Passing data to templates in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the rendered output of this Express route with EJS template?

Given the Express route below and an EJS template that displays <%= message %>, what will the user see in the browser?

Express

app.get('/greet', (req, res) => {
  res.render('greet', { message: 'Hello, friend!' });
});

A The page shows: <%= message %>

B The page shows: message

C The page is blank with no text

D The page shows: Hello, friend!

💡 Hint

Remember that res.render passes data to the template, and EJS replaces <%= %> with the value.

The message variable is passed to the EJS template. The template uses <%= message %> which outputs the string 'Hello, friend!'. So the user sees that text.

📝 Syntax

intermediate

Which option correctly passes multiple variables to a Pug template?

Choose the correct way to pass title and user variables to a Pug template in Express.

Express

app.get('/profile', (req, res) => {
  // pass variables here
});

A res.render('profile', title = 'User Profile', user = req.user);

B res.render('profile', { title: 'User Profile', user: req.user });

C res.render('profile', ['title', 'User Profile', 'user', req.user]);

D res.render('profile', { 'title': 'User Profile' }, { 'user': req.user });

💡 Hint

Remember that res.render takes the template name and a single object with all variables.

Option B correctly passes an object with both title and user properties. Other options use invalid syntax or multiple objects which is not allowed.

🔧 Debug

advanced

Why does this Express route fail to pass data to the Handlebars template?

Examine the code below. The template does not receive the name variable. What is the cause?

Express

app.get('/hello', (req, res) => {
  const name = 'Alice';
  res.render('hello');
});

A The <code>name</code> variable is not passed in the <code>res.render</code> call.

B The variable <code>name</code> must be declared with <code>var</code> to be passed.

C The route should use <code>res.send</code> instead of <code>res.render</code>.

D The template engine does not support variables named <code>name</code>.

💡 Hint

Check how variables are passed to templates in Express.

The variable name is declared but never sent to the template. You must include it in the object passed to res.render.

❓ state_output

advanced

What is the output when passing nested data to a template?

Given this Express route and a Pug template that uses user.name and user.age, what will be rendered?

Express

app.get('/user', (req, res) => {
  res.render('user', { user: { name: 'Bob', age: 30 } });
});

A The page shows: Name: Bob, Age: 30

B The page shows: Name: undefined, Age: undefined

C The page shows: Name: [object Object], Age: [object Object]

D The page is blank with no user info

💡 Hint

Remember how nested objects are accessed in templates.

The user object with name and age is passed correctly. The template accesses these properties and displays their values.

🧠 Conceptual

expert

Which option best explains why data passed to templates must be plain objects?

Why should the data passed to Express templates be plain JavaScript objects rather than complex types like functions or class instances?

A Templates require data to be JSON strings, so objects must be serialized first.

B Express automatically converts all data to strings, so complex types are ignored.

C Templates can only safely render simple data structures; complex types may cause errors or unexpected output.

D Passing functions or classes improves performance but is discouraged for security reasons.

💡 Hint

Think about how templates access and display data.

Templates expect plain objects with properties they can read and display. Complex types like functions or class instances may not serialize or render properly, causing errors or confusing output.

Practice

1. In Express, how do you pass data to a template when rendering it?

easy

A. By modifying the template file before rendering

B. By setting variables directly on req object

C. By sending data in the URL query string

D. By passing an object as the second argument to res.render()

Solution

1. Step 1: Understand res.render() usage

   Express uses res.render() to render templates and can accept data as the second argument.

2. Step 2: Passing data as an object

   The data is passed as an object with keys representing variable names in the template.

3. Final Answer:

   By passing an object as the second argument to res.render() -> Option D

4. Quick Check:

   Data passed via res.render() object = By passing an object as the second argument to res.render() [OK]

Hint: Use res.render(view, dataObject) to pass data [OK]

Common Mistakes:

• Trying to set data on req instead of res.render()
• Passing data as a string instead of an object
• Modifying template files directly for dynamic data

2. Which of the following is the correct syntax to pass a variable named title with value 'Home' to a template named index?

easy

A. res.render('index', ['title', 'Home']);

B. res.render('index', { title: 'Home' });

C. res.render('index', title = 'Home');

D. res.render('index', 'title', 'Home');

Solution

1. Step 1: Check res.render() parameters

   The first parameter is the template name as a string, the second is an object with key-value pairs.

