Functions allow using a certain fragment of the code multiple times without describing it again. Declare the function (i.e. describe a fragment of code) only once and then call the function by its name.  

Example

The rusdate function is described in the built-in template. The code of this function changes the numerical value of a month into a character value: 

<% function rusdate(date) { %> // This function changes a numerical value of a month into a character value 
    <% if (!date || date == '') { return; }%> // If the date is not specified or it is empty, an empty value will be returned
    <% var c = date.split(' ')[0].split('-'); %> // The data in the format DD-MM-YYYYY will be divided  
    <% month = {
   	 '01': 'January',
   	 '02': 'February',
   	 '03': 'March',
   	 '04': 'April',
   	 '05': 'May',
   	 '06': 'June',
   	 '07': 'July',
   	 '08': 'August',
   	 '09': 'September',
   	 '10': 'October',
   	 '11': 'November',
   	 '12': 'December'
    } %>
    <%= c[2] %> <%= month[c[1]] %> <%= c[0] %> г. // A numeracal value of MM will be changed into a month
<% } %>
JS

This function is called in several sections of the template: 

Call the rusdate function in the invoice title

<div class="header">Invoice <%= payment.number %> from <% rusdate(payment.createdate) %></div>
JS

Call the rusdate function in the payment description

Payment for <% if (payment.$postpay && payment.$postpay != 'on') { %> advance<% } %> payment N <%= payment.number %> from <% rusdate(payment.createdate) %>
CODE

Describing a custom function 

As an example, let's create a function to say thank you to customers who make an order for more than 1 000 euro. The function code: 

The opt function

<% function opt(data) { %> //Declaration of the function. The function name is "opt". 
    <% var u = ''; %> // Declaration of the empty variable. 
    <% if (data > 1000 ) { %> //A condition to check the result. 
   	 <% u =  'Thank you, the  amount:' + data; %> // Set the variable value. 
    <% return u; %> // Return the result to the template engine.
    <% } %>    
	else {
    <% u = data; %> } // Do not add "Thank you" if the total amount is less than 1 000 euro.
    <% return u; %>
<% } %>
JS

After you have added the function, add its call into the description of the string "Total":

<!-- Total -->
<tr class="total">
	<td class="desc">Total</td>
	<td class="amount"><%= payment.paymethodamount %></td>
</tr>
<% if (payment.taxrate) { %>
	<tr class="total">
		<td class="desc">VAT included<%=number_format(payment.taxrate,'0', '.', '')%>%</td>
		<td class="amount"><%= opt(payment.paymethodamount) %></td> // Call of the opt. function. payment.paymethodamount is used as an example
	</tr>
<% } %>
<!-- Total -->
JS

After the invoice template code is changed, the document will show different information depending on the order amount.