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Fetch API

The Fetch API provides a generic definition of Request and Response objects (and other things involved with network requests). It also defines related concepts such as CORS and the HTTP Origin header semantics, supplanting their separate definitions elsewhere. Fetch returns a Promise, an object represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

Asynchronous functions

The magic of async functions is that we can write asynchronous code that looks like synchronous code. It's still asynchronous code but instead of results and errors landing inside callback functions, errors are thrown naturally as exceptions and results naturally land on the next line of code.

The key features of async functions are:

  • async functions implicitly create and return promises.
  • In an async function, await consumes promises, marking a point where the code will wait asynchronously for the promise to settle.
  • While the function is waiting for the promisse to settle, the thread can run other code.
  • Exceptions are rejections, and rejections are exceptions; returns are resolutions, and fulfillments are results (that is, if you await a promise, you see the promise's fulfillment value as the result of the await expression).

Fetch API in action

The simplest use of fetch() takes one argument - the path to the resource you want to fetch - and does not directly return the JSON response body but instead returns a Promise that resolves with a Response object.

The Response object, in turn, does not directly contain the actual JSON response body but is instead a representation of the entire HTTP response. So, to extract the JSON body content from the Response object, we use the json() method, which returns a second promise that resolves with the result of parsing the response body text as JSON.

const url = 'https://api.andrespecht.dev/movies';

const options = {
  method: 'GET',
  mode: 'cors'
};

async function getMovies() {
  try {
    const response = await fetch(url, options);
    if (!response.ok) {
      throw new Error(`${response.statusText} (${response.status})`);
    }
    // Parsing the reponse as JSON
    const data = await response.json();
    // Printing the movies
    console.log(data.response);
  } catch(error) {
    console.log(error);
  }
};

getMovies();

CORS

Cross-Origin Resource Sharing (CORS) is a protocol that enables scripts running on a browser client to interact with resources from a different origin. This is useful because, thanks to the same-origin policy followed by fetch, JavaScript can only make calls to URLs that live on the same origin as the location where the script is running. For example, if a JavaScript app wishes to make an AJAX call to an API running on a different domain, it would be blocked from doing so thanks to the same-origin policy.

References