Need help with Basic I/O function

Can I get some help here? I am a bit of a newbie to nodejs but I know enough I think to do this. I can get this to work on nodejs locally, however, as a basic i/o function I get an error. Basically, when I invoke the basic function I would like to run a function that invokes other functions or modules and once its completed return something in the output. however, I get this error (below is my example code)

the error I get is

{
    "error": "ReferenceError: variablea is not defined\n    at module.exports (/catalyst/index.js:15:19)\n    at CUSTOMER_ROUTE (/var/runtime/index.js:143:9)\n    at processTicksAndRejections (internal/process/task_queues.js:95:5)"
}

const catalyst = require('zcatalyst-sdk-node');
module.exports = (context, basicIO) => {
const app = catalyst.initialize(context);
//This app variable is used to access the catalyst components.
//You can refer the SDK docs for code samples.

function runfunction() {

var variablea = 'I am a variable';

}

runfunction();
basicIO.write(variablea);

context.close();
}