display current logged in user name and data in ERPNext/Frappe Print Format

<!-- TO print current logged in user email id -->
<div class="row">
    <div class="col-xs-5 text-right"><big><b>Print By(email id)</b>  </big></div>
    <div class="col-xs-7 "><big>{{ frappe.user }} </big> </div>
</div> 

<!-- TO print current logged in user fist name -->
{% set u = frappe.get_doc("User", frappe.user) %} 
<div class="row">
    <div class="col-xs-5 text-right"><big><b>Print By(user name)</b>  </big></div>
    <div class="col-xs-7 "><big>{{ u.first_name }} </big> </div>
</div>

hiding fields based on some condition

To hide fields on form

frappe.ui.form.on("Quotation", "onload", function(frm) {
       cur_frm.set_df_property("shipping_rule", "hidden",true);
       msgprint("hi");
});


To hide fields in child table:

frappe.ui.form.on("Quotation", "onload", function(frm) {
       //cur_frm.set_df_property("shipping_rule", "hidden",true);
       cur_frm.fields_dict["items"].grid.set_column_disp('description', false)
       msgprint("hi");
});


TO held fileds in table, you can use eval property of field
eval:doc.item_code == "sample item"
eval:doc.item_code.substr(0,3)== "720"
 

Substring in javascript

Substring in Javascript:

The substr() method returns the characters in a string beginning at the specified location through the specified number of characters.

Syntax

str.substr(start[, length])

Frappe Example

frappe.ui.form.on("Quotation", "validate", function(frm) {
       var a = frm.doc.customer;
       a= a.substr(0,3);
        msgprint(a);
});

Above script returns first three letter  of customer name.


reference:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/slice