ClassNames Usage and Conventions

1. Import and Component Props Pattern

• Import classNames from 'classnames' alongside other third-party imports
• The className prop must be optional: className?: string
• Always merge the consumer's className last so it can override defaults:

className={classNames('default-classes', className)}

2. Breakpoint Order (Mobile-First)

Order classes from smallest to largest screen size — always:

base (no prefix) → xs: → s: → sm: → md: → m: → lg: → xl: → 2xl:

Check the project's tailwind.config for the actual breakpoint names — they may differ.

3. Class Organization Rules

• Group by breakpoint — each breakpoint gets its own string argument
• Order utilities logically within each group: layout → spacing → typography → colors → effects
• No trailing comma after the last classNames argument
• Breakpoint grouping controls the arguments inside classNames; it does not by itself justify extracting the result into a variable

4. Patterns

Basic Breakpoint Ordering

className={classNames(
  'base-style-1 base-style-2',
  'xs:xs-style-1 xs:xs-style-2',
  'sm:sm-style-1 sm:sm-style-2',
  'md:md-style-1 md:md-style-2',
  'lg:lg-style-1 lg:lg-style-2',
  'xl:xl-style-1 xl:xl-style-2',
  '2xl:2xl-style-1 2xl:2xl-style-2'
)}

Conditional Classes + Passed className Prop

type Props = {
  size?: ButtonSize;
  variant?: ButtonVariant;
  className?: string;
};

export const Button: React.FC<Props> = ({
  children,
  size = ButtonSize.MEDIUM,
  variant = ButtonVariant.PRIMARY,
  className,
  ...props
}) => (
  <button
    className={classNames(
      "block rounded-lg text-center transition",
      "disabled:opacity-50",
      { "px-3 py-2 text-sm-medium": size === ButtonSize.SMALL },
      { "px-4 py-3 text-md-medium": size === ButtonSize.MEDIUM },
      {
        "bg-orange-500 text-white hover:bg-orange-700":
          variant === ButtonVariant.PRIMARY,
      },
      {
        "outline outline-gray-500 hover:text-orange-500":
          variant === ButtonVariant.SECONDARY,
      },
      { "pointer-events-none": Boolean(props?.disabled) },
      className,
    )}
    type="button"
    {...props}
  >
    {children}
  </button>
);

Multiple className Props for Sub-elements

Expose separate className props for each styleable sub-element:

type Props = {
  title: ReactNode;
  children: ReactNode;
  className?: string;
  headerClassName?: string;
  contentClassName?: string;
};

export const Card: React.FC<Props> = ({
  title,
  children,
  className,
  headerClassName,
  contentClassName,
}) => (
  <div className={classNames("rounded-lg border", className)}>
    <div className={classNames("border-b p-4", headerClassName)}>{title}</div>
    <div className={classNames("p-4", contentClassName)}>{children}</div>
  </div>
);

One-Use Responsive Styles Stay Inline

Keep a class set next to the element when it is used only once. Use an inline classNames(...) call to group breakpoint-specific strings without creating a one-use variable. This preserves locality: a reader can understand the element without jumping to another declaration.

<Button
  className={classNames(
    'max-lg:hidden',
    'lg:ml-auto lg:inline-flex'
  )}
>
  Action
</Button>

Do not extract desktopActionClassName from this example merely because the classes are responsive, the call spans several lines, or a variable would shorten the JSX.

Shared Styles (Extract Reused Class Sets)

Extract a class set only when the resulting variable is referenced more than once. Reuse is the reason for the abstraction; length or breakpoint grouping alone is not. Keep one-use class sets inline even when their classNames(...) call spans several lines.

When multiple elements share the same base classes, extract to a variable:

const Buttons = () => {
  const buttonClassName = classNames(
    'flex w-full shrink-0 items-center justify-center transition-colors',
    'lg:cursor-pointer'
  );

  return (
    <div className="flex flex-col gap-3">
      <button className={classNames(buttonClassName, 'button-primary')}>
        {/* Content */}
      </button>
      <button className={classNames(buttonClassName, 'button-secondary')}>
        {/* Content */}
      </button>
    </div>
  );
};

Placement of Extracted Class Sets

Define a class variable inside the component, near its usages, when it is shared only by elements in that component. Keeping the declaration local makes ownership and relevance clear; moving trivial classNames work outside the render has no meaningful performance benefit.

Move a class set to module scope only when it is intentionally shared by multiple components or functions in the file, or when it represents a genuine file-level constant. Use constant naming such as MOBILEACTIONCLASS_NAME so the broader scope is explicit.

export const Actions: React.FC = () => {
  const mobileActionClassName = classNames('w-full', 'lg:hidden');

  return (
    <div>
      <Button className={mobileActionClassName}>Save</Button>
      <Button className={mobileActionClassName}>Cancel</Button>
    </div>
  );
};

5. Quick Checklist

• classNames imported from 'classnames'
• className?: string prop is optional
• Consumer className merged at the end
• Breakpoints ordered mobile-first (base → xs → sm → md → lg → xl → 2xl)
• Each breakpoint in its own string argument
• Utilities ordered: layout → spacing → typography → colors → effects
• No trailing comma after last argument
• One-use class sets kept inline with their element
• Extracted class sets referenced more than once
• Component-specific class variables defined inside the component near their usages
• Module-level class constants used only for genuine file-level sharing and named as constants