# Debugging in Tern

The debugger starts where the failure is. **Debug this test** on the Tests page launches the failing JavaScript or
TypeScript test with a breakpoint on the line that failed; in the source editor, `F9` and `F5` do the same for any
JavaScript, TypeScript or Python file in the project. The debugger runs the file on disk, so save edits before launch.

## Launch

| From | Target |
| --- | --- |
| Workspace toolbar **Debug** *file* | The file in the editor. |
| Toolbar **Open debugger**, project menu **Open debugger**, palette **Debug project source** | The target picker. |
| **Debug this test** (`d`) on the project Tests page | The selected test, in test mode, with a breakpoint at its failure line. |
| An `elide://debug` link | The linked file. |
| The source editor's **Debug** overlay, `F5` | The file in the editor. |

The target picker lists the project's files in groups: **Debugged this session**, **Recently tested**, **Tests**,
**Entry points** and **Sources**. `/` opens it, typing filters, `Enter` launches. A JavaScript or TypeScript test
launches in test mode, through the test runner; a Python test file runs as a script.

## Breakpoints

| Where | How |
| --- | --- |
| Breakpoints tab | Enter `file:line` (for example `src/main.mjs:12`) and choose **Add breakpoint**, before launch or while stopped. |
| While stopped | `b` adds a breakpoint at the selected frame's line. |
| Source editor | `F9` toggles a breakpoint on the caret line. |

A breakpoint the debugger has not verified shows the adapter's message. A session holds up to 256 breakpoints.

## Step and inspect

The page has three tabs: **Inspect** (stack and scopes), **Breakpoints** and **Output**; `Tab` and `Shift+Tab`
move between them. The head shows why the target stopped: `Paused at entry`, `Breakpoint reached`, `Step complete`,
`Paused` or `Exception`.

| Key | Action |
| --- | --- |
| `F5` | Continue. |
| `F6` | Pause a running target. |
| `⇧F5` | Stop the target. |
| `F10` | Step over. |
| `F11` | Step into. |
| `⇧F11` | Step out. |
| `j` `k`, `↑` `↓` | Select a frame (Inspect). |
| `b` | Breakpoint at the selected frame's line. |
| `Esc`, `Backspace` | Back out of an opened value (Inspect). `Esc` also dismisses an error, and quits when nothing is open. |
| `q`, `Ctrl+D` | Quit. |

Values are read only when you ask: **Inspect values** reads a scope's values, and selecting a value with children
opens it. Reading a value can run guest getters. Values clear when the target resumes. Source links open a frame's
file at its line.

## Source editor

In the [source editor](/docs/tern-workspace#source-editor), the **Debug** overlay marks breakpoints and the paused line in
the gutter and tints their rows. Before launch it shows **Launch** (`F5`); run and step controls appear while a target
exists.

| Key | Action |
| --- | --- |
| `F9` | Toggle a breakpoint on the caret line. |
| `F5` | Launch, or resume when paused. |
| `F6` | Pause. |
| `⇧F5` | Stop. |
| `F10`, `F11`, `⇧F11` | Step over, into, out. |
| `F7` | Move the caret back to the paused line. |

Breakpoints follow their lines as you edit.

## Shared control

The debugger runs in the project session, so other debugger views, the source editor and Tern's agent see the same
target. One view controls execution; the others observe.

| Action | Effect |
| --- | --- |
| **Take control** | Moves control to this view and drops the previous owner's queued commands. |
| Closing the controlling view | Another view has 30 seconds to take control; then the target stops. |
| **Stop** | Ends the target at once. |

A debug-test run cannot overlap an ordinary test run writing to the same project reports.

## Other debuggers

To debug outside Tern, or to debug Java and Kotlin, attach a CDP, DAP or JDWP client: see
[Debugging](/docs/debugging#attach-a-debugger).

## Next

The test passes. [Coverage](/docs/tern-coverage) shows which lines it reached and which it did not.