Check if a key is down?

问题:

Is there a way to detect if a key is currently down in JavaScript?

I know about the "keydown" event, but that's not what I need. Some time AFTER the key is pressed, I want to be able to detect if it is still pressed down.

P. S. The biggest issue seems to be that after some period of time the key begins to repeat, firing off keydown and keyup events like a fiend. Hopefully there is just a simple isKeyDown(key) function, but if not then this issue will need to be overcome / worked around.

回答1:

Is there a way to detect if a key is currently down in JavaScript?

Nope. The only possibility is monitoring each keyup and keydown and remembering.

after some period of time the key begins to repeat, firing off keydown and keyup events like a fiend.

It shouldn't. You'll definitely get keypress repeating, and in many browsers you'll also get repeated keydown, but if keyup repeats, it's a bug.

Unfortunately it is not a completely unheard-of bug: on Linux, Chromium, and Firefox (when it is being run under GTK+, which it is in popular distros such as Ubuntu) both generate repeating keyup-keypress-keydown sequences for held keys, which are impossible to distinguish from someone hammering the key really fast.

回答2:

In addition to using keyup and keydown listeners to track when is key goes down and back up, there are actually some properties that tell you if certain keys are down.

window.onmousemove = function (e) {
  if (!e) e = window.event;
  if (e.shiftKey) {/*shift is down*/}
  if (e.altKey) {/*alt is down*/}
  if (e.ctrlKey) {/*ctrl is down*/}
  if (e.metaKey) {/*cmd is down*/}
}

This are available on all browser generated event objects, such as those from keydown, keyup, and keypress, so you don't have to use mousemove.

I tried generating my own event objects with document.createEvent('KeyboardEvent') and document.createEvent('KeyboardEvent') and looking for e.shiftKey and such, but I had no luck.

I'm using Chrome 17 on Mac

回答3:

My solution:

var keys = {};
window.onkeyup = function(e) { keys[e.keyCode] = false; }
window.onkeydown = function(e) { keys[e.keyCode] = true; }

I can now check if any key is pressed anywhere else in the script by checking

keys["code of the key"]

If it's true, the key is pressed.

回答4:

I don't believe there is anything like an isKeyDown function, but you could write your own.

Basically, create an array whose length is the number of keys you want to monitor. Then using the documents/pages/controls keyUp and keyDown events, update the array with that key's state.

Then write a function that checks if a certain key is down and returns a bool.

var keyEnum = { W_Key:0, A_Key:1, S_Key:2, D_Key:3 };
var keyArray = new Array(4);

function onKeyDown()
{
    // Detect which key was pressed
    if( key == 'w' )
        keyArray[keyEnum.W_Key] = true;
    // Repeat for each key you care about...
}

function onKeyUp()
{
    // Detect which key was released
    if( key == 'w' )
        keyArray[keyEnum.W_Key] = false;
    // Repeat for each key you care about...
}

function isKeyDown(key)
{
    return keyArray[key];
}

That should accomplish what you want.

回答5:

Other people have asked this kind of question before (though I don't see any obvious dupes here right now).

I think the answer is that the keydown event (and its twin keyup) are all the info you get. Repeating is wired pretty firmly into the operating system, and an application program doesn't get much of an opportunity to query the BIOS for the actual state of the key.

What you can do, and perhaps have to if you need to get this working, is to programmatically de-bounce the key. Essentially, you can evaluate keydown and keyup yourself but ignore a keyupevent if it takes place too quickly after the last keydown... or essentially, you should delay your response to keyup long enough to be sure there's not another keydown event following with something like 0.25 seconds of the keyup.

This would involve using a timer activity, and recording the millisecond times for previous events. I can't say it's a very appealing solution, but...

