Image optimization in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is image optimization in mobile apps?

Image optimization means making images smaller in file size without losing much quality. This helps apps load faster and use less data.

beginner

Why should you use resizeMode in React Native's Image component?

resizeMode controls how the image fits inside its container. Using it properly avoids stretching or pixelation, improving appearance and performance.

intermediate

What is the benefit of using Image.prefetch() in React Native?

Image.prefetch() loads images into cache before they are shown. This makes images appear faster when needed, improving user experience.

intermediate

How does using different image formats affect optimization?

Formats like WebP or HEIC offer better compression than JPEG or PNG. Using modern formats reduces file size and speeds up loading.

beginner

What is lazy loading of images?

Lazy loading means loading images only when they are about to appear on screen. This saves memory and speeds up app start.

Which React Native property helps control how an image fits its container?

A resizeMode

B imageStyle

C source

D cacheControl

resizeMode sets how the image scales or stretches inside its container.

What does Image.prefetch() do?

A Compresses the image file

B Loads image into cache before display

C Changes image format

D Removes image from memory

Prefetching loads images early to make them appear faster when needed.

Which image format is known for better compression and smaller file size?

A JPEG

B PNG

C WebP

D BMP

WebP offers better compression than JPEG or PNG, reducing file size.

What is the main advantage of lazy loading images?

A Caches all images at once

B Increases image quality

C Changes image format

D Improves app startup speed

Lazy loading delays image loading until needed, speeding up app start.

Which of these is NOT a good practice for image optimization?

A Loading all images at app start

B Using modern image formats

C Using appropriate image sizes

D Prefetching images before display

Loading all images at start wastes memory and slows app launch.

Explain how you would optimize images in a React Native app to improve performance and user experience.

Think about loading speed, memory use, and image quality.

Describe the benefits of using Image.prefetch() and lazy loading in mobile apps.

Focus on how these techniques affect app speed and resource use.

Practice

1. What is the main benefit of optimizing images in a React Native app?

easy

A. It increases the app's file size

B. It changes the app's background color

C. It adds animations to images automatically

D. It makes the app load faster and use less data

Solution

1. Step 1: Understand image optimization purpose

   Optimizing images reduces their size and improves loading speed.

2. Step 2: Identify the benefit in app performance

   Smaller images use less data and make the app faster to load.

3. Final Answer:

   It makes the app load faster and use less data -> Option D

4. Quick Check:

   Image optimization = faster app and less data [OK]

Hint: Optimized images speed up apps and save data [OK]

Common Mistakes:

• Thinking optimization changes colors
• Believing optimization adds animations
• Assuming optimization increases file size

2. Which of the following is the correct way to set an image's resize mode in React Native?

easy

A. <Image source={uri} resizeMode='zoom' />

B. <Image source={uri} resizeMode='cover' />

C. <Image source={uri} mode='cover' />

D. <Image source={uri} resize='cover' />

Solution

1. Step 1: Recall correct prop name for resizing

   The correct prop to control image resizing is resizeMode.

2. Step 2: Check valid resizeMode values

   Valid values include 'cover', 'contain', 'stretch', but not 'zoom'.

3. Final Answer:

   <Image source={uri} resizeMode='cover' /> -> Option B

4. Quick Check:

   resizeMode='cover' is correct syntax [OK]

Hint: Use resizeMode prop with valid values like 'cover' [OK]

Common Mistakes:

• Using 'resize' instead of 'resizeMode'
• Using invalid values like 'zoom'
• Using 'mode' prop which does not exist

3. What will happen if you use a very large image without resizing in a React Native app?

medium

A. The app may become slow and use more memory

B. The app will load faster because large images are better quality

C. The image will automatically resize to fit the screen

D. The app will crash immediately

Solution

1. Step 1: Understand impact of large images

   Large images consume more memory and take longer to load.

2. Step 2: Identify app behavior with large images

   This can slow down the app but usually does not crash it immediately.

3. Final Answer:

   The app may become slow and use more memory -> Option A

4. Quick Check:

   Large images = slower app and more memory use [OK]

Hint: Large images slow app and use more memory [OK]

Common Mistakes:

• Thinking large images load faster
• Assuming images auto resize always
• Believing app crashes immediately

4. You wrote this code but the image does not resize as expected:

<Image source={{uri: 'image.png'}} resizeMode='stretch' />

What is the likely mistake?

medium

A. The source prop should be a string, not an object

B. The resizeMode value 'stretch' is invalid

C. The image style is missing width and height

D. The image URI is incorrect

Solution

1. Step 1: Check image resizing requirements

   In React Native, images need explicit width and height styles to resize properly.

2. Step 2: Verify code props and values

   The source and resizeMode are correct; missing styles cause no resize effect.

3. Final Answer:

   The image style is missing width and height -> Option C

4. Quick Check:

   Missing width/height stops resizing [OK]

Hint: Always set width and height styles for images [OK]

Common Mistakes:

• Thinking resizeMode 'stretch' is invalid
• Assuming source must be string only
• Ignoring need for width and height styles

5. You want to optimize images for different screen sizes in your React Native app. Which approach is best?

hard

A. Use multiple image versions and select with require() based on screen size

B. Use only one large image and rely on resizeMode='contain'

C. Load images from the internet without resizing

D. Use SVG images only, ignoring bitmap formats

Solution

1. Step 1: Understand responsive image needs

   Different screen sizes need different image resolutions for best quality and performance.

2. Step 2: Choose best method to handle multiple sizes

   Using multiple image versions and selecting them with require() based on screen size is efficient.

3. Final Answer:

   Use multiple image versions and select with require() based on screen size -> Option A

4. Quick Check:

   Multiple versions + require() = best optimization [OK]

Hint: Select image versions by screen size with require() [OK]

Common Mistakes:

• Using one large image wastes memory
• Ignoring resizing causes poor quality or slow app
• Assuming SVG works for all images