Template partials and layouts in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a layout in Express template engines?

A layout is a main template that defines the common structure of your web pages, like header and footer, so you don't repeat the same code in every page.

beginner

What are partials in Express templates?

Partials are small reusable template pieces, like a navigation bar or footer, that you can include in multiple pages to keep your code DRY (Don't Repeat Yourself).

beginner

How do you include a partial in an EJS template?

You use the syntax <%- include('partialName') %> to insert the partial template named 'partialName' into your current template.

beginner

Why use layouts and partials in Express apps?

They help keep your templates organized, avoid repeating code, and make it easier to update common parts like headers or footers in one place.

intermediate

How does Express know which layout to use when rendering a page?

You can set a default layout in your template engine setup or specify a layout file when rendering a view, so Express wraps your page content inside that layout.

What is the main purpose of a layout in Express templates?

A To define a common page structure for multiple views

B To store user data

C To handle server requests

D To style the page with CSS

Layouts provide a shared structure like headers and footers for multiple pages.

Which syntax is used to include a partial in an EJS template?

A <partial src='partialName' />

B {{> partialName }}

C {% include 'partialName' %}

D <%- include('partialName') %>

EJS uses <%- include('partialName') %> to insert partial templates.

Why are partials useful in template design?

A They replace CSS files

B They improve server speed

C They allow reusing small template parts across pages

D They store user sessions

Partials help reuse common template pieces like navbars or footers.

How can you specify a layout when rendering a view in Express?

A By using a CSS file named layout.css

B By passing a layout option in res.render

C By naming the file layout.html

D By setting a global variable in JavaScript

You can specify the layout file by passing { layout: 'layoutName' } in res.render.

Which of these is NOT a benefit of using layouts and partials?

A Automatically improving database queries

B Making templates easier to maintain

C Reducing repeated code

D Organizing common page elements

Layouts and partials help with templates, not database queries.

Explain how layouts and partials work together in an Express app to build web pages.

Think about how a website header and footer appear on many pages.

Describe the steps to include a navigation bar partial in an EJS template using Express.

Focus on file creation and the include syntax.

Practice

1. What is the main purpose of using template partials in Express apps?

easy

A. To define the main layout structure for all pages

B. To reuse small template parts like headers or footers across pages

C. To write JavaScript logic inside templates

D. To connect the app to a database

Solution

1. Step 1: Understand what partials are

   Partials are small pieces of templates like headers or footers that you can reuse.

2. Step 2: Differentiate partials from layouts

   Layouts provide the main page structure, while partials are smaller reusable parts inside layouts or pages.

3. Final Answer:

   To reuse small template parts like headers or footers across pages -> Option B

4. Quick Check:

   Partials = reusable small template parts [OK]

Hint: Partials = small reusable pieces like header/footer [OK]

Common Mistakes:

• Confusing partials with layouts
• Thinking partials define full page structure
• Assuming partials handle backend logic

2. Which syntax correctly includes a partial named header in an EJS template?

easy

A. <%- include('header') %>

B. <%= partial('header') %>

C. <% include header %>

D. <@include header>

Solution

1. Step 1: Recall EJS partial include syntax

   EJS uses <%- include('partialName') %> to include partials with unescaped output.

2. Step 2: Check each option

   <%- include('header') %> matches the correct syntax with parentheses and quotes around 'header'.

3. Final Answer:

   <%- include('header') %> -> Option A

4. Quick Check:

   EJS partial include = <%- include('name') %> [OK]

Hint: EJS partials use <%- include('name') %> syntax [OK]

Common Mistakes:

• Omitting quotes or parentheses in include
• Using <%= instead of <%- for unescaped output
• Confusing syntax with other template engines

3. Given this Express setup using EJS with a layout:

app.set('view engine', 'ejs');

// In route handler:
res.render('index', { title: 'Home' });

And the layout layout.ejs contains:

<html>
<head><title><%= title %></title></head>
<body>
<%- body %>
</body>
</html>

If index.ejs has <h1>Welcome</h1>, what will the rendered HTML look like?

medium

A. Welcome

B. Home<h1>Welcome</h1>

C. HomeWelcome

D. Error: body is undefined

Solution

1. Step 1: Understand layout and body insertion

   The layout uses <%- body %> to insert the rendered content of index.ejs unescaped.

2. Step 2: Check variable substitution

   The title variable is passed as 'Home', so <%= title %> becomes 'Home'. The body inserts <h1>Welcome</h1> as HTML.

3. Final Answer:

   <html><head><title>Home</title></head><body><h1>Welcome</h1></body></html> -> Option C

4. Quick Check:

   Layout inserts title and body correctly [OK]

Hint: Layout inserts body unescaped with <%- body %> [OK]

Common Mistakes:

• Using escaped output <%= body %> causing HTML to show as text
• Not passing title variable to template
• Assuming body variable is automatic without layout support

4. You try to use a layout in Express with EJS but get an error: ReferenceError: body is not defined. What is the likely cause?

medium

A. You forgot to use a layout engine or middleware that injects the body content

B. You used <%= body %> instead of <%- body %>

C. You named your layout file incorrectly

D. You did not install EJS package

Solution

1. Step 1: Understand how layouts work in Express with EJS

   EJS alone does not support layouts natively; you need middleware like express-ejs-layouts to inject body content.

2. Step 2: Identify cause of ReferenceError

   If body is undefined, it means the layout engine or middleware that sets body is missing or not configured.

3. Final Answer:

   You forgot to use a layout engine or middleware that injects the body content -> Option A

4. Quick Check:

   Missing layout middleware = body undefined error [OK]

Hint: Layouts need middleware to define body variable [OK]

Common Mistakes:

• Assuming EJS supports layouts without extra setup
• Confusing escaped vs unescaped output errors
• Ignoring middleware installation or configuration

5. You want to create a consistent site with a header, footer, and main content area using Express and EJS. Which approach best uses partials and layouts together?

hard

A. Use only partials for header, footer, and content without any layout file

B. Include header and footer partials directly in every page template without a layout

C. Write all HTML in each page template without partials or layouts

D. Create a layout.ejs with header and footer partials included, and insert page content in a <%- body %> area

Solution

1. Step 1: Understand roles of layouts and partials

   Layouts provide the main page structure; partials are reusable pieces like header and footer.

2. Step 2: Combine layout and partials for best practice

   Put header and footer partials inside the layout, then insert page-specific content in the body area.

3. Final Answer:

   Create a layout.ejs with header and footer partials included, and insert page content in a <%- body %> area -> Option D

4. Quick Check:

   Layout + partials = clean, consistent site structure [OK]

Hint: Use layout for structure, partials for reusable parts [OK]

Common Mistakes:

• Duplicating header/footer in every page instead of partials
• Skipping layout causing inconsistent page structure
• Mixing content and layout roles improperly