回答6:

/*
Tracks what keys are currently down on the keyboard
*/

function keyboard_module(onUpdate){
    var kb = {};
    var unicode_mapping = {};
    document.onkeydown = function(e){
        var unicode=e.charCode? e.charCode : e.keyCode
        var key = getKey(unicode);
        kb[key] = true;
        if(onUpdate){
            onUpdate(kb);
        }
    }

    document.onkeyup = function(e){
        var unicode=e.charCode? e.charCode : e.keyCode
        var key = getKey(unicode);
        delete kb[key];
        if(onUpdate){
            onUpdate(kb);
        }
    }

    function getKey(unicode){
        if(unicode_mapping[unicode]){
            var key = unicode_mapping[unicode];
        }else{
            var key= unicode_mapping[unicode] = String.fromCharCode(unicode);
        }
        return key;
    }
    return kb;
}

function testing(kb){
    console.log('These are the down keys', kb);
}


var keyboard = keyboard_module(testing);

....
//somewhere else in the code
if(keyboard['K']){/*do something special */}

回答7:

The following code is what I'm using:

var altKeyDownCount = 0;
window.onkeydown = function (e) {
    if (!e) e = window.event;
    if (e.altKey) {
        altKeyDownCount++;
        if (30 < altKeyDownCount) {
            $('.key').removeClass('hidden');
            altKeyDownCount = 0;
        }
        return false;
    }
}

window.onkeyup = function (e) {
    if (!e) e = window.event;
    altKeyDownCount = 0;
    $('.key').addClass('hidden');
}

When the user keeps holding down the Alt key for some time (about 2 seconds), a group of labels (class='key hidden') appears. When the Alt key is released, the labels disappear. jQuery and Bootstrap are both used.

回答8:

Look at this answer, and use onkeyup and onkeydown. Here is more specific info about those events.

回答9:

I know this is very old question, however there is a very lightweight (~.5Kb) JavaScript library that effectively "patches" the inconsistent firing of keyboard event handlers when using the DOM API.

The library is Keydrown.

Here's the operative code sample that has worked well for my purposes by just changing the key on which to set the listener:

kd.P.down(function () {
  console.log('The "P" key is being held down!');
});

kd.P.up(function () {
  console.clear();
});

// This update loop is the heartbeat of Keydrown
kd.run(function () {
  kd.tick();
});

I've incorporated Keydrown into my client-side JavaScript for a proper pause animation in a Red Light Green Light game I'm writing. You can view the entire game here. (Note: If you're reading this in the future, the game should be code complete and playable :-D!)

I hope this helps.

回答10:

Ended up here to check if there was something builtin to the browser already, but it seems there isn't. This is my solution (very similar to Robert's answer):

"use strict";

let is_key_down = (() => {
    let state = {};

    window.addEventListener('keyup', (e) => state[e.key] = false);
    window.addEventListener('keydown', (e) => state[e.key] = true);

    return (key) => state.hasOwnProperty(key) && state[key] || false;
})();

You can then check if a key is pressed with is_key_down('ArrowLeft').

回答11:

I scanned the above answers and the proposed keydown/keyup approach works only under special circumstances. If the user alt-tabs away, or uses a key gesture to open a new browser window or tab, then a keydown will be registered, which is fine, because at that point it's impossible to tell if the key is something the web app is monitoring, or is a standard browser or OS shortcut. Coming back to the browser page, it'll still think the key is held, though it was released in the meantime. Or some key is simply kept held, while the user is switching to another tab or application with the mouse, then released outside our page.

Modifier keys (Shift etc.) can be monitored via mousemove etc. assuming that there is at least one mouse interaction expected when tabbing back, which is frequently the case.

For most all other keys (except modifiers, Tab, Delete, but including Space, Enter), monitoring keypress would work for most applications - a key held down will continue to fire. There's some latency in resetting the key though, due to the periodicity of keypress firing. Basically, if keypress doesn't keep firing, then it's possible to rule out most of the keys. This, combined with the modifiers is pretty airtight, though I haven't explored what to do with Tab and Backspace.

I'm sure there's some library out there that abstracts over this DOM weakness, or maybe some DOM standard change took care of it, since it's a rather old question.

回答12:

$('#mytextbox').keydown(function (e) {
            if (e.keyCode == 13) {
                if (e.altKey) {
                    alert("alt is pressed");
                }
            }
 });

if you press alt + enter, you will see the alert.