Variable assignment in flows in Postman - Build an Automation Script

Assign and use variables in Postman request flow

Preconditions (2)

Step 1: Open the first request in the collection

Step 2: Send the request and capture a value from the JSON response (e.g., userId)

Step 3: Assign the captured value to a collection variable named 'userId'

Step 4: Open the second request in the collection

Step 5: Use the collection variable 'userId' in the URL or headers of the second request

Step 6: Send the second request

✅ Expected Result: The first request runs successfully and assigns the 'userId' variable. The second request uses the assigned 'userId' variable correctly and runs successfully.

Automation Requirements - Postman test scripts

Assertions Needed:

Verify the first request response status is 200

Verify the 'userId' variable is assigned and not empty

Verify the second request response status is 200

Best Practices:

Use pm.response.json() to parse JSON response

Use pm.collectionVariables.set() to assign variables

Use pm.collectionVariables.get() to retrieve variables

Add assertions to verify response status and variable assignment

Keep variable names clear and consistent

Automated Solution

Postman

/* First request test script */
pm.test('Status code is 200', () => {
    pm.response.to.have.status(200);
});

const jsonData = pm.response.json();
if (jsonData.userId) {
    pm.collectionVariables.set('userId', jsonData.userId);
    pm.test('userId variable is set', () => {
        pm.expect(pm.collectionVariables.get('userId')).to.not.be.empty;
    });
} else {
    pm.test('userId exists in response', () => {
        pm.expect.fail('userId not found in response');
    });
}

/* Second request test script */
pm.test('Status code is 200', () => {
    pm.response.to.have.status(200);
});

The first request test script checks that the response status is 200, meaning the request succeeded.

It then parses the JSON response using pm.response.json() and checks if userId exists.

If userId is found, it assigns it to a collection variable named userId using pm.collectionVariables.set().

It also asserts that the variable is not empty to confirm assignment.

If userId is missing, the test fails with a clear message.

The second request test script simply verifies that the response status is 200, assuming the request URL or headers use the {{userId}} variable from the collection variables.

This approach ensures the variable is assigned and used correctly across requests in the flow.

Common Mistakes - 4 Pitfalls

Using environment variables instead of collection variables without updating the environment

Not checking if the response contains the expected key before assigning variable

Hardcoding variable values instead of capturing from response

Not adding assertions to verify variable assignment

Bonus Challenge

Now add data-driven testing by running the first request with 3 different user credentials and verify variable assignment each time

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