How to filter a list using lodash if the filter function is asynchronous

lodash may not be the best tool for this job. I recommend you use async.

https://github.com/caolan/async#filter

Example: fs.exists is an asynchronous function which checks for the existence of a file then calls a callback.

async.filter(['file1','file2','file3'], fs.exists, function(results){
    // results now equals an array of the existing files
});