here's how to create promise-based functions

promise.js

// say take a number and return its double

// resolve : the success call . which goes in the first then
// reject : the error call, which goes in .catch

function double(number){
  return new Promise((resolve, reject) => {
    if(typeof number == 'number'){
      resolve(number * 2)
    } else reject("the argument should a Number")
  })
}

// outputs 8
double(4).then(d => console.log(d))
.catch(err => console.log(err))