Fix unnecessary mutation of options

src/utils.test.ts

@@ -205,28 +205,38 @@ describe('getBrowserType', () => {
 })
 
 describe('getBrowserOptions', () => {
-  it('should return undefined for empty options', async () => {
+  it('should return empty object for empty options', () => {
     const options = getBrowserOptions(CHROMIUM)
-    expect(options).toBe(undefined)
+    expect(options).toStrictEqual({})
   })
 
-  it('should return root options', async () => {
+  it('should return root options', () => {
     const launchOptions = { headless: false }
     const options = getBrowserOptions(CHROMIUM, launchOptions)
-    expect(options).toBe(launchOptions)
+    expect(options).toStrictEqual(launchOptions)
   })
 
-  it('should return options for defined browser', async () => {
+  it('should return options for defined browser', () => {
     const launchOptions = { headless: false, chromium: { headless: true } }
     const options = getBrowserOptions(CHROMIUM, launchOptions)
     expect(options).toStrictEqual({ headless: true })
   })
 
-  it('should return root options for other browser', async () => {
+  it('should return root options for other browser', () => {
     const launchOptions = { headless: false, chromium: { headless: true } }
     const options = getBrowserOptions(FIREFOX, launchOptions)
     expect(options).toStrictEqual({ headless: false })
   })
+
+  it('should not mutate original options', () => {
+    const launchOptions = { headless: false, chromium: { headless: true } }
+    const options = getBrowserOptions(FIREFOX, launchOptions)
+    expect(options).toStrictEqual({ headless: false })
+    expect(launchOptions).toStrictEqual({
+      headless: false,
+      chromium: { headless: true },
+    })
+  })
 })
 
 describe('checkBrowserEnv', () => {

src/utils.ts

@@ -164,17 +164,14 @@ export function getBrowserOptions<T>(
   browserName: BrowserType,
   options?: Options<T>,
 ): T {
-  let result: Options<T> | undefined = options
-  if (result) {
-    if (result[browserName]) {
-      result = deepMerge(result, result[browserName]!)
-    }
-    BROWSERS.forEach((browser) => {
-      delete result![browser as BrowserType]
-    })
-    return result
-  }
-  return result as T
+  let result: Options<T> = options ? { ...options } : ({} as T)
+  if (result[browserName]) {
+    result = deepMerge(result, result[browserName]!)
+  }
+  BROWSERS.forEach((browser) => {
+    delete result![browser as BrowserType]
+  })
+  return result
 }
 
 export const getSkipFlag = (