How to get watched history of youtube user using Youtube Javascript API?

This no longer seems possible with the V3 API. The watchHistory playlist ID is now always set to 'HL'.

Here's the general procedure:

1) Get the "watch History" playlist of a given user's channel (as a channel ID is the key to getting user info). Note that this will ONLY work when a user is authenticated via oAuth2.

https://www.googleapis.com/youtube/v3/channels?part=contentDetails&mine=true&key={YOUR_API_KEY}

With that response, there should be a "watchHistory" playlist ... take it and call the playlistItems endpoint:

https://www.googleapis.com/youtube/v3/playlistItems?part=snippet&playlistId=HLTFxEo9ofKM2Siifxoy5V_A&key={YOUR_API_KEY}

Unfortunately, many users are reporting there's a bug in the watch history playlist through the API right now:

https://code.google.com/p/gdata-issues/issues/detail?id=4642

So your mileage may vary.