gab-social/app/javascript/gabsocial/components/character_counter.js

import { length } from 'stringz'

/**
 * Renders a character counter
 * @param {string} props.text - text to use to measure
 * @param {number} props.max - max text allowed
 */
export default class CharacterCounter extends PureComponent {

  static propTypes = {
    text: PropTypes.string.isRequired,
    max: PropTypes.number.isRequired,
  }

  render () {
    const { text, max } = this.props

    const actualRadius = '16'
    const radius = 12
    const circumference = 2 * Math.PI * radius
    const diff = Math.min(length(text), max) / max
    const dashoffset = circumference * (1 - diff)

    return (
      <div className={[_s.default, _s.mr10, _s.justifyContentCenter, _s.alignItemsCenter].join(' ')}>
        <svg width={actualRadius * 2} height={actualRadius * 2} viewBox={`0 0 ${actualRadius * 2} ${actualRadius * 2}`}>
          <circle fill='none' cx={actualRadius} cy={actualRadius} r={radius} fill="none" stroke="#e6e6e6" strokeWidth="2" />
          <circle style={{
              strokeDashoffset: dashoffset,
              strokeDasharray: circumference,
            }}
            fill='none'
            cx={actualRadius}
            cy={actualRadius}
            r={radius}
            strokeWidth="2"
            strokeLinecap='round'
            stroke='#21cf7a'
          />
        </svg>
      </div>
    )
  }

}