Javascript Promises

Promises are special objects that can continue operations in asynchronous mode. They use a resolve and reject callbacks to finalize tasks.

Syntaxis

const myTask = new Promise((resolve, reject) => {
  // TODO: Do some complex operation, when you finish resolve the task
  // Example: Download some file and resolve with the result or the error
  download(url, (error, result) => {
    if (error) {
      reject(error) // Resolve with error
      return
    }
    resolve(result) // Resolve without error
  })
})

Call the promise and handle the flow

When the promise is resolved you can continue operation flow into a then callback or handle the error into the catch callback.

myTask.then(result => {
  // TODO: Continue with the result passing into `resolve` callback
  console.log(`Success with result`, result)
}).catch(error => {
  // TODO: Handle the error to avoid crashing.
  console.error(`This is the error generated: ${error}`)
  // TODO: Continue here (when error was generated)
})

Create a Promise chain

downloadFile(url).then(file => {
  return saveFile(file)
}).then(result => {
  return notifyFileSaved(result)
}).catch(error => {
  console.log(`There was an error in some place of the chain. Error is ${error}`)
})

Create your own functions returning promises

You can make functions that return promises.

function waitFor(time) {
  return new Promise(resolve => setTimeout(resolve, time))
}

console.log("Waiting for 10s...")
waitFor(10000).then(() => {
  console.log("Now 10s have passed")
  // TODO: Continue operations
})