feat(browser): implement locator.nth() (#7137)

Co-authored-by: Vladimir <sleuths.slews0s@icloud.com>

Author: xeger

docs/guide/browser/locators.md

@@ -387,6 +387,69 @@ It is recommended to use this only after the other locators don't work for your
 
 - [testing-library's `ByTestId`](https://testing-library.com/docs/queries/bytestid/)
 
+## nth
+
+```ts
+function nth(index: number): Locator
+```
+
+This method returns a new locator that matches only a specific index within a multi-element query result. Unlike `elements()[n]`, the `nth` locator will be retried until the element is present.
+
+```html
+<div aria-label="one"><input/><input/><input/></div>
+<div aria-label="two"><input/></div>
+```
+
+```tsx
+page.getByRole('textbox').nth(0) // ✅
+page.getByRole('textbox').nth(4) // ❌
+```
+
+::: tip
+Before resorting to `nth`, you may find it useful to use chained locators to narrow down your search.
+Sometimes there is no better way to distinguish than by element position; although this can lead to flake, it's better than nothing.
+:::
+
+```tsx
+page.getByLabel('two').getByRole('input') // ✅ better alternative to page.getByRole('textbox').nth(3)
+page.getByLabel('one').getByRole('input') // ❌ too ambiguous
+page.getByLabel('one').getByRole('input').nth(1) // ✅ pragmatic compromise
+```
+
+## first
+
+```ts
+function first(): Locator
+```
+
+This method returns a new locator that matches only the first index of a multi-element query result.
+It is sugar for `nth(0)`.
+
+```html
+<input/> <input/> <input/>
+```
+
+```tsx
+page.getByRole('textbox').first() // ✅
+```
+
+## last
+
+```ts
+function last(): Locator
+```
+
+This method returns a new locator that matches only the last index of a multi-element query result.
+It is sugar for `nth(-1)`.
+
+```html
+<input/> <input/> <input/>
+```
+
+```tsx
+page.getByRole('textbox').last() // ✅
+```
+
 ## Methods
 
 All methods are asynchronous and must be awaited. Since Vitest 3, tests will fail if a method is not awaited.

packages/browser/src/client/tester/locators/index.ts

@@ -185,6 +185,18 @@ export abstract class Locator {
     return this.elements().map(element => this.elementLocator(element))
   }
 
+  public nth(index: number): Locator {
+    return this.locator(`nth=${index}`)
+  }
+
+  public first(): Locator {
+    return this.nth(0)
+  }
+
+  public last(): Locator {
+    return this.nth(-1)
+  }
+
   public toString(): string {
     return this.selector
   }

test/browser/fixtures/locators/blog.test.tsx

@@ -22,5 +22,10 @@ test('renders blog posts', async () => {
 
   expect(screen.getByRole('listitem').all()).toHaveLength(3)
 
+  expect(screen.getByRole('listitem').nth(0).element()).toHaveTextContent(/molestiae ut ut quas/)
+  await expect.element(screen.getByRole('listitem').nth(666)).not.toBeInTheDocument()
+  expect(screen.getByRole('listitem').first().element()).toHaveTextContent(/molestiae ut ut quas/)
+  expect(screen.getByRole('listitem').last().element()).toHaveTextContent(/eum et est/)
+
   expect(screen.getByPlaceholder('non-existing').query()).not.toBeInTheDocument()
 })