I have the following code:
function download(url, callback) {
setTimeout(() => {
// script to download the picture here
console.log(`Downloading ${url} ...`);
callback();
}, 3* 1000);
}
download(url);
Why do I need to have a callback function. Can't I just create another function and call that function from within the download function? I don't see the point people saying that callbacks are needed for async programming.
Callbacks are necessary when a value depends on the response of a promise. Often when we request data from other sources, such as an external API, we don’t always know when our data will be served back.
I think what your example is alluding to would be something like this:
function download(url, callback) {
console.log(`Downloading ${url} ...`);
fetch(url)
.then((response) => {
callback(null, response)
})
.catch((error) => {
callback(err, null)
});
}
download("http://example.com/movies.json", function(err, response){
// Do something with the response data
});
Can't I just create another function and call that function from within the download function?
It would make more sense to pass your other function as the callback, like so:
function handleMovieData(err, response) {
// Do something with the response data
}
download("http://example.com/movies.json", handleMovieData);
Nick Parsons' comment explains this well
EDIT: Alternatively to passing in a callback function, you could utilize async/await (untested)
async function download(url) {
console.log(`Downloading ${url} ...`);
return new Promise(function(resolve, reject) {
fetch(url)
.then((response) => {
resolve(response)
})
.catch((error) => {
reject(err)
});
})
}
const movieData = await download("http://example.com/movies.json");
handleMovieData(movieData);
download()rather than waiting 3 seconds to call it. The callback here doesn't represent the start of the download process, but rather the end of it. So once the download which takes x amount of time completes, your callback would get called. Here setTimeout is being used to mimic the download process which takes x amount of time. The code inside the callback would then have access to the data retrieved from performing the download, such as the downloaded content