Parses text as time in JavaScript

parseTime.js

function parseTime(s, pmByDefault) {
    const match = (s || '').toLowerCase().match(/^[^0-9]*([0-9]+)([^0-9]*([0-9]*)(.*))$/);
    if (!match) {
        return null;
    }
  
    let mins = 0;
    let hours = 0;

    if (match[3]) { // strings like '12:34'
        hours = parseInt(match[1].substring(0, 2));
        mins = parseInt(match[3].substring(0, 2));
    } else { // only one number specified
        hours = parseInt(match[1].substring(0, 4));
        if (hours > 99) { // convert 1234 to 12:34, 123 to 1:23
            mins = hours % 100;
            hours = ~~(hours / 100);
        }
    }
  
    if (hours === 24 && !mins) {
        hours = 0; // allow 24:00
    } else if (hours > 23) {
        hours = 23;
    } else if (hours >= 1 && hours <= 12) {
      
       const amPm = (match[3] ? match[4] : match[2])?.match(/[a-z]/)?.[0] || '';
      
       // converts from pm to 24 h format by adding 12. If number is 12 do nothing.
       // 12:00 pm in 12h format to => 12:00 in 24 h format.
       if (amPm === 'p' && hours != 12) {
           hours += 12;
       }
       // converts from 12:00 am in 12h format to => 00:00 in 24 h format.
       else if (amPm === 'a' && hours == 12) {
          hours -= 12;
       }
       // force p.m. in end-time input when start-time is after noon
       else if (amPm !== 'a' && pmByDefault && (hours < 12 || !mins)) {
          hours += 12;
       }
    }

    if (mins > 59) {
        mins = 59;
    }

    return hours + ':' + (mins < 10 ? '0' : '') + mins
}
  
const expect = actual => ({
    toBe: expected => {
        if (actual !== expected) {
            console.error(actual + ' should be ' + expected)
        }
    }
});
    

expect(parseTime('10am')).toBe('10:00');
expect(parseTime('12pm')).toBe('12:00')
expect(parseTime('12 am')).toBe('0:00')
expect(parseTime('1 p.m.')).toBe('13:00')
expect(parseTime('1:3 p.m.')).toBe('13:03')
expect(parseTime('1:30 p.m.')).toBe('13:30')
expect(parseTime('11:59 am')).toBe('11:59')
expect(parseTime('11:59 pm')).toBe('23:59')
expect(parseTime('xxx 12 30 a.m')).toBe('0:30')
expect(parseTime('1234am')).toBe('0:34')
expect(parseTime('1234')).toBe('12:34')
expect(parseTime('1234:5647')).toBe('12:56')
expect(parseTime('123456')).toBe('12:34')
expect(parseTime('9:2030')).toBe('9:20')
expect(parseTime('10-30 a..m....')).toBe('10:30')
expect(parseTime('10-30 p..m....')).toBe('22:30')
expect(parseTime('10p')).toBe('22:00')
expect(parseTime('10:')).toBe('10:00')
expect(parseTime('')).toBe(null)
expect(parseTime('invalid time')).toBe(null)
expect(parseTime('0')).toBe('0:00')
expect(parseTime('5')).toBe('5:00')
expect(parseTime('05')).toBe('5:00')
expect(parseTime('15')).toBe('15:00')
expect(parseTime('15 am')).toBe('15:00')
expect(parseTime('0 5')).toBe('0:05')
expect(parseTime('time is 5:15')).toBe('5:15')
expect(parseTime('24:00')).toBe('0:00')
expect(parseTime('24:01')).toBe('23:01')
expect(parseTime('23:58')).toBe('23:58')
expect(parseTime('25:99')).toBe('23:59')
expect(parseTime('99:999')).toBe('23:59')
expect(parseTime('999:999')).toBe('23:59')
expect(parseTime('123')).toBe('1:23')
expect(parseTime('1234')).toBe('12:34')
expect(parseTime('1230 p')).toBe('12:30')
expect(parseTime('1 at 30 pm')).toBe('13:30')
expect(parseTime('1 at 30')).toBe('1:30')
expect(parseTime('1 p 30')).toBe('1:30')
expect(parseTime('1230 a')).toBe('0:30')
expect(parseTime('9:20a', true)).toBe('9:20')
expect(parseTime('9:20', true)).toBe('21:20')
expect(parseTime('time is 10:20:30.40 post meridiem')).toBe('22:20')

console.log('Tests finished')