Run fn when Promise.all on remaining arguments gets resolved.

doWhen.js

// Sugar.
// Instead of
// 
//  Promise.all([2, calculateValB(3), calculateValC(4)]).then((result) => {
//    var res = calculateResult(result[0], result[1], result[2]);
//    /* process result */
//  });
// 
// write
// 
//  doThen(calculateResult, 2, calculateValB(3), calculateValC(4)).then(/* process result */);
// 
// or
// 
//  var calculate = doThen.bind(null, calculateResult);
//  
//  calculate(2, calcFnB(3), calcFnC(4)).then(/* process result */);

var doThen = (fn, ...args) => { return Promise.all(args).then(fn.apply.bind(fn, null)); };

// ES5:
function doWhen(fn /*, ...args*/) {
  var args = Array.prototype.slice.call(argument, 1);
  return Promise.all(args).then(fn.apply.bind(fn, null));
}

example.js

// defer helper
function defer() {
  var deferred = {};
  deferred.promise = new Promise(function(resolve, reject) {
    deferred.resolve = resolve;
    deferred.reject = reject;
  });
  return deferred;
}

// doThen
var doThen = (fn, ...args) => { return Promise.all(args).then(fn.apply.bind(fn, null)); };

// value B: sync
function calcFnB(b) {
  return b + 2;
}

// value C: async
function calcFnC(c) {
  var deferred = defer();
  window.setTimeout(function() {
    deferred.resolve(c * 2);
  }, 1000);
  return deferred.promise;
}

// final calculation: sync
function calcFn(a,b,c) {
  return a + b + c;
}

// call: calcFn(2, B, C) when calcFnC is resolved.
doThen(calcFn, 2, calcFnB(3), calcFnC(4)).then(function(res) {
  console.log('Result:', res);
});