Demonstration on how Promises work in Javascript

promisedemo.js

// creating a new promise by instantiating the Promise Class
let somePromise = new Promise(function(resolve, reject){
  // the resolve and reject arguments are callback functions to be called.
  // resolve: to be called when the promise is successful. We can pass data to it for the subsequent the 'then' method
  // reject: to be called when the promise is unsuccessful. Again we can pass data to it for the 'catch' method
  try{
    // to mimick asynchoronous behaviour,
    // trigger the timeout callback in 1 sec
    setTimeout(function(){
      console.log('hey there 1 sec later.')

      // 'success promise data' is passed into the 'then' callback function
      resolve('success promise data')

    }, 1000)
  }catch(error){
    // in case of any errors, we will handle them here
    // this is the best place to use the reject callback
    reject('oops there is an error.')
  }
  
});

somePromise.then(function(response){
  // this function will only be triggered when the promise is successfully resolved. 
  // the 'response' argument would be whatever we passed into the resolve callback up there ^
  // in this case it would be the String 'hey there 1 sec later.'
  
  console.log(response); // expected output: 'hey there 1 sec later.'
}).catch(function(error){
  // on the other hand, this function will only be triggered when the promise is not fulfilled. eg when there is an error
  // if the promise failed, we would expect the 'error' argument to be: 'oops there is an error.'
  
  console.log(error); // expected output: 'oops there is an error.'  if the catch block runs. 
})