sos-nextjs/components/form/decimalInput.js

import React, {useState, useRef} from 'react'
import useMeasure from 'use-measure'

import styles from './styles.module.css'

function truncateFixed(num, fixed) {
  var re = new RegExp('^-?\\d+(?:\.\\d{0,' + (fixed || -1) + '})?');
  return num.toString().match(re)[0];
}

export default function Input({label, prefix, numDecimals, error, hint, name, value, onChange, onBlur, isValid}){
  const [currentDecimals, setCurrentDecimals] = useState((numDecimals === undefined || value === undefined || value === '') ? -1 : numDecimals)
  const currentValue = value === undefined ? 0 : typeof value === 'number' ? value : typeof value === 'string' ? parseInt(value || '0', 10) : 0

  const spanRef = useRef();
  const {width} = useMeasure(spanRef);

  const updateParent = value => onChange({target: {value}})

  const onKeyDown = (ev) => {
    const {key} = ev
    switch(key) {
      case 'Backspace':
        setCurrentDecimals(dec => Math.max(dec - 1, -1))
        if(currentDecimals > 0) {
          updateParent(parseFloat(truncateFixed(currentValue, currentDecimals - 1)))
        } else {
          updateParent(Math.floor(currentValue / 10))
        }
        break;

      case '.':
        setCurrentDecimals(0)
        break;

      case '0':
      case '1':
      case '2':
      case '3':
      case '4':
      case '5':
      case '6':
      case '7':
      case '8':
      case '9':
        const digit = parseInt(key, 10)
        // If we're still in the whole numbers
        if(currentDecimals < 0) {
          updateParent(currentValue * 10 + digit)
        } else if (currentDecimals === 0) {
          // Add decimal
          setCurrentDecimals(dec => dec + 1)
          updateParent(parseFloat(currentValue.toFixed(0) + '.' + digit))
        } else if (currentDecimals < numDecimals) {
          // Add to existing decimals
          setCurrentDecimals(dec => dec + 1)
          updateParent(parseFloat(currentValue.toFixed(currentDecimals) + digit))
        } else {
          // Can we play a tick noise of some sort?
        }
        break;

      default:
        return;
    }

    ev.preventDefault();
  }

  const filledText = ((prefix + ' ') || '')
    + (Math.floor(currentValue) || '0')
    + (currentDecimals >= 0 ? '.' : '')
    + (currentDecimals > 0 ? currentValue.toFixed(currentDecimals).split('.')[1] : '')

  const remainingText =
      (currentDecimals < 0 ? '.' : '')
    + (numDecimals !== undefined && currentDecimals < numDecimals ? '0'.repeat(numDecimals - Math.max(currentDecimals, 0)) : '')

  return (
    <div className={styles.formElementContainer}>
      <label htmlFor={name}>{label}:</label>
      <div className={styles.complexInput + ((isValid && !error)?'':' ' + styles.invalid)}>
        <input style={{textIndent: width}} type="text" name={name} value="" onChange={()=>{}} onKeyDown={onKeyDown} onBlur={onBlur} />
        <div className={styles.inputDecorators}>
          <span ref={spanRef} className={styles.numFilled}>{filledText}</span>
          <span className={styles.numRemaining}>{remainingText}</span>
        </div>
      </div>
      {hint && <span className={styles.hint}>{error || (isValid ? '' : hint)}</span>}
    </div>
  )
}