Why Dynamic response stubbing in Cypress? - Purpose & Use Cases

The Big Idea

What if you could control server answers instantly to test anything you want without waiting?

The Scenario

Imagine testing a web app that fetches user data from a server. You manually change the server data each time to test different user profiles. This means waiting, refreshing, and hoping the server responds as expected.

The Problem

Manually changing server data is slow and tiring. It's easy to make mistakes or forget to reset data. Tests become flaky because the server might be slow or down. You waste time waiting instead of testing.

The Solution

Dynamic response stubbing lets you fake server responses right inside your test. You control what data the app gets instantly, without changing the real server. This makes tests fast, reliable, and easy to repeat.

Before vs After

✗ Before

Visit app -> Wait for server -> Change server data manually -> Refresh -> Check UI

✓ After

cy.intercept('/api/user', { fixture: 'user1.json' }).as('getUser');
cy.visit('/');
cy.wait('@getUser');
// UI shows user1 data

What It Enables

Dynamic response stubbing makes testing flexible and fast by letting you control server answers on the fly.

Real Life Example

Testing a shopping cart app where you stub different product lists to check how the cart updates without needing real products on the server.

Key Takeaways

Manual server changes slow down and complicate testing.

Dynamic stubbing fakes server responses instantly inside tests.

This leads to faster, more reliable, and repeatable tests.

Practice

1. What is the main purpose of dynamic response stubbing in Cypress tests?

easy

A. To change server responses during tests to simulate different scenarios

B. To speed up the browser rendering process

C. To automatically generate test data from the server

D. To record user interactions for playback

Solution

1. Step 1: Understand dynamic response stubbing

   Dynamic response stubbing allows tests to modify server replies on the fly to simulate various conditions.

2. Step 2: Identify the main goal

   This helps test how the app behaves with different data or errors without relying on real server responses.

3. Final Answer:

   To change server responses during tests to simulate different scenarios -> Option A

4. Quick Check:

   Dynamic stubbing = simulate different server replies [OK]

Hint: Dynamic stubbing means changing server replies during tests [OK]

Common Mistakes:

• Confusing stubbing with speeding up rendering
• Thinking it generates test data automatically
• Mixing it up with recording user actions

2. Which of the following is the correct way to stub a dynamic response using cy.intercept in Cypress?

easy

A. cy.intercept('GET', '/api/data', { status: 200, body: { value: 42 } })

B. cy.intercept('GET', '/api/data', (res) => { res.send({ value: 42 }) })

C. cy.intercept('GET', '/api/data', () => { return { value: 42 } })

D. cy.intercept('GET', '/api/data', (req) => { req.reply({ body: { value: 42 } }) })

Solution

1. Step 1: Recall cy.intercept syntax for dynamic stubbing

   To modify the response dynamically, use a callback with the request object and call req.reply() with the new body.

2. Step 2: Check each option

   cy.intercept('GET', '/api/data', (req) => { req.reply({ body: { value: 42 } }) }) correctly uses (req) => { req.reply({ body: ... }) }. cy.intercept('GET', '/api/data', { status: 200, body: { value: 42 } }) is static stubbing, C returns a value incorrectly, D uses res.send which is not Cypress syntax.

3. Final Answer:

   cy.intercept('GET', '/api/data', (req) => { req.reply({ body: { value: 42 } }) }) -> Option D

4. Quick Check:

   Dynamic stub uses req.reply() callback [OK]

Hint: Use req.reply() inside intercept callback for dynamic response [OK]

Common Mistakes:

• Using static object instead of callback for dynamic stubbing
• Returning response instead of calling req.reply()
• Using incorrect callback parameter names

3. Given the following Cypress code, what will be the response body when the app requests '/api/user'?

cy.intercept('GET', '/api/user', (req) => {
  const id = req.query.id
  if (id === '1') {
    req.reply({ body: { name: 'Alice' } })
  } else {
    req.reply({ body: { name: 'Guest' } })
  }
})

The app calls fetch('/api/user?id=1'). What is the response?

medium

A. Empty response body

B. { name: 'Guest' }

C. { name: 'Alice' }

D. An error occurs because req.query is undefined

Solution

1. Step 1: Analyze the intercept callback logic

   The callback checks req.query.id. If it equals '1', it replies with { name: 'Alice' }, else { name: 'Guest' }.

2. Step 2: Check the request URL query

   The app calls '/api/user?id=1', so req.query.id is '1'. Therefore, the response is { name: 'Alice' }.

