This site was built to compliment the existing NetSuite Professionals Slack Community by adding a means for members to easily find answers to common questions and to create a searchable database of those questions and answers.

NetSuite Professionals

That will require the line parameter as well. So your assignment will look like:
 var inventoryDetailSubrecord = SALESORDER.getSublistSubrecord({
                        sublistId: 'item',
                        fieldId: 'inventorydetail',
                        line: x
                    });

thanks Nik. the SO I am loading is in Dynamic mode and using getSublistSubrecord throws error: Cannot find function getSublistSubrecord in object dynamic record

Since you are in dynamic mode, you need to select the line first, put a `selectSublistLine` before your `getCurrentSublistSubrecord`