ant-design/components/button/button-group.tsx

import * as React from 'react';
import classNames from 'classnames';
import splitObject from '../_util/splitObject';

const prefix = 'ant-btn-group-';

type ButtonSize = 'small' | 'large'

interface ButtonGroupProps {
  size?: ButtonSize;
  style?: React.CSSProperties;
  className?: string;
}

export default function ButtonGroup(props: ButtonGroupProps) {
  const [{ size, className }, others] = splitObject(props, ['size', 'className']);

  // large => lg
  // small => sm
  const sizeCls = ({
    large: 'lg',
    small: 'sm',
  })[size] || '';

  const classes = classNames({
    'ant-btn-group': true,
    [prefix + sizeCls]: sizeCls,
    [className]: className,
  });

  return <div {...others} className={classes} />;
}