Splash Scripts Tutorial


Scripting support is an experimental feature for early adopters; API could change in future releases.


Splash can execute custom rendering scripts written in Lua programming language. This allows to use Splash as a browser automation tool similar to PhantomJS. To execute a script and get the result back send it to execute endpoint in a lua_source argument.


Most likely you’ll be able to follow Splash scripting examples even without knowing Lua. Nevertheless, the language worths learning - with Lua you can, for example, write Redis, Nginx, Apache, World of Warcraft scripts, create mobile apps using Moai or Corona SDK or use state of the arts Deep Learning framework Torch7. It is easy to get started and there are good online resources available like Learn Lua in 15 minutes tutorial and Programming in Lua book.

Let’s start with a basic example:

function main(splash)
    local title = splash:runjs("document.title")
    return {title=title}

If we submit this script to execute endpoint in a lua_source argument, Splash will go to example.com website, wait until it loads, then wait 0.5s more, then get page title (by evaluating a JavaScript snippet in page context), and then return the result as a JSON encoded object.


Splash UI provides an easy way to try scripts: there is a code editor for Lua and a button to submit a script to execute. Visit (or whatever host/port Splash is listening to).

Entry Point: the “main” Function

The script must provide “main” function; this function is called by Splash. The result of this function is returned as an HTTP response. Script could contain other helper functions and statements, but ‘main’ is required.

In the first example ‘main’ function returned a Lua table (an associative array similar to JavaScript Object or Python dict). Such results are returned as JSON. This will return {"hello":"world!"} string as an HTTP response:

function main(splash)
    return {hello="world!"}

Script can also return a string:

function main(splash)
    return 'hello'

Strings are returned as-is (unlike tables they are not encoded to JSON). Let’s check it with curl:

$ curl ''

“main” function receives an object that allows to control the “browser tab”. All Splash features are exposed using this object. By a convention, this argument is called “splash”, but you are not required to follow this convention:

function main(please)
    return "ok"

Where Are My Callbacks?

Here is a part of the first example:

local title = splash:runjs("document.title")

The code looks like a standard procedural code; there are no callbacks or fancy control flow structures. It doesn’t mean Splash works in a synchronous way; under the hood it is still async. When you call splash.wait(0.5), Splash switches from the script to other tasks, and comes back after 0.5s.

It is possible to use loops, conditional statements, functions as usual in Splash scripts; this enables a more straightforward code.

Let’s check an example PhantomJS script:

var users = ["PhantomJS", "ariyahidayat", /*...*/];

function followers(user, callback) {
    var page = require('webpage').create();
    page.open('http://mobile.twitter.com/' + user, function (status) {
        if (status === 'fail') {
            console.log(user + ': ?');
        } else {
            var data = page.evaluate(function () {
                return document.querySelector('div.profile td.stat.stat-last div.statnum').innerText;
            console.log(user + ': ' + data);
function process() {
    if (users.length > 0) {
        var user = users[0];
        users.splice(0, 1);
        followers(user, process);
    } else {

The code is arguably tricky: process function implements a loop by creating a chain of callbacks; followers function doesn’t return a value (it would be more complex to implement) - the result is logged to the console instead.

A similar Splash script:

users = {'PhantomJS', 'ariyahidayat'}

function followers(splash, user)
    local ok, msg = splash:go('http://mobile.twitter.com/' .. user)
    if not ok then
        return "?"
    return splash:runjs([[
        document.querySelector('div.profile td.stat.stat-last div.statnum').innerText;

function process(splash, users)
    local result = {}
    for idx, user in ipairs(users) do
        result[user] = followers(splash, user)
    return result

function main(splash)
    local users = process(splash, users)
    return {users=users}


  • some Lua knowledge is helpful to be productive in Splash Scripts: ipairs, [[multi-line strings]] or string concatenation via .. could be unfamiliar;
  • in Splash variant followers function can return a result (a number of twitter followers); also, it doesn’t need a “callback” argument;
  • instead of a page.open callback which receives “status” argument there is a “blocking” splash:go call which returns “ok” flag;
  • error handling is different: in case of an HTTP 4xx or 5xx error PhantomJS doesn’t return an error code to page.open callback - example script will try to get the followers nevertheless because “status” won’t be “fail”; in Splash this error will be detected and ”?” will be returned;
  • process function can use a standard Lua for loop without a need to create a recursive callback chain;
  • instead of console messages we’ve got a JSON HTTP API;
  • apparently, PhantomJS allows to create multiple page objects and run several page.open requests in parallel (?); Splash only provides a single “browser tab” to a script via its splash parameter of main function (but you’re free to send multiple concurrent requests with Lua scripts to Splash).

