Client Side Scripting:

Make fields read-only after saving

cur_frm.cscript.custom_refresh = function(doc) {
    // use the __islocal value of doc, to check if the doc is saved or not
    cur_frm.set_df_property("myfield", "read_only", doc.__islocal ? 0 : 1);
}

Hide a field based on some condition

cur_frm.cscript.custom_refresh = function(doc) {
    cur_frm.toggle_display("myfield1", doc.myfield2=="some_value");
}

Get Values from Server

A standard way to query values from server side.

wn.call({
    method:"webnotes.client.get_value",
    args: {
        doctype:"Delivery Note Item",
        filters: {
            parent:"DN00038",
            item_code:"Ser/003"
        },
        fieldname:["qty", "stock_uom"]
    }, 
    callback: function(r) { 
        console.log(r);

        // set the returned value in a field
        cur_frm.set_value(fieldname, r.message);
    }
})

Make attachments mandatory:

cur_frm.cscript.custom_validate = function(doc) {
    if(!doc.__islocal) {
        if(!doc.file_list) {
            var msg = wn._("Please attach atleast 1 file");
            msgprint(msg);
            throw msg;
        }
    }
}

Set Naming System For Item Code

cur_frm.cscript.custom_validate = function(doc) {
    // clear item_code (name is from item_code)
    doc.item_code = "";

    // first 2 characters based on item_group
    switch(doc.item_group) {
        case "Test A":
            doc.item_code = "TA";
            break;
        case "Test B":
            doc.item_code = "TB";
            break;
        default:
            doc.item_code = "XX";
    }

    // add next 2 characters based on brand
    switch(doc.brand) {
        case "Brand A":
            doc.item_code += "BA";
            break;
        case "Brand B":
            doc.item_code += "BB";
            break;
        default:
            doc.item_code += "BX";
    }
}

Using add_fetch to pull link details

Add this line in the Custom Script (not in any function) for the target DocType (where you want to value to be fetched).

// set employee_name field to employee_name value from employee link field
// function add_fetch(link_fieldname, source_fieldname, target_fieldname)
cur_frm.add_fetch('employee','employee_name','employee_name')

Fetching Values:

Example, get default "cash_bank_account" when mode_of_payment is updated

On client side:

cur_frm.cscript.mode_of_payment = function(doc) {
	cur_frm.call({
		method: "get_bank_cash_account",
		args: { mode_of_payment: doc.mode_of_payment }
	});
}

On server side:

# whitelist allows method to be called from web request
@webnotes.whitelist()
def get_bank_cash_account(mode_of_payment):
	return {
		"cash_bank_account": webnotes.conn.get_value("Mode of Payment", 
			mode_of_payment, "default_account")
	}

Fetching a table row's values in the form

on client side

// client script (example: in sales_invoice.js)
cur_frm.cscript.wash_type = function(doc, cdt, cdn) {
    var d = locals[cdt][cdn];
    if(d.wash_type) {
        cur_frm.call({
	    child: d,
	    method: "get_item_qty",
	    args: {
	        item_code: d.item_code,
	        wash_type: d.wash_type
	    }
	});
    }
}

on server side

# server script (example: in sales_invoice.py)
# not a method of DocType object of 
    
# whitelist allows method to be called from web request
@webnotes.whitelist()
def get_item_qty(item_code, wash_type):
    # do something
    fieldvalue1 = "something"

    # return a dict of row's fieldnames and values to be updated in that table row
    return { "fieldname1":  fieldvalue1 }

Date Validation: Do not allow past dates in a date field

cur_frm.cscript.custom_validate = function(doc) {
	if (doc.from_date < get_today()) {
		msgprint("You can not select past date in From Date");
		validated = false;
	}
}

Allow user only single purpose of stock entry:

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;
	}
}

Validation on Stock Entry based on Warehouse Detail

cur_frm.cscript.custom_validate = function(doc) {
	if(user_roles.indexOf("Material Manager")==-1) {
	
		var restricted_in_source = wn.model.get("Stock Entry Detail", 
			{parent:cur_frm.doc.name, s_warehouse:"Restricted"});
			
		var restricted_in_target = wn.model.get("Stock Entry Detail", 
			{parent:cur_frm.doc.name, t_warehouse:"Restricted"})
	
		if(restricted_in_source.length || restricted_in_target.length) {
			msgprint("Only Material Manager can make entry in Restricted Warehouse");
			validated = false;
		}
	}
}

Calculate Incentive in Sales Team table based on some custom logic

cur_frm.cscript.custom_validate = function(doc) {
	// calculate incentives for each person on the deal
	total_incentive = 0
	$.each(wn.model.get("Sales Team", {parent:doc.name}), function(i, d) {

		// calculate incentive
		var incentive_percent = 2;
		if(doc.grand_total > 400) incentive_percent = 4;

		// actual incentive
		d.incentives = flt(doc.grand_total) * incentive_percent / 100;
		total_incentive += flt(d.incentives)
	});
	
	doc.total_incentive = total_incentive;
}

Cancel permission based on grand total

cur_frm.cscript.custom_before_cancel = function(doc) {
	if (user_roles.indexOf("Accounts User")!=-1 && user_roles.indexOf("Accounts Manager")==-1
			&& user_roles.indexOf("System Manager")==-1) {
		if (flt(doc.grand_total) > 10000) {
			msgprint("You can not cancel this transaction, because grand total \
				is greater than 10000");
			validated = false;
		}
	}
}

Remove a Standard button from Form's toolbar

cur_frm.cscript.custom_refresh = function() {
	if(!cur_frm.doc.__islocal && cur_frm.doc.owner === wn.boot.profile.name) {
		cur_frm.appframe.buttons.Submit.remove();
	}
}

Assign Expected Delivery Date as x days after Sales Order Date

cur_frm.cscript.custom_sales_order_date = function(doc) {
	cur_frm.set_value("expected_delivery_date", wn.datetime.add_days(doc.sales_order_date, x));
}

cur_frm.cscript.custom_onload = cur_frm.cscript.custom_sales_order_date;

Prevent back-dating for Resolution Date in Customer Issues

if (doc.resolution_date && wn.datetime.get_day_diff(new Date(), wn.datetime.str_to_obj(doc.resolution_date)) > 0) { 
        validated = false;
         msgprint("Resolution Date cannot be a past date"); // or any other message you want..
     }

Material Receipt in WarehouseX must be made against Material Request

cur_frm.cscript.custom_validate = function(doc) {
	if(doc.purpose == "Material Receipt") {
		$.each(frappe.model.get("Stock Entry Detail", {parent:doc.name}), function(i, d) {
			if(d.t_warehouse=="WarehouseX" && !d.material_request) {
				msgprint("You must receive against Material Request");
				validated = false;
				return;
			}
		})
	}
}

Change CSS Properties for some specific fields in Doctype.

  frappe.ui.form.on("Doctype Name", {
            refresh: function(frm) {
                            set_css(frm);
            }
  });

  var set_css = function (frm) {
      let el = document.querySelectorAll("[data-fieldname='field_name']")[1];
      el.style.height ="50px";
      el.style.width ="120px";
      el.backgroundColor ="red";
  }