Wsrtjtyk

I'm trying to access a value set in a function in another function, then I want to return this value at the end, I've got a console.log to show the value and the log shows the value just fine, but when trying to set it I get an error stating: TypeError: Cannot read property '0' of undefined

It seems that when I try to return the value (I do this by setting the function as a variable) a fucntion with no result is shown. Not sure how I can show the result form my setValue function, any ideas? :) My express code:

function showBalance(cardNumber) {

  connection.query(

    "SELECT Balance from userCards WHERE CardNumber  = '" + cardNumber + "'",

    function(err, rows) {

      if (err) {

        throw err;

      } else {

        setValue(rows);

      }

    }

  );

  function setValue(value) {

    console.log('Balance object is:', value[0].Balance);

    const ownBalance = value[0].Balance;

    console.log('The user balance is: ', ownBalance);

    return ownBalance;

  }

  const returnBalance = setValue();

  return returnBalance;

}

module.exports = showBalance;

In the line const returnBalance = setValue() you call setValue with 0 arguments. This will cause the script to fail, as you cannot do value[0] when value is undefined.

You can't make your outer function return a value that comes from the anonymous inner callback. This is because the outer function returns before the callback is invoked. You need to do one of the following:

• Pass a function as an argument to your outer function that represents what should happen after the callback fires. You call this function from the callback and pass the value to it. This is called continuation passing style.




• Use an API that supports Promises, and have your outer function return a Promise containing the value instead of the value itself. You can adapt a callback based API to promises using promisify: https://nodejs.org/dist/latest-v8.x/docs/api/util.html#util_util_promisify_original




• Use async/await to write your asynchronous code in a style that is very similar to synchronous code. This also uses Promises under the hood, so it requires a compatible API or promisify as above.