How to run Cron Job in Node.js application that uses cluster module?

If are using PM2,
You can use an environment variable provided by PM2 itself called NODE_APP_INSTANCE which requires PM2 2.5 or greater.

NODE_APP_INSTANCE environment variable can be used to determine difference between process, for example you may want to run a cronjob only on one process, you can just do this

if(process.env.NODE_APP_INSTANCE == 0) {
//schedule your cron job here since this part will be executed for only one cluster
}
,

Since two processes can never have the same number.

More Info on PM2 official doc here.