How to assert json value postman

PostmanHow-ToBeginner · 3 min read

How to Assert JSON Value in Postman Tests Easily

In Postman, you can assert JSON values by writing test scripts using pm.test and pm.response.json(). Extract the JSON response, then use pm.expect to check if a specific key has the expected value.

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Syntax

Use pm.test to define a test case with a description. Inside it, call pm.response.json() to parse the JSON response body. Then use pm.expect with a matcher like .to.eql() to assert the value.

pm.test("Test description", function () { ... }): Defines a test.
pm.response.json(): Parses the response body as JSON.
pm.expect(actual).to.eql(expected): Checks if actual equals expected.

javascript

pm.test("Check JSON value", function () {
    const jsonData = pm.response.json();
    pm.expect(jsonData.key).to.eql("expectedValue");
});

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Example

This example shows how to assert that the JSON response has a key status with value success. It runs after the API response is received.

javascript

pm.test("Status is success", function () {
    const jsonData = pm.response.json();
    pm.expect(jsonData.status).to.eql("success");
});

Output

PASS Status is success

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Common Pitfalls

Common mistakes include:

Not parsing the response JSON before accessing keys.
Using incorrect key names or case sensitivity.
Comparing values with wrong types (e.g., string vs number).
Not handling nested JSON objects properly.

Always check the exact JSON structure and use correct keys.

javascript

/* Wrong way: missing JSON parse */
pm.test("Wrong test", function () {
    pm.expect(pm.response.json().status).to.eql("success"); // This will fail if pm.response.body is not parsed
});

/* Right way: parse JSON first */
pm.test("Right test", function () {
    const jsonData = pm.response.json();
    pm.expect(jsonData.status).to.eql("success");
});

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Quick Reference

Action	Code Snippet	Description
Define test	pm.test("desc", function () { ... })	Create a test case with description
Parse JSON	const jsonData = pm.response.json();	Get JSON object from response body
Assert value	pm.expect(jsonData.key).to.eql(value);	Check if key equals expected value
Check nested	pm.expect(jsonData.parent.child).to.eql(value);	Assert nested JSON key value
Check type	pm.expect(typeof jsonData.key).to.eql('string');	Assert data type of value

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Key Takeaways

Always parse the response JSON with pm.response.json() before assertions.

Use pm.test to group assertions with clear descriptions.

Use pm.expect(actual).to.eql(expected) for exact value checks.

Check JSON keys carefully for correct spelling and case.

Handle nested JSON by accessing keys step-by-step.