Normalize a NetSuite entity ID to a valid customer internal ID reference using SuiteScript

suitescript_retrieve_customer_id.js

// Author <mike@suitesync.io>

// ensure that an entity ID is actually a customer ID

// http://blog.prolecto.com/2016/02/12/how-to-distinguish-a-netsuite-entity-as-a-customer-vs-a-project/
function retrieveCustomerId(entityId) {
  var isAdvancedJobs = nlapiGetContext().getFeature('ADVANCEDJOBS');
  var customerId = entityId;

  if(!isAdvancedJobs) {
    entityType = nlapiLookupField('customer', entityId, 'stage');

    if(entityType == 'JOB'){
      // undocumented `kcustomer`
      customerId = nlapiLoadRecord(entityType, entityId).getFieldValue('kcustomer');
    }
  }

  return customerId;
}