Javascript Promises

Create a Javascript Promise

A promise in JavaScript is exactly what it sounds like – you use it to make a promise to do something, usually asynchronously.

When the task completes, you either fulfill your promise or fail to do so. Promise is a constructor function, so you need to use the new keyword to create one.

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

const downloadFile = new Promise((resolve,reject) => {
});

Complete a Promise with resolve and reject

A promise has three states: pending, fulfilled, and rejected.

The promise you created in the last challenge is forever stuck in the pending state because you did not add a way to complete the promise.

The resolve and reject parameters given to the promise argument are used to do this.

resolve is used when you want your promise to succeed, and reject is used when you want it to fail.

const downloadFile = new Promise((resolve, reject) => {
  let responseFromServer;
  
  if(responseFromServer) {
    resolve("File is downloaded");
  } else {  
    reject("Failed to download file");
  }
});

Handle a Fulfilled Promise with then

Promises are most useful when you have a process that takes an unknown amount of time in your code (i.e. something asynchronous), often a server request.

When you make a server request it takes some amount of time, and after it completes you usually want to do something with the response from the server.

This can be achieved by using the then method. The then method is executed immediately after your promise is fulfilled with resolve

const downloadFile = new Promise((resolve, reject) => {
   let responseFromServer = true;
    
  if(responseFromServer) {
    resolve("File is downloaded").then(result => {
      console.log(result);
    });
  } else {  
    reject("Failed to download file");
  }
});

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.

const downloadFile  = new Promise((resolve, reject) => {
  let responseFromServer = false;
    
  if(responseFromServer) {
    resolve("File is downloaded");
  } else {  
    reject("Failed to download file");
  }
});

downloadFile.then(result => {
  console.log(result);
}).catch(error => {
  console.log(error)
});