Promise Q: Little trick for stop and return a value in a sequence

q_stoppable.js

var Q = require('q');

var S = new (function() {
  var STOPPED = function(value) {
    this.value = value
  };
  
  this.return = function(value) {
    throw new STOPPED(value)
  }
  
  this.stoppable = function(promise) {
    return promise.fail(function(msg) {
      if (msg instanceof STOPPED) {
        return Q.resolve(msg.value);
      } else {
        return Q.reject(msg);
      }
    })
  }
})

S.stoppable(
  Q.resolve('first value')
  .then(function(value) {
    console.log(value)
    S.return("ended");
  })
  .then(function() { 
    console.log('never here')
  })
)
.then(function(value) {
  console.log(value)
})
.fail(function(err) {
  console.log("fail", err.stack);
})

// Print
// first value
// ended