reducer-test.js

const Promise = require('bluebird');

let array = [
  10, 10, 10, 10, 10, 10
];

return Promise.reduce(array,
  (total, key, idx) => {
    console.log(idx);
    if(total == 20)
      return Promise.reject(total);
    else {
      return Promise.resolve(total+key);
    }
  }, 0
)
.then( x => console.log("then", x))
.catch( x => console.log("catch", x))