Return Variable From Function

Question

I am a beginner in JS and I have difficulty understanding why the output of my var is "Undefined." My JS runs a GET to download a JSON file and with a function, I am trying to read the file and return the first line:

invest.stock('APPLE', function (err, data) {
 if (err) { console.log('ERROR', err); return; }
 var APPLE_price = data.order[0]['price'];
 console.log(APPLE_price); //until here works fine
});
 console.log(APPLE_price); //output "Undefined" var

I tried declare the var previously, I created a statement to wait the var (because was a async function) but nothing works.

• Matt Greenberg: Look at javascript variable scope.
• Rohìt Jíndal: @Felipe, I added answer, Hope it will help you to understand the function level scope of the variable in javascript.

Answer 1

Declare the variable outside of the function first:

var APPLE_price;
invest.stock('APPLE', function (err, data) {
 if (err) { console.log('ERROR', err); return; }
 APPLE_price = data.order[0].price;
 console.log(APPLE_price); //until here works fine
});
setTimeout(() => console.log(APPLE_price), 2000);

But it would be far more elegant to use a callback or a Promise:

function getApplePrice() {
 return new Promise((resolve, reject) => {
 invest.stock('APPLE', function(err, data) {
 if (err) {
 reject(err);
 return;
 }
 resolve(data.order[0].price);
 });
 });
}
getApplePrice().then(applePrice => {
 console.log('applePrice is ' + applePrice);
})

Answer 2

your problem here is the variable scoop it's called the local scoop, you can't use

var APPLE_price outside the function

you can find a reference here for javascript scoops JavaScript Scopes for that.In this situation, you can declare the variable outside the function

var Name = " john";
function myFunction() {
// code here can use Name 
 }
// code here can use Name

Answer 3

The issue here is scope.

Since the scope of APPLE_PRICE is limited to the function it is in you cannot access it outside the function. But best to read some tutorials on variable scoping in JavaScript.

Answer 4

var APPLE_price = data.order[0]['price'];

Here, APPLE_price is a Local variable (Function-level scope). As it is declared within a function. Hence, only accessible within that function or by functions inside that function.

If you want access APPLE_price outside of a function you need to declare it outside the function.

var APPLE_price;
invest.stock('APPLE', function (err, data) {
 if (err) { console.log('ERROR', err); return; }
 APPLE_price = data.order[0]['price'];
 console.log(APPLE_price); // value of data.order[0]['price']
});
 console.log(APPLE_price); // value of APPLE_price under invest.stock function.