CSS-in-JS considerations in NextJS - Step-by-Step Execution

Concept Flow - CSS-in-JS considerations

Write JS component

↓

Define styles in JS

↓

Attach styles to elements

↓

Render component

↓

Styles injected into DOM

↓

Browser applies styles

↓

User sees styled UI

This flow shows how CSS-in-JS works by defining styles inside JavaScript, attaching them to elements, rendering, and then injecting styles into the DOM for the browser to apply.

Execution Sample

NextJS

import styled from 'styled-components';

const Button = styled.button`background: blue; color: white; padding: 1rem;`;

export default function App() {
  return <Button>Click me</Button>;
}

This code creates a styled button using CSS-in-JS with styled-components and renders it in a Next.js component.

Execution Table

Step	Action	Evaluation	Result
1	Import styled from styled-components	Module imported	styled function ready
2	Define Button with styles	Template literal parsed	Button component with CSS rules created
3	Render App component	JSX evaluated	Button element created with class name
4	Inject styles into DOM	CSS rules inserted in <style> tag	Styles available for browser
5	Browser applies styles	CSS rules matched to Button class	Button appears blue with white text and padding
6	User sees styled button	UI rendered	Button visible with correct styles

💡 Rendering completes and styles are applied to the button element

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
styled	undefined	function available	function available	function available	function available
Button	undefined	styled button component	component instance created	component instance created	component instance rendered
App render output	undefined	undefined	<Button /> element	<Button /> element with styles injected	<Button /> element styled and visible

Key Moments - 3 Insights

Why do styles appear in the DOM only after rendering the component?

Because CSS-in-JS libraries generate and inject styles dynamically during render, as shown in execution_table step 4.

How does the browser know which styles to apply to the button?

The styled component creates a unique class name attached to the button element, and the injected CSS targets that class, as seen in steps 3 and 5.

Can styles defined in JS affect other components unexpectedly?

No, because CSS-in-JS scopes styles to components using unique class names, preventing style leaks, demonstrated by the isolated Button styles.

Visual Quiz - 3 Questions

Look at the execution_table, at which step are the CSS rules inserted into the DOM?

A Step 4

B Step 2

C Step 5

D Step 3

💡 Hint

Check the 'Inject styles into DOM' action in execution_table step 4

According to variable_tracker, what is the state of the Button variable after step 3?

A undefined

B styled button component

C component instance created

D component instance rendered

💡 Hint

Look at the Button row under 'After Step 3' in variable_tracker

If the styles were not injected into the DOM, what would the user see?

A A blue button with white text

B A button with default browser styles

C No button at all

D An error message

💡 Hint

Refer to execution_table step 5 and 6 about browser applying styles

Concept Snapshot

CSS-in-JS lets you write CSS inside JavaScript components.
Styles are scoped to components using unique class names.
During rendering, styles are injected dynamically into the DOM.
This avoids global CSS conflicts and enables dynamic styling.
Common libraries: styled-components, emotion.
In Next.js, CSS-in-JS works seamlessly with React components.

Full Transcript

CSS-in-JS is a way to write CSS styles directly inside JavaScript components. The process starts by importing a styling function, like styled from styled-components. Then, you define a styled component by writing CSS rules inside a template literal. When the component renders, it creates an element with a unique class name. The CSS rules are injected into the DOM dynamically, so the browser can apply them. This ensures styles are scoped only to that component, preventing conflicts. The user sees the styled UI after the browser applies these styles. This approach fits well with Next.js and React, allowing dynamic and modular styling.