Added an initialiserFn parameter to the 'get' function.

[nickgrealy]

Hi all,

I had a problem, trying to demultiplex a call chain, and realised I could solve it using the lru-cache with maxAge and promises - awesome!

To that end, I thought the library would benefit from having a get(key, initialiserFn) function, so that:

• the existing get(key) signature would behave as per existing functionality
• a user can specify an optional initialiser function, to populate the cached value (if not present)
• the initialiser function may be an asynchronous/expensive operation/take a long time/be rate limited
• in this way, the initialiser is invoked once, and it's response is cached for multiple calls
• this works well, for initialiser functions that return a Promise
• if maxAge is specified, the value would eventually be evicted

Here's an actual example I'm using it for (using socket.io):

server.js

// setup an LRU cache (with eviction policy)...
const PROFILE_CACHE = LRU({ max: 50, maxAge: 1000 * 60 })

// receives "lots" of async requests from a client...
socket.on('getUserProfileRequest', userId => {
  PROFILE_CACHE.get(userId, () => getUserProfile(userId))
    .then(profile => socket.emit('getUserProfileResponse', profile))
})

// calls upstream server, does async/expensive/long running work, returns a Promise...
function getUserProfile(userId) { /* ... */ }