Lambda@Edge Origin Request Event Handler - www to non-www permanent redirect

origin_request.js

'use strict';

// Since you may want to have other redirects in the handler this 
// function will simplify it a bit.
function redirect (url) {
	return Promise.resolve({
		body: '',
		status: '301',
		statusDescription: 'Permanently moved',
		headers: {
			location: [{
				key: 'Location',
				value: url
			}]
		}
	});
};


// Export the event handler.
exports.handler = (event, context) => {
	
	// Log every request.
	console.log( JSON.stringify(event) );

	let request,
		host,
		query = ''
	
	// Get the request data from the event.
	request = event.Records[0].cf.request;

	// Get the host header.
	host = request.headers.host[0].value;

	// Make sure you're passing back the query string if there is one.
	if( 'querystring' in request && request.querystring.length) {
		query = '?'+request.querystring;
	}
	
	// Redirect www to non-www.
	if( host && host.length && host.startsWith('www.') ) {
		return redirect( `https://${host.replace('www.', '')}${request.uri}${query}` );
	}

	// The Host header has been whitelisted so that it's passed in with this request.
  	// If there is no redirect make sure the host header value is set back the the S3 bucket
	// so CloudFront know where to get your html file.
  	request.headers['host'] = [{ key: 'host', value: request.origin.s3.domainName }];

	// Return request to CloudFront
	return Promise.resolve(request);

};

If your trying to create a redirect from non-www to www replace line 42 though 44 with this:

if( host && host.length && !host.startsWith('www.') ) {
    return redirect( `https://${'www.'}${request.uri}${query}` );
}