Fetching API and setting a variable to the res - node.js

Question

const fetch = require('node-fetch');
let body = { a: 1 };
const stopId = 413
fetch(`https://api.ashx?stopId=${stopId}`, {
method: 'post',
body: JSON.stringify(body),
headers: { 'Content-Type': 'application/json' },
})
.then(res => res.json())
.then(json => body = json);
console.log(body)
I'm getting the output: { a: 1 } Instead of the API JsonResponse, however when I use .then(json => console.log(json)); I get the desired response..
I've tried to use await fetch, to pause the code till the promise returned then to console.log body but it needs to be an async function.. Does anyone know how I can assign the let body a new value before proceeding to the code below? Or would there be a way to return from .then ?
So I could do something like: (I know this doesn't work)
function fetchStop(stopId){
fetch(`https://api.ashx?stopId=${stopId}`, {
method: 'post',
body: JSON.stringify(body),
headers: { 'Content-Type': 'application/json' },
})
.then(res => res.json())
.then(json => return body);
}
console.log(fetchStop(stopId))
Any solutions or explanations/insights on how these things work is much appreciated, very much a noob with async and promises

Answer 1

The fetch executes asynchronously and you can access the result only in the callback.
Here, the console.log(body) executes soon after a network call is initiated.
const fetch = require('node-fetch');
let body = { a: 1 };
const stopId = 413
fetch(`https://api.ashx?stopId=${stopId}`, {
method: 'post',
body: JSON.stringify(body),
headers: { 'Content-Type': 'application/json' },
})
.then(res => res.json())
.then(json => body = json);
console.log(body)
To access the result,
function fetchStop(stopId){
return fetch(`https://api.ashx?stopId=${stopId}`, {
method: 'post',
body: JSON.stringify(body),
headers: { 'Content-Type': 'application/json' },
})
.then(res => res.json())
}
fetchStop(stopId).then(result => console.log(result))

Answer 2

You are using promise for fetching data from your URL https://api.ashx?stopId=${stopId}. Since this will take time and is asynchronous (nonblocking) so while it is fetching data code will move to console.log(body) and print the previous body (body = { a: 1 };). Because code flow moves to console.log before the promise gets executed, this promise will take time to fetch data. So you have to console.log within then itself. Because that's the point when your promise is getting executed later in time. You can do it easily using async await
const yourFunction = async () => {
const fetch = require('node-fetch');
let body = { a: 1 };
const stopId = 413;
const { hostname: location } = window.location;
const data = {
method: 'post',
body: JSON.stringify(body),
headers: { 'Content-Type': 'application/json' },
}
const response = await fetch(`https://api.ashx?stopId=${stopId}`, data);
if (!response.ok) throw Error(response.message);
try {
body = await response.json();
return;
} catch (err) {
throw err;
}
};