javascript - Implementing Promise.series as alternative to Promise.all -

i saw example implementation of promise.all - runs promises in parallel - implementing promise.all

note functionality looking akin bluebird's promise.mapseries http://bluebirdjs.com/docs/api/mapseries.html

i making attempt @ creating promise.series, have seems work intended (it totally wrong, don't use it, see answers):

promise.series = function series(promises){      return new promise(function(resolve,reject){      const ret = promise.resolve(null);     const results = [];      promises.foreach(function(p,i){          ret.then(function(){             return p.then(function(val){                results[i] = val;             });          });     });      ret.then(function(){          resolve(results);     },      function(e){         reject(e);      });      });  }   promise.series([     new promise(function(resolve){             resolve('a');     }),     new promise(function(resolve){             resolve('b');     })     ]).then(function(val){         console.log(val);     }).catch(function(e){         console.error(e.stack);     }); 

however, 1 potential problem implementation if reject promise, doesn't seem catch it:

 promise.series([     new promise(function(resolve, reject){             reject('a');   // << reject here     }),     new promise(function(resolve){             resolve('b');     })     ]).then(function(val){         console.log(val);     }).catch(function(e){         console.error(e.stack);     }); 

does know why error doesn't caught , if there way fix promises?

according comment, made change:

promise.series = function series(promises){      return new promise(function(resolve,reject){      const ret = promise.resolve(null);     const results = [];      promises.foreach(function(p,i){          ret.then(function(){             return p.then(function(val){                results[i] = val;             },             function(r){                 console.log('rejected');                 reject(r);   // << handle rejected promises here             });          });     });      ret.then(function(){          resolve(results);     },      function(e){         reject(e);      });      });  } 

but still doesn't work expected...

the promise returned then in foreach loop not handle potential errors.

as pointed out in comment @slezica, try use reduce rather foreach, chains promises together.

promise.series = function series(promises) {     const ret = promise.resolve(null);     const results = [];      return promises.reduce(function(result, promise, index) {          return result.then(function() {             return promise.then(function(val) {                results[index] = val;             });          });     }, ret).then(function() {         return results;     }); } 

keep in mind promises "running" @ point though. if want run promises in series, should adjust function , pass in array of functions return promises. this:

promise.series = function series(providers) {     const ret = promise.resolve(null);     const results = [];      return providers.reduce(function(result, provider, index) {          return result.then(function() {             return provider().then(function(val) {                results[index] = val;             });          });     }, ret).then(function() {         return results;     }); }