global variable is not updated inside function

Question

Given the following Code

var base64 = ''
const AWS = require('aws-sdk')
let awsConfig = {
  'region': 'us-west-2',
  'endpoint': 'http://dynamodb.us-west-2.amazonaws.com',
  'accessKeyId': 'ABCDEFGHIJKLMANOPQRSTUVWXYZ',
  'secretAccessKey': 'ABCDABSDJFGJALGJADSLGJALKDJGALKSDJGLA'
}

AWS.config.update(awsConfig)

let docClient = new AWS.DynamoDB.DocumentClient()

let fetchOneByKey = function () {
  var params = {
    TableName: 'test',
    Key: {
      'key1': 'abc'
    }
  }
  docClient.get(params, function (err, data) {
    if (err) {
      console.log('test::fetchOneByKey::error - ' + err)
    } else {
      base64 = data.Item.b.toString('base64')
    }
  })
}

fetchOneByKey()
console.log(base64)

For some reason my variable base64 prints nothing. This is weird because when I do console.log(data.Item.b.toString('base64') it prints the base64 string so i know the data is not empty for sure.

For some reason my variable is not getting updated and I am not sure why. I am clearly calling the function fetchOneByKey() so in theory base64 should get updated but it is not.

You're attempting to log the `base64` variable before the callback for `docClient.get()` runs. — Tyler Roper, Feb 10 '20 at 18:03
Your `console.log` is outside the callback of where `base64` is assigned. — Phix, Feb 10 '20 at 18:03
The variable is updated, but you're reading the value before the update happens. — Teemu, Feb 10 '20 at 18:03
@TylerRoper How do i ensure the update happens so my var gets updated? — Dinero, Feb 10 '20 at 18:04
Anything that relies on the result of your `docClient.get()` should be initiated within the callback or after `await`ing the result. — Tyler Roper, Feb 10 '20 at 18:05
Was just about to flag the duplicate as it was closed with the same one. The linked question should provide some insight. — Tyler Roper, Feb 10 '20 at 18:07
Either you will have to use promise or callback. follow [snippet](https://jsfiddle.net/bdc7r3m5/) — AZ_, Feb 10 '20 at 18:12

Mirakurun · Accepted Answer · 2020-02-10T18:29:47.987

2

docClient.get() is asynchronous. By calling console.log() immediately after calling the function, you are executing it before the read from DB is finished, and thus your variable isn't updated.

Call console.log(base64) here

else {
  base64 = data.Item.b.toString('base64')
  console.log(base64)
}

If you want to get rid of the callback and use promises instead, you can use promisify. With that you can turn it into a promise and use it with async/await or chain it with .then().

Or you can make it as promise manually.

let fetchOneByKey = function () {
  var params = {
    TableName: 'test',
    Key: {
      'key1': 'abc'
    }
  }
  return new Promise((resolve, reject) => {
    docClient.get(params, function (err, data) {
      if (err) {
        console.log('test::fetchOneByKey::error - ' + err)
        reject(err)
      } else {
        base64 = data.Item.b.toString('base64')
        resolve(base64)
      }
    })
  }
}

then you can say base64 = await fetchOneByKey(); or if you are not inside an async function you can do fetchOneByKey().then(base64 => console.log(base64))

EDIT: as @AZ_ mentioned, aws-sdksupports promises

let fetchOneByKey = async function () {
  var params = {
    ...
  }
  const data = await docClient.get(params).promise();
  base64 = data.Item.b.toString('base64');
  return base64
}

edited Feb 10 '20 at 18:29

answered Feb 10 '20 at 18:07

Mirakurun

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yes i know that works but i want to use that variable outside the function. – Dinero Feb 10 '20 at 18:08
@Dinero Use promisify and make that callback into a promise. Then you can use async/await or you can chain it with .then() – Mirakurun Feb 10 '20 at 18:10
can you edit and show? i have never used that – Dinero Feb 10 '20 at 18:11
I edited. Can you try it now? – Mirakurun Feb 10 '20 at 18:18
1

`aws-sdk` already provides the promise wrapper. so no need a separate `new Promise...`. – AZ_ Feb 10 '20 at 18:21
@AZ_ that's even better. I'm not familiar with aws-sdk. – Mirakurun Feb 10 '20 at 18:24
@AZ_ feel free to post yours as a answer too – Dinero Feb 10 '20 at 18:24
already added the snippet in comments. – AZ_ Feb 10 '20 at 18:24

global variable is not updated inside function

1 Answers1