2. Step 2: Validate object syntax

   Passing { title: 'Home' } correctly sends the variable title with value 'Home'.

3. Final Answer:

   res.render('index', { title: 'Home' }); -> Option B

4. Quick Check:

   Object with key-value = res.render('index', { title: 'Home' }); [OK]

Hint: Pass data as an object literal in second argument [OK]

Common Mistakes:

• Using assignment instead of object literal
• Passing array instead of object
• Passing multiple string arguments instead of one object

3. Given this Express code snippet:

app.get('/', (req, res) => {
  res.render('home', { user: 'Alice', age: 30 });
});

What variables can the home template access?

medium

A. Both user with 'Alice' and age with 30

B. Only age with value 30

C. Only user with value 'Alice'

D. No variables, template receives no data

Solution

1. Step 1: Identify data passed to template

   The object passed to res.render() has keys user and age with values 'Alice' and 30.

2. Step 2: Understand template variable availability

   The template can access all keys in the object as variables.

3. Final Answer:

   Both user with 'Alice' and age with 30 -> Option A

4. Quick Check:

   All keys in object = Both user with 'Alice' and age with 30 [OK]

Hint: All keys in object become template variables [OK]

Common Mistakes:

• Assuming only one variable is passed
• Thinking template cannot access numbers
• Believing data must be nested inside another object

4. What is wrong with this Express code snippet?

app.get('/profile', (req, res) => {
  res.render('profile', user: 'Bob');
});

medium

A. Callback function should have three parameters

B. Template name should be a variable, not string

C. Missing curly braces around data object

D. Cannot pass data to templates in Express

Solution

1. Step 1: Check res.render() syntax

   The second argument must be an object, so data should be inside curly braces.

2. Step 2: Identify missing braces

   Here, user: 'Bob' is passed without braces, causing syntax error.

3. Final Answer:

   Missing curly braces around data object -> Option C

4. Quick Check:

   Data must be an object literal = Missing curly braces around data object [OK]

Hint: Always wrap data in { } when passing to res.render() [OK]

Common Mistakes:

• Passing key-value pairs without braces
• Using variables instead of strings for template name
• Adding extra parameters to callback unnecessarily

5. You want to render a template dashboard that shows a user's name and a list of their tasks. Which code correctly passes this data to the template?

hard

A. res.render('dashboard', { name: 'Eve', tasks: ['Buy milk', 'Call Bob'] });

B. res.render('dashboard', 'name', 'Eve', 'tasks', ['Buy milk', 'Call Bob']);

C. res.render('dashboard', { 'Eve', ['Buy milk', 'Call Bob'] });

D. res.render('dashboard', { user: 'Eve', taskList: 'Buy milk, Call Bob' });

Solution

1. Step 1: Identify correct object structure

   The data must be passed as an object with keys matching template variables.

2. Step 2: Check data types for variables

   name is a string and tasks is an array, both correctly passed in res.render('dashboard', { name: 'Eve', tasks: ['Buy milk', 'Call Bob'] });.

3. Step 3: Evaluate other options

   The other options use incorrect syntax such as multiple separate arguments, invalid object literals without keys, or mismatched variable names and data types (string instead of array).

4. Final Answer:

   res.render('dashboard', { name: 'Eve', tasks: ['Buy milk', 'Call Bob'] }); -> Option A

5. Quick Check:

   Correct object with array value = res.render('dashboard', { name: 'Eve', tasks: ['Buy milk', 'Call Bob'] }); [OK]

Hint: Pass arrays inside object to send lists to templates [OK]

Common Mistakes:

• Passing multiple arguments instead of one object
• Using wrong data types for lists
• Mismatching variable names expected by template