How to detect keyboard modifier (Ctrl or Shift) through JavaScript

key codes:

shift   16
ctrl    17
alt     18

your jQuery:

$(document).keydown(function (e) {
    if (e.keyCode == 18) {
        alert("ALT was pressed");
    }
});

JavaScript Madness: Keyboard Events

You can use the following:

document.onkeypress = function(evt) {
  evt = evt || window.event;
  etv = evt;
  switch (etv.keyCode) {
    case 16:
      // Code to do when Shift presed
      console.log('Pressed [SHIFT]');
      break;
    case 17:
      // Code to do when CTRL presed
      console.log('Pressed [CTRL]');
      break;
    case 32:
      // Code to do when ALT presed
      console.log('Pressed [ALT]');
      break;
  }
};

I needed a solution for this too, so found some stuff that worked, cleaned it up to be a lot less code, and ES6... JSFiddle link

function isCapsLock(event=window.event) {
  const code = event.charCode || event.keyCode;

  if (code > 64 && code < 91 && !event.shiftKey) {
    return true;
  }

  return false;
}

document.getElementById("text").addEventListener("keypress", event => {
  const status = document.getElementById("status");
  if (isCapsLock(event)) {
    status.innerHTML = "CapsLocks enabled";
    status.style.color = "red";
  } else {
    status.innerHTML = "CapsLocks disabled";
    status.style.color = "blue";
  }
}, false);

<input type="text" id="text" /><br>
<span id="status"></span>

This is a very old question. gilly3's answer is valid only if we have at hand an event object of type KeyboardEvent passed as a function argument. How to detect the current control key state if we have not event object available such as in this function?

function testModifierKey() {
  // have I some modifier key hold down at this running time?
}

I found the solution after a long search from https://gist.github.com/spikebrehm/3747378 of spikebrehm. his solution is tracing the modifier key state at any time using jQuery with a global variable.

The global variable window.modifierKey can be used in any circonstance without requiring event object.

function testModifierKey() {
  // have I have some modifier key hold down at this executing time?
  if(window.modifierKey) {
    console.log("Some modifier key among shift, ctrl, alt key is currently down.");
    // do something at this condition... for example, delete item without confirmation.
  } else {
    console.log("No modifier key is currently down.");
    // do something at other condition... for example, delete this item from shopping cart with confirmation.
  }
}

Here is his script to load in your HTML document:

// source: https://gist.github.com/spikebrehm/3747378
// modifierKey used to check if cmd+click, shift+click, etc. 
!function($, global){
  var $doc = $(document);
  var keys;

  global.modifierKey = false;

   global.keys = keys = {
      'UP': 38,
      'DOWN': 40,
      'LEFT': 37,
      'RIGHT': 39,
      'RETURN': 13,
      'ESCAPE': 27,
      'BACKSPACE': 8,
      'SPACE': 32
  };

  // borrowed from Galleria.js
  var keyboard = {
    map: {},
    bound: false,

    press: function(e) {
      var key = e.keyCode || e.which;
      if ( key in keyboard.map && typeof keyboard.map[key] === 'function' ) {
        keyboard.map[key].call(self, e);
      }
    },

    attach: function(map){
      var key, up;

      for(key in map) {
        if (map.hasOwnProperty(key)) {
          up = key.toUpperCase();
          if (up in keyboard.keys) {
            keyboard.map[keyboard.keys[up]] = map[key];
          } else {
            keyboard.map[up] = map[key];
          }
        }
      }
      if (!keyboard.bound) {
        keyboard.bound = true;
        $doc.bind('keydown', keyboard.press);
      }
    },

    detach: function() {
      keyboard.bound = false;
      keyboard.map = {};
      $doc.unbind('keydown', keyboard.press);
    }
  };

  $doc.keydown(function(e) {
    var key = e.keyCode || e.which;
    if (key === 16 || key === 91 || key === 18 || key === 17) {
      modifierKey = true;
    } else {
      modifierKey = false;
    }
  });

  $doc.keyup(function(e) {
    modifierKey = false;
  });
}(jQuery, window);