Sample Custom Script for Validation

erpnext-sample-validation-script.js

// this function is called by erpnext 
// in the browser before saving

cur_frm.cscript.custom_validate = function(doc) {
  if(user=="user1@example.com" && doc.purpose!="Material Receipt") {
    msgprint("You are only allowed Material Receipt");
    validated = false;
  }
}