Croner

// Run a function at the interval set by a cron expression
let job = Cron('* * * * * *', () => {
    console.log('This will run every second');
});

// What date is next sunday?
let nextSunday = Cron('0 0 0 * * 7').next();
console.log(nextSunday.toLocaleDateString());

// How many days left to christmas eve?
let msLeft = Cron('59 59 23 24 DEC *').next() - new Date();
console.log(Math.floor(msLeft/1000/3600/24) + " days left to next christmas eve");

// ESM Import ...
import Cron from "croner";

// ... or CommonJS Require
const Cron = require("croner");

import Cron from "https://cdn.jsdelivr.net/gh/hexagon/croner@4/src/croner.js";

Cron("* * * * * *", () => {
    console.log("This will run every second.");
});

import { Cron } from "https://cdn.jsdelivr.net/gh/hexagon/croner@4/src/croner.js";

let scheduler : Cron = new Cron("* * * * * *", () => {
    console.log("This will run every second.");
});

<script src="https://cdn.jsdelivr.net/npm/croner@4/dist/croner.min.js"></script>

<script type="module">
    import Cron from "https://cdn.jsdelivr.net/npm/croner@4/dist/croner.min.mjs";

    // ... see usage section ...
</script>

var job = Cron("* * * * * *" , /*optional*/ { maxRuns: 1 } , /*optional*/ () => {} );

// If function is omitted in constructor, it can be scheduled later
job.schedule(() => {});        

// States
let nextRun = job.next( /*optional*/ previousRun );    // Get a Date object representing next run
let prevRun = job.previous( );    
let msToNext = job.msToNext( /*optional*/ previosRun ); // Milliseconds left to next execution
let isRunning = job.running();

// Control scheduled execution
job.pause();                
job.resume();
job.stop();

// ┌──────────────── (optional) second (0 - 59)
// │ ┌────────────── minute (0 - 59)
// │ │ ┌──────────── hour (0 - 23)
// │ │ │ ┌────────── day of month (1 - 31)
// │ │ │ │ ┌──────── month (1 - 12, JAN-DEC)
// │ │ │ │ │ ┌────── day of week (0 - 6, SUN-Mon) 
// │ │ │ │ │ │       (0 to 6 are Sunday to Saturday; 7 is Sunday, the same as 0)
// │ │ │ │ │ │
// * * * * * *

// Run a function according to pattern
Cron('0-4 */5 1,2,3 * JAN-MAR SAT', function () {
    console.log('This will run the first five seconds every fifth minute');
    console.log('of hour 1,2 and 3 every saturday in January to March.');
});

// Find next month
let nextMonth = Cron('0 0 0 1 * *').next(),
    nextSunday = Cron('0 0 0 * * 7').next(),
    nextSat29feb = Cron("0 0 0 29 2 6").next();

console.log("First day of next month: " +  nextMonth.toLocaleDateString());
console.log("Next sunday: " +  nextSunday.toLocaleDateString());
console.log("Next saturday at 29th of february: " +  nextSat29feb.toLocaleDateString());  // 2048-02-29

var job = Cron(
    '* * * * *', 
    { 
        maxRuns: Infinity, 
        startAt: "2021-11-01T00:00:00", 
        stopAt: "2021-12-01T00:00:00",
        timezone: "Europe/Stockholm"
    },
    function() {
        console.log('This will run every minute, from 2021-11-01 to 2021-12-01 00:00:00 in Europe/Stockholm.');
    }
);

let job = Cron('* * * * * *', (self) => {
    console.log('This will run every second. Pause on second 10. Resume on second 15. And quit on second 20.');
    console.log('Current second: ', new Date().getSeconds());
    console.log('Previous run: ' + self.previous());
    console.log('Next run: ' + self.next());
});

Cron('10 * * * * *', {maxRuns: 1}, () => job.pause());
Cron('15 * * * * *', {maxRuns: 1}, () => job.resume());
Cron('20 * * * * *', {maxRuns: 1}, () => job.stop());