Mocking data fetching in NextJS - Step-by-Step Execution

Concept Flow - Mocking data fetching

Start Component Render

↓

Call fetch function

↓

Is mock enabled?

Yes→Return mock data

↓

Return real data

↓

Use data in component

↓

Render UI with data

↓

End

The component starts rendering, calls a fetch function that checks if mock data should be used. If yes, it returns mock data immediately; otherwise, it fetches real data. Then the component uses this data to render the UI.

Execution Sample

NextJS

export async function fetchData(mock = false) {
  if (mock) {
    return { name: 'Mock User', age: 30 };
  }
  const res = await fetch('/api/user');
  return res.json();
}

This function fetches user data. If mock is true, it returns fake data immediately; otherwise, it fetches real data from an API.

Execution Table

Step	Action	mock parameter	Data Returned	Notes
1	Call fetchData(mock=false)	false	Calls fetch('/api/user')	No mock, real fetch starts
2	Await fetch response	false	Response object	Waiting for real data
3	Parse JSON from response	false	{ name: 'Real User', age: 25 }	Real data received
4	Return data to component	false	{ name: 'Real User', age: 25 }	Component uses real data
5	Call fetchData(mock=true)	true	{ name: 'Mock User', age: 30 }	Mock enabled, returns mock data immediately
6	Return data to component	true	{ name: 'Mock User', age: 30 }	Component uses mock data
7	End	-	-	No further actions

💡 Execution stops after returning data to the component either from mock or real fetch.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 5	Final
mock	false	false	false	true	true
data	undefined	fetch response pending	{ name: 'Real User', age: 25 }	{ name: 'Mock User', age: 30 }	{ name: 'Mock User', age: 30 }

Key Moments - 2 Insights

Why does the function return mock data immediately when mock=true?

Because the if condition at step 5 detects mock=true and returns the mock object without waiting for any fetch, as shown in execution_table row 5.

What happens when mock=false in terms of data fetching?

The function performs a real fetch call to the API, waits for the response, parses JSON, and then returns real data, as detailed in execution_table rows 1 to 4.

Visual Quiz - 3 Questions

Look at the execution table, what data is returned at step 3?

A { name: 'Real User', age: 25 }

B { name: 'Mock User', age: 30 }

C fetch response object

D undefined

💡 Hint

Check the 'Data Returned' column at step 3 in the execution_table.

At which step does the function return mock data?

A Step 2

B Step 5

C Step 4

D Step 6

💡 Hint

Look for the step where mock=true and mock data is returned in the execution_table.

If mock parameter is always false, what changes in the variable_tracker?

A data becomes mock data at final

B mock changes to true after step 3

C mock stays false, data never becomes mock data

D mock toggles between true and false

💡 Hint

Refer to the 'mock' variable values in variable_tracker when mock=false.

Concept Snapshot

Mocking data fetching in Next.js:
- Use a parameter or flag to decide mock vs real fetch
- If mock enabled, return fake data immediately
- Else, fetch real data asynchronously
- Component uses returned data to render UI
- Helps test UI without real backend calls

Full Transcript

This visual execution shows how a Next.js data fetching function can return either mock data or real data based on a parameter. When the mock flag is true, the function returns a predefined object immediately, skipping any network call. When false, it performs a real fetch to an API endpoint, waits for the response, parses JSON, and returns the real data. The component then uses this data to render the UI. This approach helps developers test UI components without relying on backend availability or network conditions.