Variables & Expressions

Loadster’s variables and expressions allow you to inject dynamic values into your scripts.

They make it possible to do things like:

  • Log in with a different username and password each time.
  • Capture a CSRF token and reuse it when submitting a secure web form.
  • Submit a search with a different search phrase each time.
  • Generate a random username for each new registration.

While testing a static website may not require any special variables or expressions, you’ll probably need them if you’re testing a dynamic web application or API.

Variables and expressions work the same in both protocol scripts and browser-scripts.

Loadster variables and expressions are denoted by the ${var} or ${expr()} syntax. Anything between the ${ and the } is interpreted as a variable or expression.

Variables can have any name you want, as long as it is alphanumeric text with no spaces. Expression functions can be any of the predefined functions that Loadster supports.

Using Expressions in Protocol Scripts

In protocol scripts, you can use variables throughout your HTTP steps, including:

  • URL
  • Page resources
  • Authentication credentials
  • Custom headers
  • Request body

Variables in the URL

Variables can be used in the primary URL for a request, or to append dynamic query string parameters.

Using a variable in the URL
Using a variable in the URL

When your script runs, ${BaseURL} will be substituted with the value of the BaseURL variable.

Variables in page resources

In the example below, a page resource with the ${CoverImage} variable has been added to the selected HTTP command.

Using a variable to load a dynamic page resource
Using a variable to load a dynamic page resource

Variables in authentication credentials

When setting up authentication for an HTTP step, you can use variables for the username and password.

Using variables for HTTP Basic Authentication
Using variables for HTTP Basic Authentication

Variables in custom headers

You can add a request and then insert the variable as the value. This example shows an Authorization header with ${AuthToken} as the value.

Setting a dynamic request header based on a variable
Setting a dynamic request header based on a variable

Variables in the request body

In the example below, several values are set as variables in the request body. This makes it possible to submit a request with different data for each bot. This also works for other body types, not just JSON.

Using variables in a request body
Using variables in a request body

Important: When using variables in a request body, the “Interpolate variables in request body” option must be selected. Otherwise, they’ll be sent as literal strings instead of interpolated variables.

Using Expressions in Browser Scripts

In browser scripts, you can use variables or expressions in the DOM selector field or any other primary field. For example, you could use a variable to supply the text to be entered in a username or password field.

Scoping of Script Variables

Each bot keeps its own separate variable context. This means that each bot has its own unique value for a given variable at any given time, and setting a variable for one bot will have no effect on other bots that might also be running the script.

Also, keep in mind that these Loadster script variables are different from JavaScript variables inside a code block or another JavaScript block. To access a script variable from JavaScript, you’ll need to use bot.getVariable('var') instead of ${var}.

Setting Variables

There are a few ways to set a variable in a Loadster script.

Dynamic Datasets allow you to seed data for your test ahead of time. For example, you might create a data set that contains a list of usernames and passwords. Each bot will then pull a unique value from the data set and use it when they run the script. Dynamic datasets are a good way to set variables when you know ahead of time what values you want to use.

Capturing Rules let you extract values on the fly from the HTTP responses coming back from your site. Capturing is a powerful way to parameterize your scripts when you don’t know the values ahead of time, and where the value might be different each time the script runs. For example, you could capture a unique registration code that your application generates after you submit a registration form, and store it in a variable for later use.

Code Blocks can set and retrieve variables programmatically, with bot.getVariable(name) and bot.setVariable(name, value). When you store a script variable in this way, the value is always cast to a string.

Expression Functions

Expressions are similar to variables in that you can include them in your script in a ${} section. But instead of just holding a value, expressions can invoke functions to generate or transform values, and you can even chain functions together to apply multiple transformations.

The expression syntax is ${func()} or ${func(var)}, where var is the name of a variable and func is the function that is applied.

Loadster supports quite a few functions out of the box.

function description
urlencode(str) Applies URL encoding to the value.
urldecode(str) Decodes a URL-encoded value.
jsescape(str) Escapes a value so it is safe to use in a JavaScript string.
jsunescape(str) Removes JavaScript escape characters from an escaped string, so it can be displayed literally.
htmlescape(str) Escapes a string so it can be used safely in HTML, applying escape sequences like < and >.
htmlunescape(str) Removes HTML escape sequences and turns them back to their literal characters.
xmlescape(str) Essentially the same as htmlescape() but for XML.
xmlunescape(str) Essentially the same as htmlunescape() but for XML.
uppercase(str) Turn all letters in a string to uppercase.
lowercase(str) Turn all letters in a string to lowercase.
timestamp() Outputs a current Unix timestamp in seconds since the epoch, e.g. 1404432584.
timestamp("yyyy-MM-dd") Outputs a current timestamp, in a format you specify. The format is based on Java’s SimpleDateFormat.
uuid() Generates a UUID/GUID, like 1e06863a-6118-426d-bf46-35858c7d797.
randomalpha(len) Generates a random string of alpha characters, of the specified length.
randomnumeric(len) Generates a random string of numbers, of the specified length.
randomalphanumeric(len) Generates a random string of mixed alpha characters and numbers, of the specified length.

Example: Expression to URL encode a query parameter

HTTP requires that special characters in a URL be “URL encoded”. Let’s say we have a variable called SearchPhrase which contains the email address of the current bot. Normally, you could reference it like this:

Substituting a literal value in the URL
Substituting a literal value in the URL

However, the email address has an @ character in it which is not always safe in URLs. To encode it, use the urlencode() function:

Substituting a safe URL-encoded value in the URL
Substituting a safe URL-encoded value in the URL

The urlencode() function in your expression will apply URL encoding to whatever value the SearchPhrase variable currently has. So if your search query is “Transfer Coin”, Loadster will substitute the correct URL encoded equivalent of “Transfer%20Coin”.

Example: Expression to escape characters in a JSON request body

Let’s say you are testing a RESTful JSON endpoint. The request is supposed to be a POST request with a payload like this:

{ "fullName": "Buford \"Mad Dog\" Tanner", "age": 26 }

Instead of submitting the same name each time, you decide to pre-populate a list of names and ages, and then parameterize the request body with variables:

{ "fullName": "${FullName}", "age": ${Age} }

But what if your data set contains double quote characters or other characters that are illegal in JSON? To be safe, you should escape them with the jsescape() function:

{ "fullName": "${jsescape(FullName)}", "age": ${Age} }

The jsescape() function guarantees that the interpolated value is safe to use in a JavaScript or JSON string.

Example: Expression to uppercase and URL encode a string

There may be occasions (hopefully not too often) that require performing multiple functions on a single value. It is quite straightforward:${urlencode(uppercase(Email))}

Operations will be applied in order, starting with the innermost function and proceeding outwards. In the example above, a username like would become USER%40COMPANY.COM.

Example: Expression to generate a timestamp

Sometimes you need to send a current date or timestamp when making a request.

  "name": "Biff Tanner",
  "age": 26,
  "dateCreated": "${timestamp("yyyy-MM-dd'T'HH:mm:ss.S'Z'")}"

This example formats the timestamp with the current date/time in JavaScript’s built-in ISO8601 format. You can specify a different format in the expression (see below).