From eee70919c65d37c0d3154371191b1a581f9deaa3 Mon Sep 17 00:00:00 2001
From: Rup Sarmah <84681565+sarmah-rup@users.noreply.github.com>
Date: Mon, 5 Oct 2026 18:39:27 +0530
Subject: [PATCH] (fix) tracker-js: apply error sampleRate the right way round

captureError skipped an error when sampleRate >= Math.random(), so
sampleRate: 1 dropped every error and sampleRate: 0 sent all of them.
Skip when Math.random() >= sampleRate instead, which matches the
documented meaning (share of errors to send).
---
 packages/tracker-js/src/Lib.ts                |  2 +-
 .../tracker-js/tests/errorSampling.test.ts    | 61 +++++++++++++++++++
 2 files changed, 62 insertions(+), 1 deletion(-)
 create mode 100644 packages/tracker-js/tests/errorSampling.test.ts

diff --git a/packages/tracker-js/src/Lib.ts b/packages/tracker-js/src/Lib.ts
index 2cb44c59e..07af3996e 100644
--- a/packages/tracker-js/src/Lib.ts
+++ b/packages/tracker-js/src/Lib.ts
@@ -400,7 +400,7 @@ export class Lib {
   }
 
   captureError(event: ErrorEvent): void {
-    if (typeof this.errorsOptions?.sampleRate === 'number' && this.errorsOptions.sampleRate >= Math.random()) {
+    if (typeof this.errorsOptions?.sampleRate === 'number' && Math.random() >= this.errorsOptions.sampleRate) {
       return
     }
 
diff --git a/packages/tracker-js/tests/errorSampling.test.ts b/packages/tracker-js/tests/errorSampling.test.ts
new file mode 100644
index 000000000..95ea2bd0e
--- /dev/null
+++ b/packages/tracker-js/tests/errorSampling.test.ts
@@ -0,0 +1,61 @@
+import { Lib } from '../src/Lib'
+import { setLocation } from './testUtils'
+
+describe('Error sampling', () => {
+  let lib: Lib
+  let sendRequest: jest.SpyInstance
+
+  const errorEvent = () =>
+    new ErrorEvent('error', {
+      message: 'Boom',
+      filename: 'app.js',
+      lineno: 1,
+      colno: 1,
+      error: new Error('Boom'),
+    })
+
+  beforeEach(() => {
+    setLocation({ pathname: '/home' })
+    lib = new Lib('test-project', { devMode: true })
+    sendRequest = jest.spyOn(lib as any, 'sendRequest').mockResolvedValue(undefined)
+  })
+
+  afterEach(() => {
+    jest.restoreAllMocks()
+  })
+
+  test('sends every error when sampleRate is 1', () => {
+    const actions = lib.trackErrors({ sampleRate: 1 })
+    jest.spyOn(Math, 'random').mockReturnValue(0.5)
+
+    lib.captureError(errorEvent())
+    actions.stop()
+
+    expect(sendRequest).toHaveBeenCalledWith('error', expect.objectContaining({ name: 'Error', message: 'Boom' }))
+  })
+
+  test('sends no errors when sampleRate is 0', () => {
+    const actions = lib.trackErrors({ sampleRate: 0 })
+    jest.spyOn(Math, 'random').mockReturnValue(0.5)
+
+    lib.captureError(errorEvent())
+    actions.stop()
+
+    expect(sendRequest).not.toHaveBeenCalled()
+  })
+
+  test('keeps roughly sampleRate share of errors', () => {
+    const actions = lib.trackErrors({ sampleRate: 0.2 })
+    const random = jest.spyOn(Math, 'random')
+
+    random.mockReturnValue(0.1)
+    lib.captureError(errorEvent())
+    expect(sendRequest).toHaveBeenCalledTimes(1)
+
+    random.mockReturnValue(0.9)
+    lib.captureError(errorEvent())
+    expect(sendRequest).toHaveBeenCalledTimes(1)
+
+    actions.stop()
+  })
+})