139 lines
4.5 KiB
Markdown
139 lines
4.5 KiB
Markdown
# Debugging
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Debugging with Puppeteer can be an arduous task. There is no _single_ method for
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debugging all possible issues since Puppeteer touches many distinct components
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of a browser such as network requests and Web APIs. On a high note, Puppeteer
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provides _several_ methods for debugging which hopefully does cover all possible
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issues.
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## Background
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In general, there are two possible sources of an issue: Code running on Node.js
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(which we call _server code_), and
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[code running in the browser](../api/puppeteer.page.evaluate)
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(which we call _client code_). There is also a third possible source being the
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browser itself (which we call _internal code_), but if you suspect this is the
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source **after attempting the methods below**, we suggest
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[searching existing issues](https://github.com/puppeteer/puppeteer/issues)
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before
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[filing an issue](https://github.com/puppeteer/puppeteer/issues/new/choose).
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## Debugging methods for all situations
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These methods can be used to debug any situation. These should be used as a
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quick sanity check before diving into more complex methods.
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### Turn off [`headless`](../api/puppeteer.browserlaunchargumentoptions)
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Sometimes it's useful to see what the browser is displaying. Instead of
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launching in
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[`headless`](../api/puppeteer.browserlaunchargumentoptions) mode,
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launch a full version of the browser with
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[`headless`](../api/puppeteer.browserlaunchargumentoptions) set to
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`false`:
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```ts
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const browser = await puppeteer.launch({headless: false});
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```
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### Puppeteer "slow-mo"
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The [`slowMo`](../api/puppeteer.browserconnectoptions) option slows down
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Puppeteer operations by a specified amount of milliseconds. It's another way to
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help see what's going on.
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```ts
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const browser = await puppeteer.launch({
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headless: false,
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slowMo: 250, // slow down by 250ms
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});
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```
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## Debugging methods for client code
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### Capture `console.*` output
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Since client code runs in the browser, doing `console.*` in client code will not
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directly log to Node.js. However, you can [listen](../api/puppeteer.page.on) for
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the [`console`](../api/puppeteer.pageeventobject) event which returns a
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payload with the logged text.
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```ts
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page.on('console', msg => console.log('PAGE LOG:', msg.text()));
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await page.evaluate(() => console.log(`url is ${location.href}`));
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```
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### Use the debugger in the browser
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1. Set [`devtools`](../api/puppeteer.browserlaunchargumentoptions) to
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`true` when launching Puppeteer:
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```ts
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const browser = await puppeteer.launch({devtools: true});
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```
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2. Add `debugger` inside any client code you want debugged. For example,
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```ts
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await page.evaluate(() => {
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debugger;
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});
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```
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The Browser will now stop in the location the `debugger` word is found in
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debug mode.
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## Debugging methods for server code
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### Use the debugger in Node.js (Chrome/Chromium-only)
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Since server code intermingles with client code, this method of debugging is
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closely tied with the browser. For example, you can step over
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`await page.click()` in the server script and see the click happen in the
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browser.
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Note that you won't be able to run `await page.click()` in DevTools console due
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to this
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[Chromium bug](https://bugs.chromium.org/p/chromium/issues/detail?id=833928), so
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if you want to try something out, you have to add it to your test file.
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1. Set [`headless`](../api/puppeteer.browserlaunchargumentoptions) to
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`false`.
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2. Add `debugger` to any server code you want debugged. For example,
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```ts
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debugger;
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await page.click('a[target=_blank]');
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```
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3. Run your server code with `--inspect-brk`. For example,
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```bash
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node --inspect-brk path/to/script.js
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```
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4. In the opened Chrome/Chromium browser, open `chrome://inspect/#devices` and
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click `inspect`.
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5. In the newly opened test browser, press `F8` to resume test execution.
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6. Now your `debugger` statement will be hit and you can debug in the test
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browser.
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### Log DevTools protocol traffic
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If all else fails, it's possible there may be an issue between Puppeteer and the
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DevTools protocol. You can debug this by setting the `DEBUG` environment
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variable before running your script. This will log internal traffic via
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[`debug`](https://github.com/visionmedia/debug) under the `puppeteer` namespace.
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```bash
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# Basic verbose logging
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env DEBUG="puppeteer:*" node script.js
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# Prevent truncating of long messages
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env DEBUG="puppeteer:*" env DEBUG_MAX_STRING_LENGTH=null node script.js
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# Protocol traffic can be rather noisy. This example filters out all Network domain messages
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env DEBUG="puppeteer:*" env DEBUG_COLORS=true node script.js 2>&1 | grep -v '"Network'
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```
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