Variable scope and precedence in Postman - Build an Automation Script

Verify variable scope and precedence in Postman scripts

Preconditions (2)

Step 1: Set a global variable named 'var1' with value 'globalValue'

Step 2: Set an environment variable named 'var1' with value 'envValue'

Step 3: Set a collection variable named 'var1' with value 'collectionValue'

Step 4: In the Pre-request Script of a request, set a local variable 'var1' with value 'localValue'

Step 5: In the request URL or headers, use {{var1}} to reference the variable

Step 6: Send the request

Step 7: In the Tests script, retrieve the value of 'var1' using pm.variables.get('var1')

Step 8: Log the retrieved value to the Postman Console

✅ Expected Result: The value logged in the Postman Console should be 'localValue', showing that local variables have the highest precedence over collection, environment, and global variables.

Automation Requirements - Postman Sandbox JavaScript

Assertions Needed:

Assert that pm.variables.get('var1') returns 'localValue' in the Tests script

Best Practices:

Use pm.variables.set() to set local variables in Pre-request Script

Use pm.collectionVariables.set(), pm.environment.set(), pm.globals.set() for other scopes

Use pm.variables.get() to retrieve the variable value considering scope precedence

Log outputs to Postman Console for debugging

Keep variable names consistent to test precedence clearly

Automated Solution

Postman

// Pre-request Script
pm.globals.set('var1', 'globalValue');
pm.environment.set('var1', 'envValue');
pm.collectionVariables.set('var1', 'collectionValue');
pm.variables.set('var1', 'localValue');

// Request URL example: https://example.com/api?param={{var1}}

// Tests Script
const varValue = pm.variables.get('var1');
console.log('Value of var1:', varValue);
pm.test('Variable scope precedence test', function () {
    pm.expect(varValue).to.eql('localValue');
});

First, we set the variable 'var1' in all scopes: global, environment, collection, and local (Pre-request Script).

Local variables set with pm.variables.set() have the highest precedence and override others.

In the request URL, {{var1}} will resolve to the local variable value 'localValue'.

In the Tests script, we retrieve the variable using pm.variables.get('var1'), which respects the scope precedence and returns 'localValue'.

The assertion checks that the retrieved value matches the expected local variable value, confirming the precedence order.

Logging to the Postman Console helps verify the actual value during test execution.

Common Mistakes - 3 Pitfalls

{'mistake': 'Setting variables only in global or environment scope and expecting local scope precedence', 'why_bad': "Local variables override other scopes, so if not set, the expected precedence won't be demonstrated.", 'correct_approach': 'Always set the local variable in the Pre-request Script using pm.variables.set() to test precedence.'}

Using pm.environment.get() or pm.globals.get() to retrieve variables ignoring scope precedence

Not clearing or resetting variables before the test, causing stale values

Bonus Challenge

Now add data-driven testing by setting 'var1' to three different values in local scope and verify the precedence each time.

Show Hint

Practice

1. In Postman, which variable scope has the highest precedence when accessing a variable value?

easy

A. Global variables

B. Environment variables

C. Collection variables

D. Local variables

Solution

1. Step 1: Understand Postman variable scopes

   Postman has multiple variable scopes: local, environment, collection, and global.

2. Step 2: Identify precedence order

   Local variables override environment, collection, and global variables when accessing a variable.

3. Final Answer:

   Local variables -> Option D

4. Quick Check:

   Highest precedence = Local variables [OK]

Hint: Local variables always override others in Postman [OK]

Common Mistakes:

• Confusing environment variables as highest
• Thinking global variables override local
• Ignoring local scope existence

2. Which of the following is the correct way to set a local variable in a Postman test script?

easy

A. pm.variables.set('varName', 'value');

B. pm.environment.set('varName', 'value');

C. pm.collectionVariables.set('varName', 'value');

D. pm.globals.set('varName', 'value');

Solution

1. Step 1: Recall methods to set variables in Postman

   Local variables are set using pm.variables.set(). Environment, collection, and global variables use their respective methods.

2. Step 2: Match method to local variable scope

   pm.variables.set() sets a local variable for the current request or script.

3. Final Answer:

   pm.variables.set('varName', 'value'); -> Option A

4. Quick Check:

   Local variable set method = pm.variables.set() [OK]

Hint: Use pm.variables.set() for local variables [OK]

Common Mistakes:

• Using pm.environment.set() for local variables
• Confusing collection and global variable setters
• Forgetting to use pm.variables for local scope

3. Given these variables set in Postman:
pm.environment.set('token', 'envToken');
pm.collectionVariables.set('token', 'colToken');
pm.variables.set('token', 'localToken');
What will pm.variables.get('token') return?

medium

A. 'envToken'

B. 'localToken'

C. 'colToken'

D. null

Solution

1. Step 1: Identify variable scopes and values

   Environment variable 'token' = 'envToken', collection variable 'token' = 'colToken', local variable 'token' = 'localToken'.

2. Step 2: Understand variable precedence when getting value

   pm.variables.get() returns the local variable value if set, ignoring environment and collection variables.

3. Final Answer:

   'localToken' -> Option B

4. Quick Check:

   Local variable value returned by pm.variables.get() [OK]

Hint: pm.variables.get() returns local variable if exists [OK]

Common Mistakes:

• Assuming environment variable is returned
• Confusing collection variable with local
• Expecting null if multiple scopes exist

4. You wrote this Postman test script:

pm.environment.set('user', 'Alice');
pm.variables.set('user', 'Bob');
console.log(pm.environment.get('user'));

But the console logs 'Alice' instead of 'Bob'. What is the issue?

medium

A. pm.environment.set() overwrites local variables

B. pm.variables.set() does not set local variables

C. pm.variables.get() should be used to get local variable

D. pm.environment.get() always reads environment variable ignoring local

Solution

1. Step 1: Analyze variable setting and getting methods

   pm.environment.set() sets environment variable; pm.variables.set() sets local variable. But pm.environment.get() reads environment variable only.

2. Step 2: Identify correct method to get local variable

   To get local variable, use pm.variables.get(), not pm.environment.get().

3. Final Answer:

   pm.variables.get() should be used to get local variable -> Option C

4. Quick Check:

   Use matching get method for variable scope [OK]

Hint: Use pm.variables.get() to read local variables [OK]

Common Mistakes:

• Using pm.environment.get() to read local variables
• Assuming set method affects all scopes
• Expecting pm.variables.set() to overwrite environment

5. You have a global variable 'count' set to '5'. In a collection run, you set a collection variable 'count' to '10'. In a pre-request script, you set a local variable 'count' to '15'. Which value will be used if your test script calls pm.variables.get('count') and why?

hard

A. '15' because local variables have highest precedence

B. '5' because global variables have highest precedence

C. '10' because collection variables override local variables

D. null because variable names conflict

Solution

1. Step 1: Review variable scopes and precedence

   Global variable 'count' = '5', collection variable 'count' = '10', local variable 'count' = '15'. Local variables override collection and global.

2. Step 2: Determine which value pm.variables.get() returns

   pm.variables.get() returns the local variable value if it exists, ignoring collection and global variables.

3. Final Answer:

   '15' because local variables have highest precedence -> Option A

4. Quick Check:

   Local variable value returned by pm.variables.get() [OK]

Hint: Local variables override collection and global variables [OK]

Common Mistakes:

• Thinking global variables override local
• Assuming collection variables override local
• Believing variable conflicts cause null