Create a new Date() object and call getFullYear():

new Date().getFullYear()  // returns the current year

Example usage: a page footer that always shows the current year:

document.getElementById("year").innerHTML = new Date().getFullYear();

footer {
  text-align: center;
  font-family: sans-serif;
}

<footer>
     <span id="year"></span> by Stack Overflow
</footer>

See also, the Date() constructor's full list of methods.

JavaScript doesn't natively implement strftime, so you'll have to do something a bit less elegant:

window.onload = function() {
    var months = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'];;
    var date = new Date();

    document.getElementById('date').innerHTML = months[date.getMonth()] + ' ' + date.getFullYear();
};

I'm assuming that you have an element with an id of date somewhere in your HTML.

const dateObj = new Date();
const month   = dateObj.getUTCMonth() + 1; // months from 1-12
const day     = dateObj.getUTCDate();
const year    = dateObj.getUTCFullYear();

const newDate = year + "/" + month + "/" + day;

// Using template literals:
const newDate = `${year}/${month}/${day}`;

// Using padded values, so that 2023/1/7 becomes 2023/01/07
const pMonth        = month.toString().padStart(2,"0");
const pDay          = day.toString().padStart(2,"0");
const newPaddedDate = `${year}/${pMonth}/${pDay}`;

or you can set new date and give the above values