function will return null from javascript post/get
New to JavaScript, and just started to read about Callback and Promise. I just don't understand the concept of "wait" until I have a value from a POST/GET request.
More specific, why won't this work?
var token = null;
/
/ var options = {headers, form, etc}
/
function getToken() {
request(options)
.then(function (parsedBody) {
this.token = parsedBody
})
.catch(function (err) {
console.log('POST-Error')
})
return token
}
console.log(getToken())
I'm using the 'request-promise' lib in nodejs. As I understand when request is called, then the token variable will be set since its a promise, right?
It's complicated and I don't blame you that you are confused. Everyone new to Node seems to get confused with that.
Your problem here is that the variable will be changed correctly - just not yet when the console.log() is run.
When a function returns a promise then it means that something will happen in the future - possibly in an instant, maybe later, but not yet at the time when that function returns the promise.
Then you can add a fulfillment and rejection handlers to the promise - those handlers will get called when the promise is actually resolved (ie there is some data returned by the request, or there is an error).
So, you cannot write:
console.log(getToken());
What you can write is:
getToken()
.then(data => console.log('Data:', data))
.catch(error => console.log('Error:', error));
But only when you actually return the promise from your function:
function getToken() {
let promise = request(options);
return promise;
}
Now, you could use an async keyword and do something like this:
async function getToken() {
let data = await request(options);
return data;
}
but this function doesn't return data - it returns a promise just like the previous example.
Remember one thing that you cannot return any value from your function that is not available yet. You can only return a promise if you want to return something before you have it. And every asynchronous function - whether it's async function , a function that explicitly returns a promise or a function that takes a callback - will return before the asynchronous action is done, without blocking the thread.
For more details see some of those answers:
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