Passing data to templates in Express - Mini Project: Build & Apply

Passing Data to Templates in Express

📖 Scenario: You are building a simple web page using Express.js that shows a list of fruits and their colors. You want to pass this data from your server code to the template so it can display the list dynamically.

🎯 Goal: Create an Express app that sends a list of fruits with their colors to a template. The template will then show each fruit and its color on the web page.

📋 What You'll Learn

Create a data object with fruit names and their colors

Set up a configuration variable for the page title

Pass the data and title to the template using res.render

Add the template code to display the fruit list and title

💡 Why This Matters

🌍 Real World

Passing data to templates is how web servers send dynamic content to users, like showing product lists or user profiles.

💼 Career

Understanding how to pass data to templates is essential for backend web developers working with Express and similar frameworks.

1

Create the fruit data object

Create a constant called fruits that is an array of objects. Each object should have name and color properties with these exact entries: { name: 'Apple', color: 'Red' }, { name: 'Banana', color: 'Yellow' }, { name: 'Grape', color: 'Purple' }.

Express

// Create the fruits array with objects for Apple, Banana, and Grape
// Your code here

Hint

Use const fruits = [ ... ] with objects inside the array.

2

Add a page title variable

Create a constant called pageTitle and set it to the string 'Fruit Colors'.

Express

const fruits = [
  { name: 'Apple', color: 'Red' },
  { name: 'Banana', color: 'Yellow' },
  { name: 'Grape', color: 'Purple' }
];

// Create a pageTitle variable with 'Fruit Colors'
// Your code here

Hint

Use const pageTitle = 'Fruit Colors';

3

Pass data to the template in the route

Inside your Express app, create a GET route for '/'. Use res.render to render the template named 'fruits' and pass an object with fruits and pageTitle as properties.

Express

const fruits = [
  { name: 'Apple', color: 'Red' },
  { name: 'Banana', color: 'Yellow' },
  { name: 'Grape', color: 'Purple' }
];

const pageTitle = 'Fruit Colors';

// Create a GET route for '/' that renders 'fruits' template
// Pass fruits and pageTitle to the template
// Your code here

Hint

Use app.get('/', (req, res) => { res.render('fruits', { fruits, pageTitle }); });

4

Create the template to display the data

In your template file named fruits.ejs, add an <h1> tag that shows the pageTitle. Then add an unordered list <ul> that loops over fruits and displays each fruit's name and color inside a <li>.

Express

Hint

Use EJS syntax: <% fruits.forEach(fruit => { %> and <%= fruit.name %>

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