inflight

Add callbacks to requests in flight to avoid async duplication

USAGE

var inflight = require('inflight')
// some request that does some stuff
function req(key, callback) {
 // key is any random string. like a url or filename or whatever.
 //
 // will return either a falsey value, indicating that the
 // request for this key is already in flight, or a new callback
 // which when called will call all callbacks passed to inflightk
 // with the same key
 callback = inflight(key, callback)
 // If we got a falsey value back, then there's already a req going
 if (!callback) return
 // this is where you'd fetch the url or whatever
 // callback is also once()-ified, so it can safely be assigned
 // to multiple events etc. First call wins.
 setTimeout(function() {
 callback(null, key)
 }, 100)
}
// only assigns a single setTimeout
// when it dings, all cbs get called
req('foo', cb1)
req('foo', cb2)
req('foo', cb3)
req('foo', cb4)