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Local UDF in JavaScript

Recently we also added the support of JavaScript-based local UDF. You can develop User-defined scalar functions (UDFs) or User-defined aggregate functions (UDAFs) with modern JavaScript (powered by V8). No need to deploy extra server/service for the UDF. More languages will be supported in the future.

Register a JS UDF

  1. Open the workspace settings page via clicking on the workspace name at top-right corner and choosing Settings.
  2. Click the Register New Function button.
  3. Specify a function name, such as second_max. Make sure the name won't conflict with built-in functions or other UDF. Description is optional.
  4. Choose the data type for input parameters and return value.
  5. Choose JavaScript as the UDF type.
  6. Specify whether the function is for aggregation or not.
  7. Enter the JavaScript source for the UDF. (We will explain more how to write the code.)
  8. Click Create button to register the function.

Develop a scalar function

A scalar function is a function that returns one value per invocation; in most cases, you can think of this as returning one value per row. This contrasts with Aggregate Functions, which return one value per group of rows.

Scalar function with 1 argument

For example, you would like to check whether the user sets a work email in their profile. Although this could be doable with plain SQL but it'll be nice if you can create a UDF to make the SQL more readable, e.g.

SELECT * FROM user_clicks where is_work_email(email)

You can use the following code to define a new function is_work_email with one input type string and return bool.

function is_work_email(values){


  1. The first line defines a function with the exactly same name as the UDF. The number of arguments should match what you specify in the UDF form.
  2. Please note the input is actually a JavaScript list. For the sake of high performance, Timeplus will reduce the number of function calls by combining the arguments together. You need to return a list with the exactly same length of the input.
  3. creates a new array populated with the results of calling a provided function on every element in the calling array (doc).
  4. email=>email.endsWith("") is the shortcut to return a bool by checking whether the email ends with "". You can add more complex logic, or write in multiple lines and end with return ...

Scalar function with 2 or more arguments

Let's enhance the previous example, by defining a list of email domains which won't be considered as work-related. e.g.

SELECT * FROM user_clicks where email_not_in(email,',,')

Similar to the last tutorial, you create a new function called email_not_in. This time you specify two arguments in string. Note: currently JS UDF doesn't support complex data types, such as array(string).

The following code implements this new function:

function email_not_in(emails,lists){
let list=lists[0].split(','); // convert string to array(string)
for(let i=0;i<list.length;i++){
return false; // if the email ends with any of the domain, return false, otherwise continue
return true; // no match, return true confirming the email is in none of the provided domains

Scalar function with no argument

Currently we don't support JS UDF without arguments. As a workaround, you can define a single argument, e.g.

SELECT *, magic_number(1) FROM user_clicks

The magic_number takes an int argument as a workaround.

function magic_number(values){

In this case, the function will return 42 no matter what parameter is specified.

Develop an aggregate function

An aggregate function returns one value per group of rows. When you register the UDF, make sure you turn on the option to indicate this is an aggregation function. Comparing to scalar functions, the life cycle is a bit more complex.

3 required and 3 optional functions

Let's take an example of a function to get the second maximum values from the group.

1initialize()YesInitialize the states.function(){
2process(args..)YesMain logic for the functionfunction(values){
3finalize()YesReturn the final aggregation resultfunction(){
return this.sec_max
4serialize()NoSerialize JS internal state to a string, so that Timeplus can persistent for failover/recovery.function(){
return JSON.stringify({'max':this.max,'sec_max':this.sec_max})
5deserialize(str)NoOpposite to serialize(). Read the string and convert back to JS internal state.function(str){
let s=JSON.parse(str);
6merge(str)NoMerges two states into one. Used for multiple shards processing.function(str){
let s=JSON.parse(str);

Example: get second largest number

The full source code for this JS UDAF is

initialize: function() {
this.max = -1.0;
this.sec_max = -1.0;

process: function(values) {
for (let i = 0; i < values.length; i++) {

_update: function(value) {
if (value > this.max) {
this.sec_max = this.max;
this.max = value;
} else if (value > this.sec_max) {
this.sec_max = value;

finalize: function() {
return this.sec_max

serialize: function() {
return JSON.stringify({
'max': this.max,
'sec_max': this.sec_max

deserialize: function(state_str) {
let s = JSON.parse(state_str);
this.max = s['max'];
this.sec_max = s['sec_max']

merge: function(state_str) {
let s = JSON.parse(state_str);

To register this function, choose JavaScript as UDF type, make sure turn on 'is aggregation'. Set the function name say second_max (you don't need to repeat the function name in JS code). Add one argument in float type and set return type to float too.

Please note, unlike JS scalar function, you need to put all functions under an object {}. You can define internal private functions, as long as the name won't conflict with native functions in JavaScript, or in the UDF lifecycle.


  • We will provide better testing tools in the future.

  • The custom JavaScript code is running in a sandbox with V8 engine. It won't impact other workspaces.

  • Compound data structure such as array or map are not supported in JS UDF. JavaScript data types are more generic and here is the mapping for JavaScript data type and data type in Timeplus:

    Timeplus Data TypesJavaScript Data Types
    date/date32/datetime/datetime64Date (in milliseconds)