how to publish mqtt messages to greengrass core devices from cloud via SDK for JavaScript?

Your Greengrass component needs to subscribe to messages from AWS IoT Core on a device specific topic, e.g fromcloud/device001, as described here.

If you just want to echo a message back to the device, you can use the AWS IoT Republishing action in the AWS IoT Rule Engine that triggers when a message arrives in AWS IoT on a topic e.g tocloud/device001 and republish it to a topic e.g fromcloud/device001 . You can use the AWS IoT Rule engine SQL function topic(2) to extract the device id from the topic name to construct the return topic.

If you trigger a Lambda function from the AWS IoT Rule Engine, you can use the iotdata.publish() REST method from one of our AWS SDKs to publish a message to the device specific topic fromcloud/device001

Hope that helps