There are great PhantomJS wrappers like CasperJS and NightmareJS which (among other things) bring a sync-looking syntax to PhantomJS scripts by providing custom control flow mini-languages. However, they all have their own gotchas and edge cases (loops? moving code to helper functions? error handling?). Splash scripts are standard Lua code.


PhantomJS itself and its wrappers are great, they deserve lots of respect; please don’t take this writeup as an attack on them. These tools are much more mature and feature complete than Splash. Splash tries to look at the problem from a different angle, but for each unique Splash feature there are ten unique PhantomJS features.

Living Without Callbacks

In Splash scripts it is not explicit which calls are async and which calls are blocking. It is a common criticism of coroutines/greenlets; check e.g. this article for a good description of the problem. However, we feel that in Splash scripts negative effects are not quite there: scripts are meant to be small, shared state is minimized, and an API is designed to execute a single command at time, so in most cases the control flow is linear.

If you want to be safe then think of all splash methods as of async; consider that after you call splash:foo() a webpage being rendered can change. Often that’s the point of calling a method, e.g. splash:wait(time) or splash:go(url) only make sense because webpage changes after calling them, but still - keep it in mind.

Currently the only async methods are splash:go and splash:wait. Most splash methods are currently not async, but thinking of them as of async will allow your scripts to work if we ever change that.


For the curious, Splash uses Lua coroutines under the hood.

Internally, “main” function is executed as a coroutine by Splash, and some of the splash:foo() methods use coroutine.yield. See http://www.lua.org/pil/9.html for Lua coroutines tutorial.

Calling Splash Methods

Unlike many languages, in Lua methods are usually separated from an object using a colon :; to call “foo” method of “splash” object use splash:foo() syntax. See http://www.lua.org/pil/16.html for more details.

There are two main ways to call Lua methods in Splash scripts: using positional and named arguments. To call a method using positional arguments use parentheses splash:foo(val1, val2), to call it with named arguments use curly braces: splash:foo{name1=val1, name2=val2}:

-- Examples of positional arguments:
splash:wait(0.5, false)
local title = splash:runjs("document.title")

-- The same using keyword arguments:
splash:wait{time=0.5, cancel_on_redirect=false}
local title = splash:runjs{source="document.title"}

-- Mixed arguments example:
splash:wait{0.5, cancel_on_redirect=false}

For the convenience all splash methods are designed to support both styles of calling. But note that generally this convention is not followed in Lua. There are no “real” named arguments in Lua, and most Lua functions (including the ones from the standard library) choose to support only one style of calling. Check http://www.lua.org/pil/5.3.html for more info.

Error Handling

There are two ways to report errors in Lua: raise an exception and return an error flag. See http://www.lua.org/pil/8.3.html.

Splash uses the following convention:

  1. for developer errors (e.g. incorrect function arguments) exception is raised;
  2. for errors outside developer control (e.g. a non-responding remote website) status flag is returned: functions that can fail return ok, reason pairs which developer can either handle or ignore.

If main results in an unhandled exception then Splash returns HTTP 400 response with an error message.

It is possible to raise an exception manually using Lua error function:

error("A message to be returned in a HTTP 400 response")

To handle Lua exceptions (and prevent Splash from returning HTTP 400 response) use Lua pcall; see http://www.lua.org/pil/8.4.html.

To convert “status flag” errors to exceptions Lua assert function can be used. For example, if you expect a website to work and don’t want to handle errors manually, then assert allows to stop processing and return HTTP 400 if the assumption is wrong:

local ok, msg = splash:go("http://example.com")
if not ok then
    -- handle error somehow, e.g.

-- a shortcut for the code above: use assert