How to early break reduce() method?

Question

How can I break the iteration of reduce() method?

for:

for (var i = Things.length - 1; i >= 0; i--) {
  if(Things[i] <=         


        

Answer 1

If you want to chain promises sequentially with reduce using the pattern below:

return [1,2,3,4].reduce(function(promise,n,i,arr){
   return promise.then(function(){
       // this code is executed when the reduce loop is terminated,
       // so truncating arr here or in the call below does not works
       return somethingReturningAPromise(n);
   });
}, Promise.resolve());

But need to break according to something happening inside or outside a promise things become a little bit more complicated because the reduce loop is terminated before the first promise is executed, making truncating the array in the promise callbacks useless, I ended up with this implementation:

function reduce(array, promise, fn, i) {
  i=i||0;
  return promise
  .then(function(){
    return fn(promise,array[i]);
  })
  .then(function(result){
    if (!promise.break && ++i

Then you can do something like this:

var promise=Promise.resolve();
reduce([1,2,3,4],promise,function(promise,val){
  return iter(promise, val);
}).catch(console.error);

function iter(promise, val) {
  return new Promise(function(resolve, reject){
    setTimeout(function(){
      if (promise.break) return reject('break');
      console.log(val);
      if (val==3) {promise.break=true;}
      resolve(val);
    }, 4000-1000*val);
  });
}