Correct way to write loops for promise.

Question

How to correctly construct a loop to make sure the following promise call and the chained logger.log(res) runs synchronously through iterat

Answer 1

Given

• asyncFn function
• array of items

Required

• promise chaining .then()'s in series (in order)
• native es6

Solution

let asyncFn = (item) => {
  return new Promise((resolve, reject) => {
    setTimeout( () => {console.log(item); resolve(true)}, 1000 )
  })
}

// asyncFn('a')
// .then(()=>{return async('b')})
// .then(()=>{return async('c')})
// .then(()=>{return async('d')})

let a = ['a','b','c','d']

a.reduce((previous, current, index, array) => {
  return previous                                    // initiates the promise chain
  .then(()=>{return asyncFn(array[index])})      //adds .then() promise for each item
}, Promise.resolve())