intro-to-es6-promises.md

Promises

• to do something, usually asynchronously.

• When the task completes, either fulfill promise or fail to do so.

• Promise is a constructor function

• use the new keyword to create one.

• It takes a function, as its argument, with two parameters - resolve and reject.

• These are methods used to determine the outcome of the promise.

• Syntax:

  const myPromise = new Promise((resolve, reject) => {

  });

• 3 states:

1. pending
2. fulfilled
3. rejected

• resolve - for promise to succeed
• reject - when you want promise to fail

ex:

const makeServerRequest = new Promise((resolve, reject) => {
  // responseFromServer represents a response from a server
  let responseFromServer;
    
  if(responseFromServer) {
    resolve("We got the data");
  } else {  
    reject("Data not received")
  }
});   

Handle a Fulfilled Promise with then

• then method is executed immediately after your promise is fulfilled with resolve

ex:

makeServerRequest.then(result => {
  console.log(result);
});

Handle a Rejected Promise with catch

• catch is the method used when your promise has been rejected.
• It is executed immediately after a promise's reject method is called.

ex:

makeServerRequest.catch(error => {
  console.log(error);
});