/baseclient.ts
+++ b/packages/core/src/baseclient.ts
@@ -9,8 +9,6 @@ import type {
   Event,
   EventDropReason,
   EventHint,
-  Hook,
-  HookStore,
   Integration,
   IntegrationClass,
   Outcome,
@@ -19,6 +17,7 @@ import type {
   SessionAggregates,
   Severity,
   SeverityLevel,
+  Transaction,
   TransactionEvent,
   Transport,
 } from '@sentry/types';
@@ -99,7 +98,8 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
   /** Holds flushable  */
   private _outcomes: { [key: string]: number } = {};
 
-  private _hooks: HookStore = {};
+  // eslint-disable-next-line @typescript-eslint/ban-types
+  private _hooks: Record<string, Function[]> = {};
 
   /**
    * Initializes this client instance.
@@ -355,24 +355,35 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
     }
   }
 
-  /**
-   * @inheritDoc
-   */
-  public on<HookType extends Hook>(hook: HookType['name'], callback: HookType['callback']): void {
+  // Keep on() & emit() signatures in sync with types' client.ts interface
+
+  /** @inheritdoc */
+  public on(hook: 'startTransaction' | 'finishTransaction', callback: (transaction: Transaction) => void): void;
+
+  /** @inheritdoc */
+  public on(hook: 'beforeEnvelope', callback: (envelope: Envelope) => void): void;
+
+  /** @inheritdoc */
+  public on(hook: string, callback: unknown): void {
     if (!this._hooks[hook]) {
       this._hooks[hook] = [];
     }
 
-    (this._hooks[hook] as HookType['callback'][]).push(callback);
+    // @ts-ignore We assue the types are correct
+    this._hooks[hook].push(callback);
   }
 
-  /**
-   * @inheritDoc
-   */
-  public emit<HookType extends Hook>(hook: HookType['name'], ...args: Parameters<HookType['callback']>): void {
+  /** @inheritdoc */
+  public emit(hook: 'startTransaction' | 'finishTransaction', transaction: Transaction): void;
+
+  /** @inheritdoc */
+  public emit(hook: 'beforeEnvelope', envelope: Envelope): void;
+
+  /** @inheritdoc */
+  public emit(hook: string, ...rest: unknown[]): void {
     if (this._hooks[hook]) {
-      // @ts-ignore it does not like ...args, but we know this is correct
-      (this._hooks[hook] as HookType['callback'][]).forEach(callback => callback(...args));
+      // @ts-ignore we cannot enforce the callback to match the hook
+      this._hooks[hook].forEach(callback => callback(...rest));
     }
   }
 
diff --git a/packages/core/test/lib/base.test.ts b/packages/core/test/lib/base.test.ts
index 85f12e3f3353..d09928566f40 100644
--- a/packages/core/test/lib/base.test.ts
+++ b/packages/core/test/lib/base.test.ts
@@ -1,4 +1,4 @@
-import { Event, Span, Transaction, TransactionHook, EnvelopeHook, Envelope } from '@sentry/types';
+import type { Envelope, Event, Span, Transaction, Client } from '@sentry/types';
 import { dsnToString, logger, SentryError, SyncPromise } from '@sentry/utils';
 
 import { Hub, makeSession, Scope } from '../../src';
@@ -1732,41 +1732,45 @@ describe('BaseClient', () => {
   });
 
   describe('hooks', () => {
-    it('should call a startTransaction hook', () => {
-      expect.assertions(1);
+    const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN });
 
-      const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN });
-      const client = new TestClient(options);
+    // Make sure types work for both Client & BaseClient
+    const scenarios = [
+      ['BaseClient', new TestClient(options)],
+      ['Client', new TestClient(options) as Client],
+    ] as const;
 
-      const mockTransaction = {
-        traceId: '86f39e84263a4de99c326acab3bfe3bd',
-      } as Transaction;
+    describe.each(scenarios)('with client %s', (_, client) => {
+      it('should call a startTransaction hook', () => {
+        expect.assertions(1);
 
