Bluebird .call()

.call(     String methodName,     [any args...] ) 

This is a convenience method for doing:

promise.then(function(obj) {     return obj[methodName].call(obj, arg...); }); 

For example (some is a built-in array method):

var Promise = require("bluebird"); var fs = Promise.promisifyAll(require("fs")); var path = require("path"); var thisPath = process.argv[2] || "."; var now = Date.now();  fs.readdirAsync(thisPath)     .map(function(fileName) {         return fs.statAsync(path.join(thisPath, fileName));     })     .call("some", function(stat) {         return (now - new Date(stat.mtime)) < 10000;     })     .then(function(someFilesHaveBeenModifiedLessThanTenSecondsAgo) {         console.log(someFilesHaveBeenModifiedLessThanTenSecondsAgo) ;     }); 

Chaining lo-dash or underscore methods (Copy-pasteable example):

var Promise = require("bluebird"); var pmap = Promise.map; var props = Promise.props; var _ = require("lodash"); var fs = Promise.promisifyAll(require("fs"));  function getTotalSize(paths) {     return pmap(paths, function(path) {         return fs.statAsync(path).get("size");     }).reduce(function(a, b) {         return a + b;     }, 0); }  fs.readdirAsync(".").then(_)     .call("groupBy", function(fileName) {         return fileName.charAt(0);     })     .call("map", function(fileNames, firstCh) {         return props({             firstCh: firstCh,             count: fileNames.length,             totalSize: getTotalSize(fileNames)         });     })     // Since the currently wrapped array contains promises we need to unwrap it and call .all() before continuing the chain     // If the currently wrapped thing was an object with properties that might be promises, we would call .props() instead     .call("value").all().then(_)     .call("sortBy", "count")     .call("reverse")     .call("map", function(data) {         return data.count + " total files beginning with " + data.firstCh + " with total size of " + data.totalSize + " bytes";     })     .call("join", "\n")     .then(console.log)