Environments and variables

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This guide shows how to use environments and variables in Advanced REST Client.
The introduction video
Environments and variables in ARC
Variables overview
Variables are used in all request fields (method, URL, headers, body) and allow defining a value in a global scope that is inserted in place when making a request.
Environments are the way of organizing variables into a namespace. It allows switching between different sets of variables. It can be used to define variables for a different context like staging and production.
Let's say you have "production" and "stage" environments for your API. Both are using different authorization keys. Environments and variables are made to manage a situation like this. Let's say your API supports the x-client-id header with some value that is used to recognize the client. Because of the separation of the environments, you have to use different keys for both environments. To deal with this create two environments for "production" and "stage" and create apiClientId variables with corresponding values. You can use the variable as ${apiClientId} in the headers editor as a value. The value is automatically added to the headers before sending the request to the server. 
Listing variables
Advanced REST Client lists your application variables created for the current environment and system variables available on your machine. You can use both keys to insert it into a request.
Variables list
To protect your data variables are masked by default. You can toggle the visibility of the values using the button at the bottom of the variables list.
You can disable application or system variables in application settings.
When switching the environment in the top toolbar you will switch between different application variables defined for this environment. System variables cannot be changed.
Empty environment
Editing variables
To add a new environment first click on "Edit variables" to open the variables editor.
Empty variables editor
Then "Add" next in the environment selector. It then renders a text field where you can enter a new environment name. There are no limitations for the environment name.
Adding an environment
When the environment is saved then you can define variables for it.
Adding a variable
You can disable a variable by toggling it on the left-hand side of the input field.
Variable names must start with a letter and only contain letters and numbers. Special characters or national accents are not supported.
The variable value is always visible in the editor mode.
The variable value can be any value, even another variable.
You can repeat these steps as many times as you need for each of your environments. After your variables are defined you can use them in the editors.
Applying variables
To apply a variable simply as ${ before variable name and } after it.
Applying variable to a request
The value is applied to the header just before sending the request
Sent HTTP message with variable applied to a header
You can put a variable into a method, URL, headers, or payload field. Variables can be used as many times as you need in a request.
Composite variables
A variable's value can use another variable. Variables make it easier to build variables that require values from multiple sources.
As an example, basic authorization requires both username and password. It might be difficult to maintain credentials in multiple requests. It's a better idea to store both the username and the password as separate variables and create a new one that combines both with some additional value.
Composite variables example
As a result, the combined value is used with the request.
Processed variable value
Functions in variables
You can use one of the predefined functions in your variables. The ARC's engine supports various String, Math, and other functions. To run a function editor press a pencil icon next to the variable value text field. It brings an editor popup with available options with configuration.
Variable value editor
It uses the JavaScript engine to evaluate the value. We will use a slice function from String class to demonstrate the principle. The slice function accepts two arguments start and end and returns a string that contains characters found between the start index and end index. We are going to "slice" our ${username} variable to get a string from position 1 to 3. We would use the following syntax: String.slice(${username}, 1, 3) where the first argument is the string we are operating on and other arguments are arguments passed to the JavaScript function.
In a programming world this would JavaScript would evaluate: "uname".slice(1, 3) which  result with na.
Evaluated function
Function execution group
ARC supports a few additional functions. They are defined in the miscellaneous tab. At the moment you can choose between now random decodeURIComponent and encodeURIComponent functions.
Now and random functions support grouping that allows you to generate a value once when making a request and to reuse generated value again using the same group name. Pass a value to either of the functions as an argument and re-use the same function call more than once to recall generated value.
Grouped generated values