-      client?.on<TransactionHook>('startTransaction', (transaction: Transaction) => {
-        expect(transaction).toEqual(mockTransaction);
-      });
+        const mockTransaction = {
+          traceId: '86f39e84263a4de99c326acab3bfe3bd',
+        } as Transaction;
 
-      client?.emit<TransactionHook>('startTransaction', mockTransaction);
-    });
+        client.on?.('startTransaction', transaction => {
+          expect(transaction).toEqual(mockTransaction);
+        });
 
-    it('should call a beforeEnvelope hook', () => {
-      expect.assertions(1);
+        client.emit?.('startTransaction', mockTransaction);
+      });
 
-      const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN });
-      const client = new TestClient(options);
+      it('should call a beforeEnvelope hook', () => {
+        expect.assertions(1);
 
-      const mockEnvelope = [
-        {
-          event_id: '12345',
-        },
-        {},
-      ] as Envelope;
+        const mockEnvelope = [
+          {
+            event_id: '12345',
+          },
+          {},
+        ] as Envelope;
 
-      client?.on<EnvelopeHook>('beforeEnvelope', (envelope: Envelope) => {
-        expect(envelope).toEqual(mockEnvelope);
-      });
+        client.on?.('beforeEnvelope', envelope => {
+          expect(envelope).toEqual(mockEnvelope);
+        });
 
-      client?.emit<EnvelopeHook>('beforeEnvelope', mockEnvelope);
+        client.emit?.('beforeEnvelope', mockEnvelope);
+      });
     });
   });
 });
diff --git a/packages/types/src/client.ts b/packages/types/src/client.ts
index 4481b7ddfea5..3839f794b8e1 100644
--- a/packages/types/src/client.ts
+++ b/packages/types/src/client.ts
@@ -2,7 +2,6 @@ import type { EventDropReason } from './clientreport';
 import type { DataCategory } from './datacategory';
 import type { DsnComponents } from './dsn';
 import type { Event, EventHint } from './event';
-import type { Hook } from './hooks';
 import type { Integration, IntegrationClass } from './integration';
 import type { ClientOptions } from './options';
 import type { Scope } from './scope';
@@ -10,6 +9,8 @@ import type { SdkMetadata } from './sdkmetadata';
 import type { Session, SessionAggregates } from './session';
 import type { Severity, SeverityLevel } from './severity';
 import type { Transport } from './transport';
+import { Transaction } from './transaction';
+import { Envelope } from './envelope';
 
 /**
  * User-Facing Sentry SDK Client.
@@ -155,11 +156,22 @@ export interface Client<O extends ClientOptions = ClientOptions> {
   /**
    * Register a callback for transaction start and finish.
    */
-  on?<HookType extends Hook>(hook: HookType['name'], callback: HookType['callback']): void;
+  on?(hook: 'startTransaction' | 'finishTransaction', callback: (transaction: Transaction) => void): void;
+
+  /**
+   * Register a callback for transaction start and finish.
+   */
+  on?(hook: 'beforeEnvelope', callback: (envelope: Envelope) => void): void;
+
+  /**
+   * Fire a hook event for transaction start and finish. Expects to be given a transaction as the
+   * second argument.
+   */
+  emit?(hook: 'startTransaction' | 'finishTransaction', transaction: Transaction): void;
 
   /*
    * Fire a hook event for envelope creation and sending. Expects to be given an envelope as the
    * second argument.
    */
-  emit?<HookType extends Hook>(hook: HookType['name'], ...args: Parameters<HookType['callback']>): void;
+  emit?(hook: 'beforeEnvelope', envelope: Envelope): void;
 }
diff --git a/packages/types/src/hooks.ts b/packages/types/src/hooks.ts
deleted file mode 100644
index f3e4ca389bed..000000000000
--- a/packages/types/src/hooks.ts
+++ /dev/null
@@ -1,20 +0,0 @@
-import type { Envelope } from './envelope';
-import type { Transaction } from './transaction';
-
-// Hooks related to transaction start/finish
-export type TransactionHook = {
-  name: 'startTransaction' | 'transactionFinish';
-  callback: (transaction: Transaction) => void;
-};
-
-// Hooks related to envelope create and send
-export type EnvelopeHook = {
-  name: 'beforeEnvelope';
-  callback: (envelope: Envelope) => void;
-};
-
-export type Hook = TransactionHook | EnvelopeHook;
-
-type HookStoreItem<T extends Hook> = Partial<Record<T['name'], T['callback'][]>>;
-
-export type HookStore = HookStoreItem<EnvelopeHook> & HookStoreItem<TransactionHook>;
diff --git a/packages/types/src/index.ts b/packages/types/src/index.ts
index 961d910adf47..7f7ed86cfc9a 100644
--- a/packages/types/src/index.ts
+++ b/packages/types/src/index.ts
@@ -96,4 +96,3 @@ export type { WrappedFunction } from './wrappedfunction';
 export type { Instrumenter } from './instrumenter';
 
