Network Requests: POSTs

Learning Goals

• Get more familiar with the Promise object and the fetch API
• Know what POST requests do and how to use them

Vocab

• The Promise Object - represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

POST with fetch

What if we want to add information to a database?

If we want to use fetch to make any other kind of request, we’ll have to add an optional init object into the function call.

fetch(url, {
  method:"POST",
})

fetch(url, {
  method: 'POST',
  body: JSON.stringify(someDataToSend), // remember how HTTP can only send and receive strings, just like localStorage?
  headers: {
  	'Content-Type': 'application/json'
  }
})
  .then(response => response.json())
  .then(json => /*do something with json*/)
  .catch(err => /*do something with the error*/);

Nice to Know: Query Strings / URL Structure

What’s all that weird stuff in the URL we’re fetching?

Fetch and XMLHttpRequest Objects take the url as one of their arguments. The URL itself can be thought of containing sub-arguments that give these request objects and methods more information. The entire anatomy of a URL can be broken down into a series of informative pieces, but the ones we’re focused on today are queries.

Anything coming after the ? in a url is part of a query. Queries can be broken down into categories and arguments. Each category / argument pair is separated by an &.

In the example from above:

fetch("https://opentdb.com/api.php?amount=1&category=27&type=multiple")

we’re querying information about the amount, category, and type of the trivia we want to receive.

Promises

“A Promise is an object representing the eventual completion or failure of an asynchronous operation”

In our case, we can think of Promises as a placeholder that will do something once it receives a response back from the trivia server.

The great thing about promises is that since they are just objects we can move them around like an object and can return them from functions.

function getTrivia(number, categoryId) {
  const root = 'https://opentdb.com/api.php';
  const url = `${root}?amount=${number}&category=${categoryId}&type=multiple`;
  const promise = fetch(url)
                  .then(response => response.json());

  return promise;
}

getTrivia(10, 27)
  .then(data => console.log(data))
  .catch(err => /* do something else */);

What is this asynchronous thing all about?

Let’s say we’re at a restaurant for a night out on the town…Here’s how the experience would go in each scenario:

• Synchronous: I order my food, everyone in the restaurant has to wait until I get my food before the next person can order.

• Asynchronous: I order my food, the order is put into a queue, other food is made in the meantime, my food is ready, and the server brings it to me.

A Non-AJAX Example: setTimeout()

console.log("Legen...");

setTimeout(() => {
  console.log("DARY!");
}, 2000);

console.log("Wait for it...");

setTimeout() is actually an asynchronous function, which executes its callback after waiting for the allotted time to expire.

Further Reading:

• The Evolution of Asynchronous JavaScript
• Fetch
• Promises
• Postman – good tool for testing out APIs
• GETs, POSTs and DELETEs, man! Intro to Postman - Instructor video for getting started with Postman