fetch is an asynchronous function; it doesn't return the response right away (you get a Promise instead).

If you want to fetch the data, you need to use the await (if the fetch is called inside another async function), or provide a callback via then method in the Promise from the fetch call:

fetch('https://power.larc.nasa.gov/api/temporal/monthly/point?parameters=ALLSKY_SFC_SW_DNI&community=RE&longitude=48.0000&latitude=27.0000&format=JSON&start=2001&end=2020')
.then(response => {
   const data = response.json();
});

Working with data from response

If you would do something like this, it won't work:

let data;

fetch('https://power.larc.nasa.gov/api/temporal/monthly/point?parameters=ALLSKY_SFC_SW_DNI&community=RE&longitude=48.0000&latitude=27.0000&format=JSON&start=2001&end=2020')
.then(response => {
   data = response.json();
});

console.log(data);

The reason is because the callback inside the then method is executed after the response has been returned (this might take seconds for example), but the console.log(data) is executed immediately after the fetch is called. This means that the data isn't assigned yet and will be probably undefined. For this reason, if you want to put data somewhere, you need to put your code inside the callback.

function processData(responseData) {
    // Here, you can put your code to process the data from response
    console.log(responseData);
}

fetch('https://power.larc.nasa.gov/api/temporal/monthly/point?parameters=ALLSKY_SFC_SW_DNI&community=RE&longitude=48.0000&latitude=27.0000&format=JSON&start=2001&end=2020')
.then(response => {
    const data = response.json();

    processData(data);
});

This way the data will be processed after it is fetched.

Fetch returns a Promise since you are sending a request to an API. This type of functions is asynchronous since we don't know when we're getting back the response from the API.

You can either use async/await or .then() and .catch() to execute your code when you get back the response.

Try this:

let returnedData;
fetch('https://power.larc.nasa.gov/api/temporal/monthly/point?parameters=ALLSKY_SFC_SW_DNI&community=RE&longitude=48.0000&latitude=27.0000&format=JSON&start=2001&end=2020')
.then((data) => {
    console.log(data)
    returnedData = data
})
.catch((error) => {
    console.log(error)
})