Master Postman Pre-request Scripts: Automate Signing, Env Switching, and Auth

Background

Business service APIs require signature logic, making manual signing in Postman cumbersome.

Testing across multiple server environments demands domain switching.

Some APIs need authentication after login.

Pre-request Script Overview

Postman runs on a Node.js runtime, allowing dynamic behavior through JavaScript code added to requests and collections. Two script locations are supported:

Pre-request Script tab: code runs before the request is sent.

Test tab: code runs after the response is received.

Script Execution Order

For a single request:

Pre-request script executes before the request is sent.

Test script executes after the response is received.

For each request in a collection, scripts run in this order:

Collection-level pre-request script runs before every request.

Folder-level pre-request script runs before each direct child request.

Collection-level test script runs after each request.

Folder-level test script runs after each direct child request.

Implementation Steps

Signing

Write a pre-request script for the collection to implement the signing logic.

In the Pre-request Script tab, add the JavaScript code for signing and optionally log output.

Open the Postman console to view the signing result.

Signing completed successfully.

Multi‑Environment Switching

Use Postman's Environments feature to define variables for different domains (e.g., test and prod).

Create environment variables such as

host

and assign values for each environment.

Define the specific variable values.

Use the variable in request URLs with the {{key}} syntax.

Switch environments to change the host dynamically.

Authenticated Requests

Some APIs require a JWT token in the header after login.

Use the Tests tab to capture the token from the login response and store it in an environment variable.

In the login request's Tests script, set the token variable.

Then use the variable in subsequent request headers.

Call the login API to populate the token variable automatically.