Lambda.js

'use strict';

console.log('Loading function');

exports.handler = (event, context, callback) => {
    console.log('Received event:', JSON.stringify(event, null, 2));
    
    /*
    See what our origin(what domain is making the request)
    */
    //console.log("Origin: " + event.headers.origin);
    
    /*
    Open up CORS for two specific domains - a CloudFront URL, and an S3 Storage bucket
    This will let the CORS requests go through at the Lambda level and return results
    */
    let origin;
    if(event.headers.origin == 'INPUT YOUR CLOUDFRONT URL' 
       || event.headers.origin == 'INPUT YOUR S3 BUCKET URL')
    {
        origin = event.headers.origin
    }
    
    /*
    This will send out the response with either a 400 error or a 200 success
    Note, also, the headers.  We're making sure to pass application/json and allow CORS domain with a varible 'origin'
    that we've defined above
    */
    const done = (err, res) => callback(null, {
        statusCode: err ? '400' : '200',
        body: err ? err.message : JSON.stringify(res),
        headers: {
            'Content-Type': 'application/json',
            'Access-Control-Allow-Origin': origin,
        },
    });

    /*
    If we see an http requst, and it is a GET request, we'll return a simple JSON array.  If not, we'll throw an
    "unsupported method" and return it as a 400 error
    */
    switch (event.httpMethod) {
        case 'GET':
            done(null, {"messages": [{"id":1,"content":"Hello"},{"id":2,"content":"Serverless"},{"id":3,"content":"AWS"}]});
            break;
        default:
            done(new Error(`Unsupported method "${event.httpMethod}"`));
    }
};

Sample AWS Lambda function for a serverless app implementation(Written in Node.JS 6.10)
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