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

What params are you passing? Invoke is synchronous by default, using invoke type RequestResponse. So unless you have set it to type Event (which is async) then you're performing this synchronously. — Jon Church
– Jon Church, Commented May 7, 2017 at 13:15

Asanka · Accepted Answer · 2017-05-29 08:46:32Z

2

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);
});

edited May 29, 2017 at 8:46

answered May 7, 2017 at 12:48

Asanka

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3 Comments

Jon Church Over a year ago

The docs state that you have to specify async execution. Am I misunderstanding them? From the lambda invoke docs "By default, the Invoke API assumes RequestResponse invocation type. You can optionally request asynchronous execution by specifying Event as the InvocationType."

Asanka Over a year ago

Here the Event InvocationType means that you can invoke lambda function and complete the api call without waiting for it to finish and give the result. But still it's a http request and at client side executes asynchronously.

Asanka Over a year ago

One thing to note is that if you use Event InvocationType you would not receive the result from data parameter in the callback

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