Writing Tests & Assertions

Manual checking is slow and error-prone. Postman lets you write automated tests that run after every request.

Where Tests Live

In Postman, go to the Tests tab (now called Scripts → Post-response in newer versions).

Tests run automatically after the response is received.

// This code runs after every request
pm.test("Status code is 200", function() {
    pm.response.to.have.status(200);
});

---

Your First Test

Testing Status Code

pm.test("Status code is 200", function() {
    pm.response.to.have.status(200);
});

• Create a GET request to https://fakestoreapi.com/products/1
• Go to Tests tab
• Paste the code above
• Click Send
• Check Test Results tab in response - should show ✅ PASS

Multiple Status Codes

pm.test("Successful response", function() {
    pm.expect(pm.response.code).to.be.oneOf([200, 201]);
});

---

Common Assertions

Status Code Tests

// Exact status
pm.test("Status is 200 OK", function() {
    pm.response.to.have.status(200);
});

// Status name
pm.test("Status is OK", function() {
    pm.response.to.have.status("OK");
});

// Created
pm.test("Resource created", function() {
    pm.response.to.have.status(201);
});

// Not found
pm.test("Product not found", function() {
    pm.response.to.have.status(404);
});

Response Time Tests

// Response under 500ms
pm.test("Response time is acceptable", function() {
    pm.expect(pm.response.responseTime).to.be.below(500);
});

// Response under 1 second
pm.test("Response time under 1 second", function() {
    pm.expect(pm.response.responseTime).to.be.below(1000);
});

Response Body Tests

// Parse JSON response
const response = pm.response.json();

// Check property exists
pm.test("Response has id", function() {
    pm.expect(response).to.have.property("id");
});

// Check property value
pm.test("Product name is correct", function() {
    pm.expect(response.title).to.equal("Fjallraven - Foldsack No. 1 Backpack");
});

// Check price is a number
pm.test("Price is a number", function() {
    pm.expect(response.price).to.be.a("number");
});

// Check price is positive
pm.test("Price is positive", function() {
    pm.expect(response.price).to.be.above(0);
});

---

E-commerce Test Examples

Test: Get Product Details

const product = pm.response.json();

pm.test("Status code is 200", function() {
    pm.response.to.have.status(200);
});

pm.test("Product has required fields", function() {
    pm.expect(product).to.have.property("id");
    pm.expect(product).to.have.property("title");
    pm.expect(product).to.have.property("price");
    pm.expect(product).to.have.property("category");
});

pm.test("Price is valid", function() {
    pm.expect(product.price).to.be.a("number");
    pm.expect(product.price).to.be.above(0);
});

pm.test("Response time is acceptable", function() {
    pm.expect(pm.response.responseTime).to.be.below(500);
});

Test: List Products

const products = pm.response.json();

pm.test("Status code is 200", function() {
    pm.response.to.have.status(200);
});

pm.test("Response is an array", function() {
    pm.expect(products).to.be.an("array");
});

pm.test("Products array is not empty", function() {
    pm.expect(products.length).to.be.above(0);
});

pm.test("Each product has required fields", function() {
    products.forEach(function(product) {
        pm.expect(product).to.have.property("id");
        pm.expect(product).to.have.property("title");
        pm.expect(product).to.have.property("price");
    });
});

Test: Add to Cart

const cart = pm.response.json();

pm.test("Cart created successfully", function() {
    pm.response.to.have.status(200);
});

pm.test("Cart has ID", function() {
    pm.expect(cart).to.have.property("id");
});

pm.test("Cart contains products", function() {
    pm.expect(cart.products).to.be.an("array");
    pm.expect(cart.products.length).to.be.above(0);
});

pm.test("Product quantities are correct", function() {
    const firstProduct = cart.products[0];
    pm.expect(firstProduct.quantity).to.be.above(0);
});

Test: User Login

const response = pm.response.json();

pm.test("Login successful", function() {
    pm.response.to.have.status(200);
});

pm.test("Token received", function() {
    pm.expect(response).to.have.property("token");
    pm.expect(response.token).to.be.a("string");
    pm.expect(response.token.length).to.be.above(10);
});

// Save token for later requests
if (response.token) {
    pm.environment.set("authToken", response.token);
    console.log("Token saved to environment");
}

---

Header Tests

pm.test("Content-Type is JSON", function() {
    pm.response.to.have.header("Content-Type");
    pm.expect(pm.response.headers.get("Content-Type")).to.include("application/json");
});

pm.test("Cache headers present", function() {
    pm.response.to.have.header("Cache-Control");
});

---

String Assertions

const product = pm.response.json();

// Contains substring
pm.test("Description mentions wireless", function() {
    pm.expect(product.description.toLowerCase()).to.include("wireless");
});

// Starts with
pm.test("ID starts with PROD", function() {
    pm.expect(product.id).to.match(/^PROD/);
});

// Email format
pm.test("Email is valid format", function() {
    pm.expect(user.email).to.match(/^[\w-]+@[\w-]+\.\w+$/);
});

---

Array Assertions

const products = pm.response.json();

// Array length
pm.test("Returns exactly 5 products", function() {
    pm.expect(products).to.have.lengthOf(5);
});

// Array contains item
pm.test("Electronics category exists", function() {
    const categories = products.map(p => p.category);
    pm.expect(categories).to.include("electronics");
});

// All items match condition
pm.test("All products have price", function() {
    const allHavePrice = products.every(p => p.price > 0);
    pm.expect(allHavePrice).to.be.true;
});

// At least one matches
pm.test("At least one product under $50", function() {
    const hasAffordable = products.some(p => p.price < 50);
    pm.expect(hasAffordable).to.be.true;
});

---

Error Response Tests

pm.test("Returns 404 for invalid product", function() {
    pm.response.to.have.status(404);
});

pm.test("Error message is descriptive", function() {
    const error = pm.response.json();
    pm.expect(error).to.have.property("message");
});

---

Using Snippets

Postman has built-in snippets! Click Snippets on the right side of the Tests tab:

• Status code is 200
• Response body: JSON value check
• Response time is less than 200ms
• Response body: Contains string

Click a snippet to insert it automatically.

---

Practice: Test Suite for Products API

Create tests for GET https://fakestoreapi.com/products:

// Response Tests
pm.test("Status code is 200", function() {
    pm.response.to.have.status(200);
});

pm.test("Response time is under 1 second", function() {
    pm.expect(pm.response.responseTime).to.be.below(1000);
});

// Data structure tests
const products = pm.response.json();

pm.test("Response is an array", function() {
    pm.expect(products).to.be.an("array");
});

pm.test("At least 10 products returned", function() {
    pm.expect(products.length).to.be.at.least(10);
});

// Data quality tests
pm.test("All products have valid structure", function() {
    products.forEach((product, index) => {
        pm.expect(product, `Product ${index}`).to.have.property("id");
        pm.expect(product, `Product ${index}`).to.have.property("title");
        pm.expect(product, `Product ${index}`).to.have.property("price");
        pm.expect(product.price, `Product ${index} price`).to.be.a("number");
        pm.expect(product.price, `Product ${index} price`).to.be.above(0);
    });
});

pm.test("No duplicate IDs", function() {
    const ids = products.map(p => p.id);
    const uniqueIds = [...new Set(ids)];
    pm.expect(ids.length).to.equal(uniqueIds.length);
});

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Key Takeaways

Next up: Environment Variables - managing different environments and reusable data!