Center a popup window on screen?

Question

How can we center a popup window opened via javascript window.open function on the center of screen variable to the currently selected screen resolution ?

Answer 1

SINGLE/DUAL MONITOR FUNCTION (credit to http://www.xtf.dk - thank you!)

UPDATE: It will also work on windows that aren't maxed out to the screen's width and height now thanks to @Frost!

If you're on dual monitor, the window will center horizontally, but not vertically... use this function to account for that.

const popupCenter = ({url, title, w, h}) => {
    // Fixes dual-screen position                             Most browsers      Firefox
    const dualScreenLeft = window.screenLeft !==  undefined ? window.screenLeft : window.screenX;
    const dualScreenTop = window.screenTop !==  undefined   ? window.screenTop  : window.screenY;

    const width = window.innerWidth ? window.innerWidth : document.documentElement.clientWidth ? document.documentElement.clientWidth : screen.width;
    const height = window.innerHeight ? window.innerHeight : document.documentElement.clientHeight ? document.documentElement.clientHeight : screen.height;

    const systemZoom = width / window.screen.availWidth;
    const left = (width - w) / 2 / systemZoom + dualScreenLeft
    const top = (height - h) / 2 / systemZoom + dualScreenTop
    const newWindow = window.open(url, title, 
      `
      scrollbars=yes,
      width=${w / systemZoom}, 
      height=${h / systemZoom}, 
      top=${top}, 
      left=${left}
      `
    )

    if (window.focus) newWindow.focus();
}

Usage Example:

popupCenter({url: 'http://www.xtf.dk', title: 'xtf', w: 900, h: 500});  

CREDIT GOES TO: http://www.xtf.dk/2011/08/center-new-popup-window-even-on.html (I wanted to just link out to this page but just in case this website goes down the code is here on SO, cheers!)