youtube-auto-liker 
This userscript automatically likes videos of channels you're subscribed to on YouTube, after you've watched at least half of the video.
Download
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This is a userscript. To use it you'll first need one of the following browser extensions:
Tampermonkey Greasemonkey Violentmonkey 
Link (Beta) - Link 
Link Link - 
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- Then install this script from one of the following links:
- GitHub (Recommended)
- Greasy Fork
- OpenUserJS
Options
You can configure the following options by editing OPTIONS within the script.
| Option | Description | Default |
|---|---|---|
CHECK_FREQUENCY |
How often to attempt liking the video. | 5000 |
WATCH_THRESHOLD |
The percentage of the video watched before liking.  |
0.5 |
HIDE_LIKE_NOTIFICATION |
Hide like notifications.  |
false |
LIKE_IF_NOT_SUBSCRIBED |
Whether to like videos when you are not subscribed. | false |
TODO
- Add options:
- Retries - How many times to retry liking upon failure.
- Whitelisted channels - A list of channels to always like the videos of, even if you are not subscribed to them.
- Blacklisted channels - A list of channels to never like the videos of, even if you are subscribed to them.
- Add a UI so that the user can configure the options without needing to manually edit the script.
Acknowledgements
- The userscript itself was inspired by this one by JoeSimmons, which hasn't worked since 2015
- The debugger code was adapted from this Stack Overflow answer by arctelix