Why handle fetch with two then methods?

JavaScript
#1

I see the fetch() pattern uses two then() methods to handle data when successful, but i dont understand why?

#2

Hello there,

I assume you are referring to something like:

fetch('some-url').then(res = > res.json()).then(data => console.log(data));

If so, this is because there are two asynchronous actions taking place:

  1. fetch returns a promise
  2. .json() returns a promise

If you want to see some of the internals, you could try:

fetch('https://freecodecamp.org/sky020').then(res => console.log(res));

Note: you are likely to get CORS errors, but the principle is still the same

Nowadays, I prefer reading through the async/await syntax:

const res = await fetch('some-url');
const data = await res.json();
console.log(data);

Hope this helps

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#3

ok that makes sense. I think i didnt evaluate that json() returns a promise and im likely to need to use it.
How come the XMLHttpRequest method doesnt use json() but directly JSON.parse() etc?

#4

Well, .json() and JSON... are not comparable. Looking at an example:

const req = new XMLHttpRequest();
req.open("GET",'some-url',true);
req.send();
req.onload = function(){
  const json = JSON.parse(req.responseText);
  // Do something with json
};

In this instance, it would be more comparable to take req.onload as being similar to the .then method. However, the main difference in this case is req.responseText is nothing but a string - it only has string methods (i.e. no .json() method).

So, if you want to use a string as an object (specifically, a JSON object), you need to convert (parse) the string into an object. Essentially (an example),

req.responseText = "{someProperty: \"some value\", key: \"value\"}";

Now, parsing text (a string) does not need to happen asynchronously. I can write my own parser:

const responseText = "{someProperty: \"some value\", key: \"value\"}";
function myJSONParser(text) {
  const propRegex = new RegExp(/\w+(?=:)/g);
  const properties = text.match(propRegex)
  const valueRegex = new RegExp(/(?<=")[^,}]+/g);
  const vals = text.match(valueRegex);
  console.log(properties, vals);
  const obj = properties.map((prop, i) => ({prop: vals[i]}))
  return obj;
}

console.log(myJSONParser(responseText));

All of that is just to say, there is no need for the parser to be asynchronous.

Hope this helps

1 Like
#5

you helped me a lot, thanks for your thorough replies!

1 Like