 export type { BrowserClientReplayOptions } from './browseroptions';
-export type { HookStore, Hook, TransactionHook, EnvelopeHook } from './hooks';

From f68eb4e0d60f7373679d46b43007ec2ef2ab33f1 Mon Sep 17 00:00:00 2001
From: Abhijeet Prasad <aprasad@sentry.io>
Date: Wed, 8 Mar 2023 18:31:32 +0100
Subject: [PATCH 09/11] fix lint

---
 packages/core/test/lib/base.test.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/core/test/lib/base.test.ts b/packages/core/test/lib/base.test.ts
index d09928566f40..7b0cd1dadc95 100644
--- a/packages/core/test/lib/base.test.ts
+++ b/packages/core/test/lib/base.test.ts
@@ -1,4 +1,4 @@
-import type { Envelope, Event, Span, Transaction, Client } from '@sentry/types';
+import type { Client,Envelope, Event, Span, Transaction } from '@sentry/types';
 import { dsnToString, logger, SentryError, SyncPromise } from '@sentry/utils';
 
 import { Hub, makeSession, Scope } from '../../src';

From 32182f7262e4a68395ee6beb2f9009360f4833b5 Mon Sep 17 00:00:00 2001
From: Abhijeet Prasad <aprasad@sentry.io>
Date: Wed, 8 Mar 2023 19:07:29 +0100
Subject: [PATCH 10/11] fix types lint

---
 packages/types/src/client.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/types/src/client.ts b/packages/types/src/client.ts
index 3839f794b8e1..71e2ac5a96d6 100644
--- a/packages/types/src/client.ts
+++ b/packages/types/src/client.ts
@@ -1,6 +1,7 @@
 import type { EventDropReason } from './clientreport';
 import type { DataCategory } from './datacategory';
 import type { DsnComponents } from './dsn';
+import type { Envelope } from './envelope';
 import type { Event, EventHint } from './event';
 import type { Integration, IntegrationClass } from './integration';
 import type { ClientOptions } from './options';
@@ -8,9 +9,8 @@ import type { Scope } from './scope';
 import type { SdkMetadata } from './sdkmetadata';
 import type { Session, SessionAggregates } from './session';
 import type { Severity, SeverityLevel } from './severity';
+import type { Transaction } from './transaction';
 import type { Transport } from './transport';
-import { Transaction } from './transaction';
-import { Envelope } from './envelope';
 
 /**
  * User-Facing Sentry SDK Client.

From 51570b580da9d7838a85b57110030fd9bd35a050 Mon Sep 17 00:00:00 2001
From: Abhijeet Prasad <aprasad@sentry.io>
Date: Wed, 8 Mar 2023 19:27:34 +0100
Subject: [PATCH 11/11] prettier

---
 packages/core/test/lib/base.test.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/core/test/lib/base.test.ts b/packages/core/test/lib/base.test.ts
index 7b0cd1dadc95..4ee65114fe79 100644
--- a/packages/core/test/lib/base.test.ts
+++ b/packages/core/test/lib/base.test.ts
@@ -1,4 +1,4 @@
-import type { Client,Envelope, Event, Span, Transaction } from '@sentry/types';
+import type { Client, Envelope, Event, Span, Transaction } from '@sentry/types';
 import { dsnToString, logger, SentryError, SyncPromise } from '@sentry/utils';
 
 import { Hub, makeSession, Scope } from '../../src';