totp/src/App.tsx

import React from 'react';
import {NumberInput, TextInput} from './Input';
import OTPOutput, {HashAlgorithm} from './OTPOutput';
import Select from './Select';

function App() {
  const [secret, setSecret] = React.useState('');
  const [step, setStep] = React.useState(30);
  const [offset, setOffset] = React.useState(0);
  const [digits, setDigits] = React.useState(6);
  const [algorithm, setAlgorithm] = React.useState<HashAlgorithm>('sha1');

  const validStep = step > 0;
  const validDigits = digits > 0 && digits < 10;
  const valid = validStep && validDigits;

  React.useEffect(() => {
    if (!validStep) return;
    const now = Date.now();
    setOffset(Math.floor(now / (1000 * step)));
  }, [validStep, step]);

  React.useEffect(() => {
    if (!validStep) return;
    const now = Date.now();
    const nextOffset = Math.floor(now / (1000 * step)) + 1;
    const nextUpdate = nextOffset * step * 1000;
    const timer = setTimeout(() => setOffset(nextOffset), nextUpdate - now);
    return () => clearTimeout(timer);
  }, [validStep, offset, step]);

  const onReset = React.useCallback(() => {
    setStep(30);
    setDigits(6);
    setAlgorithm('sha1');
  }, []);

  return (
    <div className="totp-app">
      <div className="totp-settings">
        <TextInput label="Secret key" value={secret} onChange={setSecret}/>
        <NumberInput label="Time step" value={step} onChange={setStep} min={1}
                     error={!validStep && 'You must enter an integer time step ≥ 1 second'}/>
        <NumberInput label="Code digits" value={digits} onChange={setDigits} min={1} max={10}
                     error={!validDigits && 'You must enter an integer digit count between 1 and 10'}/>
        <Select label="Algorithm" value={algorithm} onChange={setAlgorithm} options={{
          sha1: 'SHA-1',
          sha256: 'SHA-256',
          sha512: 'SHA-512',
        }}/>
        <button type="button" className="btn btn-secondary" onClick={onReset}>Reset</button>
      </div>
      {valid && <OTPOutput secret={secret} offset={offset} algorithm={algorithm} digits={digits}/>}
    </div>
  );
}

export default App;