user4682: I don't have a great understanding of how bitcoin mining works, but how useful would a JavaScript miner be for collecting revenue from visitors? Stupid question, but if I put the user in as part of a mining pool, must their computer actually find a hash in order for me to make revenue? Is this at all practical?

user1891: Not practical at all. Most probably the users would be just pissed of. Right now it is not even practical to mine with GPU-s, not to mention CPU-s and for sure JS.
Regarding the question: 
"if I put the user in as part of a mining pool, ..."
No, it is not important that he would find a hash. It is working this way. Your all workers are calculating hashes (does not matter if you find the right one or no). If a miner mined something at that time, the miner calculates how much work you have done and gives you the amount of bitcoins. So if you contributed 5% - you will get 5% of 25BTC and in case of some pulls fees. If pull mined nothing, you get nothing.
So in total, if you do not want to piss of users, do not do this.

user1409: It would not at all be useful for Bitcoin mining. However, it may be useful for other digital cryptocurrencies, such as Litecoin or PPCoin.

user4358: Javascript has access to OpenGL ES, including shaders, so it has access to programmable parts of the GPU at nearly native speeds. I think that a smart kid might be able to build a fragment shader that does SHA256 hashing, which outputs to the stencil buffer, so that Javascript has a 2-way communication channel with the GPU.
So yeah, you could probably use javascript to mine at really nice speeds. Maybe 70% or 80% of what an OpenCL GPU miner does?
[Edit] oh look, someone already built a fragment shader that does that
[Edit 2] and an actual JS/WebGL application

4682: <p>I don't have a great understanding of how bitcoin mining works, but how useful would a JavaScript miner be for collecting revenue from visitors? Stupid question, but if I put the user in as part of a mining pool, must their computer actually find a hash in order for me to make revenue? Is this at all practical?</p>
 1891: <p>Not practical at all. Most probably the users would be just pissed of. Right now it is not even practical to mine with GPU-s, not to mention CPU-s and for sure JS.</p>

<p>Regarding the question: 
"if I put the user in as part of a mining pool, ..."</p>

<p>No, it is not important that he would find a hash. It is working this way. Your all workers are calculating hashes (does not matter if you find the right one or no). If a miner mined something at that time, the miner calculates how much work you have done and gives you the amount of bitcoins. So if you contributed 5% - you will get 5% of 25BTC and in case of some pulls fees. If pull mined nothing, you get nothing.</p>

<p>So in total, if you do not want to piss of users, do not do this.</p>
 1409: <p>It would not at all be useful for Bitcoin mining. However, it <em>may</em> be useful for other digital cryptocurrencies, such as Litecoin or PPCoin.</p>
 4358: <p>Javascript has access to OpenGL ES, including shaders, so it has access to <em>programmable</em> parts of the GPU at nearly native speeds. I think that a smart kid might be able to build a <em>fragment shader</em> that does SHA256 hashing, which outputs to the stencil buffer, so that Javascript has a 2-way communication channel with the GPU.</p>

<p>So yeah, you could probably use javascript to mine at really nice speeds. Maybe 70% or 80% of what an OpenCL GPU miner does?</p>

<p>[Edit] <a href="https://bitcointalk.org/index.php?topic=4618.msg191488#msg191488">oh look, someone already built a fragment shader that does that</a><br>
[Edit 2] <a href="https://bitcointalk.org/index.php?topic=27056.0">and an actual JS/WebGL application</a></p>
