How to execute a Javascript function only after multiple other functions have completed?

Question

My specific problem is that I need to execute a (potentially) large number of Javascript functions to prepare something like a batch file (each function call adds some infor

Answer 1

If you want to keep it simple, you can use a counter-based callbacks system. Here's a draft of a system that allows when(A, B).then(C) syntax. (when/then is actually just sugar, but then again the whole system arguably is.)

var when = function() {
  var args = arguments;  // the functions to execute first
  return {
    then: function(done) {
      var counter = 0;
      for(var i = 0; i < args.length; i++) {
        // call each function with a function to call on done
        args[i](function() {
          counter++;
          if(counter === args.length) {  // all functions have notified they're done
            done();
          }
        });
      }
    }
  };
};

Usage:

when(
  function(done) {
    // do things
    done();
  },
  function(done) {
    // do things
    setTimeout(done, 1000);
  },
  ...
).then(function() {
  // all are done
});