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.

@Felipe, I added answer, Hope it will help you to understand the function level scope of the variable in javascript. — Rohìt Jíndal
– Rohìt Jíndal, Commented Mar 28, 2018 at 7:05

CertainPerformance · Accepted Answer · 2018-03-28 00:51:09Z

2

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);
})

answered Mar 28, 2018 at 0:51

CertainPerformance

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Comments

ayed maissen · Accepted Answer · 2018-03-28 01:24:15Z

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

answered Mar 28, 2018 at 1:24

ayed maissen

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Comments

SharpCode · Accepted Answer · 2018-03-28 00:55:27Z

0

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.

answered Mar 28, 2018 at 0:55

SharpCode

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Comments

Rohìt Jíndal · Accepted Answer · 2018-03-28 07:03:47Z

0

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.

answered Mar 28, 2018 at 7:03

Rohìt Jíndal

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