How to Use Pre-request Script in Postman: Simple Guide

In Postman, use the Pre-request Script tab to write JavaScript code that runs before your API request. This script can set variables, modify request data, or prepare authentication dynamically before sending the request.

Syntax

The Pre-request Script in Postman uses JavaScript to run code before the request is sent. You can access and set variables using pm.variables.set('key', value) and read variables with pm.variables.get('key'). Use console.log() to print messages in the Postman Console for debugging.

javascript

pm.variables.set('token', '123abc');
const token = pm.variables.get('token');
console.log('Token is:', token);

Output

Token is: 123abc

Example

This example shows how to generate a timestamp before the request and save it as a variable. The timestamp can then be used in the request URL or headers.

javascript

const timestamp = new Date().toISOString();
pm.variables.set('currentTimestamp', timestamp);
console.log('Timestamp set:', timestamp);

Output

Timestamp set: 2024-06-01T12:34:56.789Z

Common Pitfalls

• Forgetting to use pm.variables.set() to save variables, which means they won't be available in the request.
• Using synchronous code only; asynchronous code like setTimeout won't delay the request.
• Not checking the Postman Console for errors or logs, making debugging harder.

javascript

/* Wrong: Variable not saved, so request can't use it */
const token = 'abc123';
console.log('Token:', token);

/* Right: Save variable to use in request */
pm.variables.set('token', 'abc123');
console.log('Token saved');

Output

Token saved

Quick Reference

Use this cheat sheet to remember key commands in Pre-request Scripts:

Action	Command
Set variable	pm.variables.set('key', value)
Get variable	pm.variables.get('key')
Log to console	console.log('message')
Set environment variable	pm.environment.set('key', value)
Set global variable	pm.globals.set('key', value)

Key Takeaways

Write JavaScript in the Pre-request Script tab to run code before sending requests.

Use pm.variables.set() and pm.variables.get() to manage variables dynamically.

Check the Postman Console to debug and verify your script outputs.

Avoid asynchronous code that delays execution; scripts run synchronously.

Use pre-request scripts to prepare data like tokens, timestamps, or headers before requests.