How to trap double key press in javascript?

When the key(s) are hit, make a note of the time. Then compare it with the time you noted the last time they key(s) were hit.

If the difference is within your threshold, consider it a double. Otherwise, don't. Rough example:

var delta = 500;
var lastKeypressTime = 0;
function KeyHandler(event)
{
   if ( String.fromCharCode(event.charCode).toUpperCase()) == 'T' )
   {
      var thisKeypressTime = new Date();
      if ( thisKeypressTime - lastKeypressTime <= delta )
      {
        doDoubleKeypress();
        // optional - if we'd rather not detect a triple-press
        // as a second double-press, reset the timestamp
        thisKeypressTime = 0;
      }
      lastKeypressTime = thisKeypressTime;
   }
}

Have a variable (perhaps first_press) that you set to true when a keypress event happens, and start a timer that will reset the variable to false after a set amount of time (however fast you want them to press the keys).

In your keypress event, if that variable is true then you have a double press.

Example:

var first_press = false;
function key_press() {
    if(first_press) {
        // they have already clicked once, we have a double
        do_double_press();
        first_press = false;
    } else {
        // this is their first key press
        first_press = true;

        // if they don't click again in half a second, reset
        window.setTimeout(function() { first_press = false; }, 500);
    }
}