How to handle ESC keydown on javascript popup window

Try something like this:

$(document).keydown(function(e) {
    // ESCAPE key pressed
    if (e.keyCode == 27) {
        window.close();
    }
});

event.key === "Escape"

No more arbitrary number codes!

document.addEventListener('keydown', function(event) {
    const key = event.key; // const {key} = event; in ES6+
    if (key === "Escape") {
        window.close();
    }
});

Mozilla Docs

Supported Browsers

It is possible to achieve with JS Without using jQuery.

window.onkeydown = function( event ) {
    if ( event.keyCode == 27 ) {
        console.log( 'escape pressed' );
    }
};

Remember that you must use the function @Gumbo posted in the popup-window... So you will need to include JQuery in the popup and execute the function there, not the window that opens the popup.