Looking at the logic of what constitutes a JavaScript promise - I can see that it is a way of separating out callbacks from asynchronous code

Promise Experiment

function P (cb) {
  var _nextFn
  var _catchFn

  /**
   * User registers a callback via
   * the 'then()' instance fn
   */
  this.then = function (cb) {
    _nextFn = cb
    return this
  }

  /**
   * User registers an error handler
   * via the 'catch()' instance fn
   */
  this.catch = function (cb) {
    _catchFn = cb
    return this
  }

  /**
   * User signals that the registered callback
   * must not be executed, and the user provides
   * a value to pass to that callback
   */
  function resolve (value) {
    try {
      _nextFn(value)
    } catch (error) {
      _catchFn(error)
    }  
  }

  /**
   * If an error is thrown then the registered
   * error handler should be invoked
   */
  function reject (error) {
    _catchFn(error)
  }

  cb(resolve, reject)
}

const p = new P( resolve => setTimeout(() => resolve('Some Value'), 1000) )
p.then( value => { throw new Error (value) } )
p.catch( error => console.log('Error caught:', error.message))

console.log('The End')