Variable is null after returned from lambda function in AWS

Question

I tried to define local variable then call lambda function which populates the value to my local variable:

var listOfAliases = null;
lambda.invoke(params, function(err, data) {
    if (err) {
        //context.fail(err);
        console.log(`This is the ERROR execution =${err} =================================`);
        prompt(err);
    } else {
        //context.succeed('Data loaded from DB: '+ data.Payload);
        listOfAliases = JSON.stringify(data.Payload);
        console.log(`This is the VALIDE execution =${data.Payload} =================================`); //I can see this in the log with proper values
        console.log(`This is the VALIDE execution(listOfAliases) =${listOfAliases} =================================`); //I can see this in the log with proper values

    }
    callback(null, JSON.parse(data.Payload));
});

console.log(`This is the DB execution listOfAliases=${listOfAliases} =================================`); //I can see this in the log with NULL value

Answer 1

The problem here is that lambda.invoke executes asynchronously and your last console.log executes before the invoke callback function completes.

If you need to access the result from outside one the asynchronous call completes, you could use a promise.

var promise = new Promise(function(resolve,reject){
  lambda.invoke(params, function(err, data) {
       if (err) {
           reject(err);    
       } else {
           resolve(JSON.stringify(data.Payload));
       }
  });
});
promise.then(function(listOfAliases){
  console.log('This is the DB execution listOfAliases ' + listOfAliases);
});