superset2/docs/developer_portal/guidelines/frontend/emotion-styling-guidelines.md

---
title: Emotion Styling Guidelines and Best Practices
sidebar_position: 2
---

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# Emotion Styling Guidelines and Best Practices

## Emotion Styling Guidelines

### DO these things:

- **DO** use `styled` when you want to include additional (nested) class selectors in your styles
- **DO** use `styled` components when you intend to export a styled component for re-use elsewhere
- **DO** use `css` when you want to amend/merge sets of styles compositionally
- **DO** use `css` when you're making a small, or single-use set of styles for a component
- **DO** move your style definitions from direct usage in the `css` prop to an external variable when they get long
- **DO** prefer tagged template literals (`css={css\`...\`}`) over style objects wherever possible for maximum style portability/consistency
- **DO** use `useTheme` to get theme variables

### DON'T do these things:

- **DON'T** use `styled` for small, single-use style tweaks that would be easier to read/review if they were inline
- **DON'T** export incomplete Ant Design components

## When to use `css` or `styled`

### Use `css` for:

1. **Small, single-use styles**
```tsx
import { css } from '@emotion/react';

const MyComponent = () => (
  <div
    css={css`
      margin: 8px;
      padding: 16px;
    `}
  >
    Content
  </div>
);
```

2. **Composing styles**
```tsx
const baseStyles = css`
  padding: 16px;
  border-radius: 4px;
`;

const primaryStyles = css`
  ${baseStyles}
  background-color: blue;
  color: white;
`;

const secondaryStyles = css`
  ${baseStyles}
  background-color: gray;
  color: black;
`;
```

3. **Conditional styling**
```tsx
const MyComponent = ({ isActive }: { isActive: boolean }) => (
  <div
    css={[
      baseStyles,
      isActive && activeStyles,
    ]}
  >
    Content
  </div>
);
```

### Use `styled` for:

1. **Reusable components**
```tsx
import styled from '@emotion/styled';

const StyledButton = styled.button`
  padding: 12px 24px;
  border: none;
  border-radius: 4px;
  background-color: ${({ theme }) => theme.colors.primary};
  color: white;

  &:hover {
    background-color: ${({ theme }) => theme.colors.primaryDark};
  }
`;
```

2. **Components with complex nested selectors**
```tsx
const StyledCard = styled.div`
  padding: 16px;
  border: 1px solid ${({ theme }) => theme.colors.border};

  .card-header {
    font-weight: bold;
    margin-bottom: 8px;
  }

  .card-content {
    color: ${({ theme }) => theme.colors.text};

    p {
      margin-bottom: 12px;
    }
  }
`;
```

3. **Extending Ant Design components**
```tsx
import { Button } from 'antd';
import styled from '@emotion/styled';

const CustomButton = styled(Button)`
  border-radius: 8px;
  font-weight: 600;

  &.ant-btn-primary {
    background: linear-gradient(45deg, #1890ff, #722ed1);
  }
`;
```

## Using Theme Variables

Always use theme variables for consistent styling:

```tsx
import { useTheme } from '@emotion/react';

const MyComponent = () => {
  const theme = useTheme();

  return (
    <div
      css={css`
        background-color: ${theme.colors.grayscale.light5};
        color: ${theme.colors.grayscale.dark2};
        padding: ${theme.gridUnit * 4}px;
        border-radius: ${theme.borderRadius}px;
      `}
    >
      Content
    </div>
  );
};
```

## Common Patterns

### Responsive Design
```tsx
const ResponsiveContainer = styled.div`
  display: flex;
  flex-direction: column;

  ${({ theme }) => theme.breakpoints.up('md')} {
    flex-direction: row;
  }
`;
```

### Animation
```tsx
const FadeInComponent = styled.div`
  opacity: 0;
  transition: opacity 0.3s ease-in-out;

  &.visible {
    opacity: 1;
  }
`;
```

### Conditional Props
```tsx
interface StyledDivProps {
  isHighlighted?: boolean;
  size?: 'small' | 'medium' | 'large';
}

const StyledDiv = styled.div<StyledDivProps>`
  padding: 16px;
  background-color: ${({ isHighlighted, theme }) =>
    isHighlighted ? theme.colors.primary : theme.colors.grayscale.light5};

  ${({ size }) => {
    switch (size) {
      case 'small':
        return css`font-size: 12px;`;
      case 'large':
        return css`font-size: 18px;`;
      default:
        return css`font-size: 14px;`;
    }
  }}
`;
```

## Best Practices

### 1. Use Semantic CSS Properties
```tsx
// ✅ Good - semantic property names
const Header = styled.h1`
  font-size: ${({ theme }) => theme.typography.sizes.xl};
  margin-bottom: ${({ theme }) => theme.gridUnit * 4}px;
`;

// ❌ Avoid - magic numbers
const Header = styled.h1`
  font-size: 24px;
  margin-bottom: 16px;
`;
```

### 2. Group Related Styles
```tsx
// ✅ Good - grouped styles
const Card = styled.div`
  /* Layout */
  display: flex;
  flex-direction: column;
  padding: ${({ theme }) => theme.gridUnit * 4}px;

  /* Appearance */
  background-color: ${({ theme }) => theme.colors.grayscale.light5};
  border: 1px solid ${({ theme }) => theme.colors.grayscale.light2};
  border-radius: ${({ theme }) => theme.borderRadius}px;

  /* Typography */
  font-family: ${({ theme }) => theme.typography.families.sansSerif};
  color: ${({ theme }) => theme.colors.grayscale.dark2};
`;
```

### 3. Extract Complex Styles
```tsx
// ✅ Good - extract complex styles to variables
const complexGradient = css`
  background: linear-gradient(
    135deg,
    ${({ theme }) => theme.colors.primary} 0%,
    ${({ theme }) => theme.colors.secondary} 100%
  );
`;

const GradientButton = styled.button`
  ${complexGradient}
  padding: 12px 24px;
  border: none;
  color: white;
`;
```

### 4. Avoid Deep Nesting
```tsx
// ✅ Good - shallow nesting
const Navigation = styled.nav`
  .nav-item {
    padding: 8px 16px;
  }

  .nav-link {
    color: ${({ theme }) => theme.colors.text};
    text-decoration: none;
  }
`;

// ❌ Avoid - deep nesting
const Navigation = styled.nav`
  ul {
    li {
      a {
        span {
          color: blue; /* Too nested */
        }
      }
    }
  }
`;
```

## Performance Tips

### 1. Avoid Inline Functions in CSS
```tsx
// ✅ Good - external function
const getBackgroundColor = (isActive: boolean, theme: any) =>
  isActive ? theme.colors.primary : theme.colors.secondary;

const Button = styled.button<{ isActive: boolean }>`
  background-color: ${({ isActive, theme }) => getBackgroundColor(isActive, theme)};
`;

// ❌ Avoid - inline function (creates new function on each render)
const Button = styled.button<{ isActive: boolean }>`
  background-color: ${({ isActive, theme }) =>
    isActive ? theme.colors.primary : theme.colors.secondary};
`;
```

### 2. Use CSS Objects for Dynamic Styles
```tsx
// For highly dynamic styles, consider CSS objects
const dynamicStyles = (props: Props) => ({
  backgroundColor: props.color,
  fontSize: `${props.size}px`,
  // ... other dynamic properties
});

const DynamicComponent = (props: Props) => (
  <div css={dynamicStyles(props)}>
    Content
  </div>
);
```