I've been messing around with the fetch()
api recently, and noticed something which was a bit quirky.
let url = "http://jsonplaceholder.typicode.com/posts/6";
let iterator = fetch(url);
iterator
.then(response => {
return {
data: response.json(),
status: response.status
}
})
.then(post => document.write(post.data));
;
post.data
returns a Promise
object.
http://jsbin.com/wofulo/2/edit?js,output
However if it is written as:
let url = "http://jsonplaceholder.typicode.com/posts/6";
let iterator = fetch(url);
iterator
.then(response => response.json())
.then(post => document.write(post.title));
;
post
here is a standard Object
which you can access the title attribute.
http://jsbin.com/wofulo/edit?js,output
So my question is: why does response.json
return a promise in an object literal, but return the value if just returned?
Why does
response.json
return a promise?
Because you receive the response
as soon as all headers have arrived. Calling .json()
gets you another promise for the body of the http response that is yet to be loaded. See also Why is the response object from JavaScript fetch API a promise?.
Why do I get the value if I return the promise from the
then
handler?
Because that's how promises work. The ability to return promises from the callback and get them adopted is their most relevant feature, it makes them chainable without nesting.
You can use
fetch(url).then(response =>
response.json().then(data => ({
data: data,
status: response.status
})
).then(res => {
console.log(res.status, res.data.title)
}));
or any other of the approaches to access previous promise results in a .then() chain to get the response status after having awaited the json body.