Responsive images in NextJS - Step-by-Step Execution

Concept Flow - Responsive images

Start: Define Image Component

↓

Set src, width, height, sizes

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Next.js Image optimizes image

↓

Browser loads image

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Browser checks viewport size

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Browser selects best image size

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Image displays responsively

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End

This flow shows how Next.js Image component uses provided sizes and dimensions to serve the best image size for the user's screen, making images responsive.

Execution Sample

NextJS

import Image from 'next/image'

export default function Example() {
  return <Image src="/photo.jpg" width={800} height={600} sizes="(max-width: 600px) 100vw, 800px" alt="Sample" />
}

This code renders a responsive image that adjusts size based on screen width using Next.js Image component.

Execution Table

Step	Action	Input/State	Result/Output
1	Component renders	src='/photo.jpg', width=800, height=600, sizes='(max-width: 600px) 100vw, 800px'	Next.js prepares image with multiple sizes
2	Browser loads page	Viewport width unknown yet	Image placeholder reserved with correct aspect ratio
3	Browser evaluates viewport width	Viewport width = 500px (example)	Matches '(max-width: 600px) 100vw' condition
4	Browser selects image size	sizes='100vw' means 100% viewport width	Loads image sized to 500px width
5	Image displays	Image loaded at 500px width	Image fits screen responsively
6	Resize browser to 700px	Viewport width = 700px	Matches fallback '800px' size
7	Browser selects image size	sizes='800px'	Loads image sized to 800px width
8	Image displays	Image loaded at 800px width	Image fits larger screen
9	End	All steps complete	Responsive image behavior achieved

💡 Execution stops after image loads and displays responsively for current viewport size.

Variable Tracker

Variable	Start	After Step 3	After Step 6	Final
viewportWidth	unknown	500	700	700
selectedImageWidth	none	500	800	800
imageDisplayed	no	yes (500px wide)	yes (800px wide)	yes (800px wide)

Key Moments - 3 Insights

Why does the image load at 500px width when the viewport is 500px?

Because the sizes attribute says for max-width 600px use 100vw (100% viewport width), so the browser picks an image sized to 500px to fit exactly.

What happens when the viewport width grows larger than 600px?

The browser uses the fallback size '800px' from sizes attribute and loads a larger image to fit the bigger screen.

Why do we provide width and height props if the image size changes?

Width and height define the original image's aspect ratio so Next.js can reserve space and avoid layout shifts while loading responsive sizes.

Visual Quiz - 3 Questions

Look at the execution table, what is the selectedImageWidth at step 4?

A 600

B 500

C 800

D 1000

💡 Hint

Check the 'selectedImageWidth' value after step 3 and step 4 in the execution_table.

At which step does the viewport width change to 700px?

A Step 6

B Step 3

C Step 2

D Step 8

💡 Hint

Look for the step where viewportWidth changes from 500 to 700 in variable_tracker and execution_table.

If the sizes attribute was '(max-width: 400px) 100vw, 600px', what image width would load at viewport 500px?

A 500px

B 400px

C 600px

D 800px

💡 Hint

Check how sizes fallback works when viewport is larger than max-width condition in execution_table.

Concept Snapshot

Next.js Responsive Images
- Use <Image> with src, width, height, sizes props
- sizes tells browser which image width to pick based on viewport
- Browser picks best image size for screen
- Width/height reserve space to prevent layout shift
- Responsive images improve load speed and look good on all devices

Full Transcript

This visual execution shows how Next.js Image component handles responsive images. The component is given a source image, fixed width and height, and a sizes attribute that tells the browser how wide the image should be at different screen widths. When the page loads, the browser checks the viewport width and uses the sizes attribute to pick the best image size to load. For example, if the viewport is 500 pixels wide and sizes says to use 100vw for max-width 600px, the browser loads a 500 pixel wide image. If the viewport grows to 700 pixels, the browser uses the fallback size of 800 pixels. The width and height props help Next.js reserve space to avoid layout shifts while the image loads. This process ensures images look sharp and load efficiently on all screen sizes.