Passing data to templates in Express

Introduction

Passing data to templates lets you show dynamic content on web pages. It helps you send information from your server to the page users see.

You want to show a user's name on a welcome page after login.

You need to display a list of products fetched from a database.

You want to show error messages or success notifications after a form submission.

You want to customize page content based on user preferences.

You want to display the current date or time on a page.

Syntax

Express

res.render('templateName', { key1: value1, key2: value2 })

The first argument is the template file name without extension.

The second argument is an object with data you want to use inside the template.

Examples

This sends a title variable with value 'Welcome Home' to the 'home' template.

Express

res.render('home', { title: 'Welcome Home' })

This sends username and age to the 'profile' template for dynamic display.

Express

res.render('profile', { username: 'Alice', age: 30 })

This sends an error message to the 'error' template to show to the user.

Express

res.render('error', { message: 'Page not found' })

Sample Program

This Express app sends a user object to the 'index' template. The template uses the data to greet the user by name and show their age.

Express

import express from 'express';
const app = express();
app.set('view engine', 'ejs');

app.get('/', (req, res) => {
  const user = { name: 'Sam', age: 25 };
  res.render('index', { user });
});

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

// index.ejs template content:
// <h1>Hello, <%= user.name %>!</h1>
// <p>You are <%= user.age %> years old.</p>

Important Notes

Make sure your template engine (like EJS) is installed and set up correctly.

Data keys become variables inside the template for easy use.

Passing complex objects is allowed, not just simple strings or numbers.

Summary

Use res.render() to send data to templates in Express.

Pass an object with keys and values as the second argument.

Templates use these keys as variables to show dynamic content.

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