promises-over-callbacks.md

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For HTTP Request, our existing solution is to use callbacks.

request(url, (error, response) => {
  // handle success or error.
});

doSomethingElse()

A few problems exist with callbacks. One is known as callback hell. A larger problem is decomposition.

The callback pattern require us to specify the task and the callback at the same time. By difference, promises allow us to specify and dispatch the request in one place.

promise = fetch(url) // fetch is a replacement for XMLHttpRequest

And then add the callback later, and in a different place.

promise.then(response => {
  // handle the response
})

This also allows us to attach multiple handlers to the same task.

promise.then(response => {
  // handle the response.
})

promise.then(response => {
  // do something else with the response.
})