Skip to content

REST API variable chaining

REST API variable chaining is a technique used in API testing and development workflows to extract values from one HTTP response and inject them into subsequent requests.^[600-developer-tools-vscode-restclient-postman.md]

This method is primarily utilized to manage stateful interactions, such as passing authentication tokens or session IDs returned from a login endpoint to other API calls that require authorization.^[600-developer-tools-vscode-restclient-postman.md]

Implementation in Tools

VSCode REST Client

In the VSCode REST Client extension, chaining is handled via a custom request syntax that captures response data into a variable^[600-developer-tools-vscode-restclient-postman.md#L37-50]. To extract a value, a request must be assigned a name using the # @name comment. Variables are then defined using the syntax @variableName = {{requestName.response.body.$.jsonPath}}^[600-developer-tools-vscode-restclient-postman.md].

For example, to extract a session ID from a login response: 

# @name loginCWS
POST {{domain}}/sessions HTTP/1.1
...

@authToken = {{loginCWS.response.body.$.id}}
The authToken variable can then be reused in headers for later requests^[600-developer-tools-vscode-restclient-postman.md].

Postman

Postman achieves variable chaining through test scripts written in JavaScript that execute after a request completes^[600-developer-tools-vscode-restclient-postman.md#L55-73]. The script parses the responseBody, extracts specific fields, and assigns them to environment variables using pm.environment.set(variableName, value)^[600-developer-tools-vscode-restclient-postman.md].

A typical script involves parsing the JSON response, logging specific values to the console for debugging, and setting them for subsequent use^[600-developer-tools-vscode-restclient-postman.md].

Sources

^[600-developer-tools-vscode-restclient-postman.md]