Image: pica

http://nodeca.github.io/pica/demo/

Check demo to see why it's awesome :) . That's not first attempt to write high quality resizer in pure js. But all previous were finished with conclusions: "Yes, algorythm works, but speed sucks". Pica's speed does not :)

To be honest:

1. it's not as fast as C, because JS still does not have SIMD. But it's still very useable. Sometime you have no alternatives to js. For example, to reduce uploads on client side.
2. It operates on RGBA buffers, not on jpg/png. It's a well implemented heavy math, to use for more high level wrappers.