3. Final Answer:

   { name: 'Alice' } -> Option C

4. Quick Check:

   Query id '1' returns Alice [OK]

Hint: Match query params in req.query to decide response [OK]

Common Mistakes:

• Assuming req.query is undefined in Cypress intercept
• Ignoring query parameters in the stub
• Confusing string and number types in comparison

4. Identify the error in this Cypress dynamic stub code:

cy.intercept('POST', '/api/login', (req) => {
  req.reply({ statusCode: 200, body: { token: 'abc123' } })
})

The test fails with an error about req.reply not being a function. What is wrong?

medium

A. The callback receives req but req.reply is not a function; should use req.continue

B. The callback parameter should be interception and interception.reply used

C. The callback parameter is correct, but req.reply is only available on res

D. The callback parameter should be named res instead of req

Solution

1. Step 1: Understand intercept callback parameter

   The callback receives an interception object, often named interception, which has the reply method.

2. Step 2: Identify the naming and usage error

   Using req as parameter is allowed but req.reply is not a function because the object is not the request but the interception. The correct approach is to name it interception and call interception.reply().

3. Final Answer:

   The callback parameter should be interception and interception.reply used -> Option B

4. Quick Check:

   Use interception.reply() in intercept callback [OK]

Hint: Use interception.reply(), not req.reply(), in intercept callback [OK]

Common Mistakes:

• Confusing request object with interception object
• Using req.reply() which does not exist
• Misnaming callback parameter causing method errors

5. You want to stub a GET request to '/api/items' so that the response body depends on a query parameter 'type'. If 'type' is 'fruit', respond with { items: ['apple', 'banana'] }. If 'type' is 'vegetable', respond with { items: ['carrot', 'lettuce'] }. Otherwise, respond with { items: [] }. Which code correctly implements this dynamic response stubbing?

hard

A. cy.intercept('GET', '/api/items', (interception) => {
const type = interception.query.type
if (type === 'fruit') {
interception.reply({ body: { items: ['apple', 'banana'] } })
} else if (type === 'vegetable') {
interception.reply({ body: { items: ['carrot', 'lettuce'] } })
} else {
interception.reply({ body: { items: [] } })
}
})

B. cy.intercept('GET', '/api/items', (req) => {
if (req.query.type === 'fruit') {
req.reply({ items: ['apple', 'banana'] })
} else if (req.query.type === 'vegetable') {
req.reply({ items: ['carrot', 'lettuce'] })
} else {
req.reply({ items: [] })
}
})

C. cy.intercept('GET', '/api/items', (req) => {
const type = req.query.type
if (type === 'fruit') {
req.reply({ body: { items: ['apple', 'banana'] } })
} else if (type === 'vegetable') {
req.reply({ body: { items: ['carrot', 'lettuce'] } })
} else {
req.reply({ body: { items: [] } })
}
})

D. cy.intercept('GET', '/api/items', (interception) => {
const type = interception.query.type
if (type === 'fruit') {
interception.send({ body: { items: ['apple', 'banana'] } })
} else if (type === 'vegetable') {
interception.send({ body: { items: ['carrot', 'lettuce'] } })
} else {
interception.send({ body: { items: [] } })
}
})

Solution

1. Step 1: Identify correct parameter and method usage

   The callback parameter should be named interception (or similar) and use interception.reply() to send the response with a body property.

2. Step 2: Check each option for correctness

   cy.intercept('GET', '/api/items', (interception) => { const type = interception.query.type if (type === 'fruit') { interception.reply({ body: { items: ['apple', 'banana'] } }) } else if (type === 'vegetable') { interception.reply({ body: { items: ['carrot', 'lettuce'] } }) } else { interception.reply({ body: { items: [] } }) } }) correctly uses interception.query.type and calls interception.reply({ body: ... }). cy.intercept('GET', '/api/items', (req) => { if (req.query.type === 'fruit') { req.reply({ items: ['apple', 'banana'] }) } else if (req.query.type === 'vegetable') { req.reply({ items: ['carrot', 'lettuce'] }) } else { req.reply({ items: [] }) } }) misses body wrapper. cy.intercept('GET', '/api/items', (req) => { const type = req.query.type if (type === 'fruit') { req.reply({ body: { items: ['apple', 'banana'] } }) } else if (type === 'vegetable') { req.reply({ body: { items: ['carrot', 'lettuce'] } }) } else { req.reply({ body: { items: [] } }) } }) uses req but req.reply is not a function. cy.intercept('GET', '/api/items', (interception) => { const type = interception.query.type if (type === 'fruit') { interception.send({ body: { items: ['apple', 'banana'] } }) } else if (type === 'vegetable') { interception.send({ body: { items: ['carrot', 'lettuce'] } }) } else { interception.send({ body: { items: [] } }) } }) uses interception.send which is not a Cypress method.

3. Final Answer:

   Option A code correctly implements dynamic response stubbing with query check and reply -> Option A

4. Quick Check:

   Use interception.reply({ body }) and check interception.query [OK]

Hint: Use interception.reply({ body }) and check interception.query.type [OK]

Common Mistakes:

• Using req.reply() which is undefined
• Omitting the body wrapper in reply
• Using interception.send() which does not exist