Here is the one-liner:

function convertTZ(date, tzString) {
    return new Date((typeof date === "string" ? new Date(date) : date).toLocaleString("en-US", {timeZone: tzString}));   
}

// usage: Asia/Jakarta is GMT+7
convertTZ("2012/04/20 10:10:30 +0000", "Asia/Jakarta") // Tue Apr 20 2012 17:10:30 GMT+0700 (Western Indonesia Time)

// Resulting value is regular Date() object
const convertedDate = convertTZ("2012/04/20 10:10:30 +0000", "Asia/Jakarta") 
convertedDate.getHours(); // 17

// Bonus: You can also put Date object to first arg
const date = new Date()
convertTZ(date, "Asia/Jakarta") // current date-time in jakarta.

   

This is the MDN Reference.

Beware the caveat: function above works by relying on parsing toLocaleString result, which is string of a date formatted in en-US locale , e.g. "4/20/2012, 5:10:30 PM". Each browser may not accept en-US formatted date string to its Date constructor and it may return unexpected result (it may ignore daylight saving).

Currently all modern browser accept this format and calculates daylight saving correctly, it may not work on older browser and/or exotic browser.

side-note: It would be great if modern browser have toLocaleDate
function, so we don’t have to use this hacky work around.