Countdown

index.tsx

import { FC, useState, useEffect } from 'react';

import { Count, Timer } from './styled';

interface IProps {
  initialTime: number;
}

const Countdown: FC<IProps> = ({ initialTime }) => {
  const [strokeDasharray, setStrokeDasharray] = useState('283');
  const [timer, setTimer] = useState(initialTime);

  let timePassed = 0;
  let timeLeft = initialTime;
  let timerInterval = null;

  const startTimer = () => {
    timerInterval = setInterval(() => {
      timePassed = timePassed += 1;
      timeLeft = initialTime - timePassed;

      setTimer(formatTime(timeLeft));
      setCircleDasharray();

      if (timeLeft === 0) onTimesUp();
    }, 1000);
  };

  const onTimesUp = () => {
    clearInterval(timerInterval);
  };

  useEffect(() => {
    startTimer();
  }, []);

  const formatTime = time => {
    return time % 60;
  };

  const calculateTimeFraction = () => {
    const rawTimeFraction = timeLeft / initialTime;

    return rawTimeFraction - (1 / 60) * (1 - rawTimeFraction);
  };

  const setCircleDasharray = () => {
    const circleDasharray = `${(calculateTimeFraction() * 283).toFixed(0)} 283`;

    setStrokeDasharray(circleDasharray);
  };

  return (
    <Count>
      <svg
        className="svg"
        viewBox="0 0 100 100"
        xmlns="http://www.w3.org/2000/svg"
      >
        <g className="circle">
          <circle className="path-elapsed" cx="50" cy="50" r="45" />
          <path
            strokeDasharray={strokeDasharray}
            className="path-remaining"
            d="
              M 50, 50
              m -45, 0
              a 45,45 0 1,0 90,0
              a 45,45 0 1,0 -90,0
            "
          />
        </g>
      </svg>

      <Timer>{timer}</Timer>
    </Count>
  );
};

export default Countdown;

styled.tsx

import styled from 'styled-components';

export const Count = styled.div`
  position: relative;
  width: 30rem;
  height: 30rem;

  .svg {
    transform: scaleX(-1);
  }

  .circle {
    fill: none;
    stroke: none;
  }

  .path-elapsed {
    stroke-width: 0.7rem;
    stroke: grey;
  }

  .path-remaining {
    stroke-width: 0.7rem;
    stroke-linecap: round;
    transform: rotate(90deg);
    transform-origin: center;
    transition: 1s linear all;
    fill-rule: nonzero;
    stroke: currentColor;
    color: green;
  }
`;

export const Timer = styled.span`
  position: absolute;
  width: 30rem;
  height: 30rem;
  top: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 4.8rem;
`;