# Mouse events

In this chapter we'll get into more details about mouse events and their properties.

Please note: such events may come not only from "mouse devices", but are also from other devices, such as phones and tablets, where they are emulated for compatibility.

## Mouse event types

We've already seen some of these events:

`mousedown/mouseup`
: Mouse button is clicked/released over an element.

`mouseover/mouseout`
: Mouse pointer comes over/out from an element.

`mousemove`
: Every mouse move over an element triggers that event.

`click`
: Triggers after `mousedown` and then `mouseup` over the same element if the left mouse button was used.

`dblclick`
: Triggers after two clicks on the same element within a short timeframe. Rarely used nowadays.

`contextmenu`
: Triggers when the right mouse button is pressed. There are other ways to open a context menu, e.g. using a special keyboard key, it triggers in that case also, so it's not exactly the mouse event.

...There are several other events too, we'll cover them later.

## Events order

As you can see from the list above, a user action may trigger multiple events.

For instance, a left-button click first triggers `mousedown`, when the button is pressed, then `mouseup` and `click` when it's released.

In cases when a single action initiates multiple events, their order is fixed. That is, the handlers are called in the order `mousedown` -> `mouseup` -> `click`.

```online
Click the button below and you'll see the events. Try double-click too.

On the teststand below all mouse events are logged, and if there is more than a 1 second delay between them they are separated by a horizontal ruler.

Also we can see the `button` property that allows to detect the mouse button, it's explained below.

<input onmousedown="return logMouse(event)" onmouseup="return logMouse(event)" onclick="return logMouse(event)" oncontextmenu="return logMouse(event)" ondblclick="return logMouse(event)" value="Click me with the right or the left mouse button" type="button"> <input onclick="logClear('test')" value="Clear" type="button"> <form id="testform" name="testform"> <textarea style="font-size:12px;height:150px;width:360px;"></textarea></form>
```

## Mouse button

Click-related events always have the `button` property, which allows to get the exact mouse button.

We usually don't use it for `click` and `contextmenu` events, because the former happens only on left-click, and the latter -- only on right-click.

From the other hand, `mousedown` and `mouseup` handlers we may need `event.button`, because these events trigger on any button, so `button` allows to distinguish between "right-mousedown" and "left-mousedown".

The possible values of `event.button` are:

| Button state | `event.button` |
|--------------|----------------|
| Left button (primary) | 0 |
| Middle button (auxiliary) | 1 |
| Right button (secondary) | 2 |
| X1 button (back) | 3 |
| X2 button (forward) | 4 |

Most mouse devices only have the left and right buttons, so possible values are `0` or `2`. Touch devices also generate similar events when one taps on them.

Also there's `event.buttons` property that has all currently pressed buttons as an integer, one bit per button. In practice this property is very rarely used, you can find details at [MDN](mdn:/api/MouseEvent/buttons) if you ever need it.

```warn header="The outdated `event.which`"
Old code may use `event.which` property that's an old non-standard way of getting a button, with possible values:

- `event.which == 1` – left button,
- `event.which == 2` – middle button,
- `event.which == 3` – right button.

As of now, `event.which` is deprecated, we shouldn't use it.
```

## Modifiers: shift, alt, ctrl and meta

All mouse events include the information about pressed modifier keys.

Event properties:

- `shiftKey`: `key:Shift`
- `altKey`: `key:Alt` (or `key:Opt` for Mac)
- `ctrlKey`: `key:Ctrl`
- `metaKey`: `key:Cmd` for Mac

They are `true` if the corresponding key was pressed during the event.

For instance, the button below only works on `key:Alt+Shift`+click:

```html autorun height=60
<button id="button">Alt+Shift+Click on me!</button>

<script>
  button.onclick = function(event) {
*!*
    if (event.altKey && event.shiftKey) {
*/!*
      alert('Hooray!');
    }
  };
</script>
```

```warn header="Attention: on Mac it's usually `Cmd` instead of `Ctrl`"
On Windows and Linux there are modifier keys `key:Alt`, `key:Shift` and `key:Ctrl`. On Mac there's one more: `key:Cmd`, corresponding to the property `metaKey`.

In most applications, when Windows/Linux uses `key:Ctrl`, on Mac `key:Cmd` is used.

That is: where a Windows user presses `key:Ctrl+Enter` or `key:Ctrl+A`, a Mac user would press `key:Cmd+Enter` or `key:Cmd+A`, and so on.

So if we want to support combinations like `key:Ctrl`+click, then for Mac it makes sense to use `key:Cmd`+click. That's more comfortable for Mac users.

Even if we'd like to force Mac users to `key:Ctrl`+click -- that's kind of difficult. The problem is: a left-click with `key:Ctrl` is interpreted as a *right-click* on MacOS, and it generates the `contextmenu` event, not `click` like Windows/Linux.

So if we want users of all operating systems to feel comfortable, then together with `ctrlKey` we should check `metaKey`.

For JS-code it means that we should check `if (event.ctrlKey || event.metaKey)`.
```

```warn header="There are also mobile devices"
Keyboard combinations are good as an addition to the workflow. So that if the visitor uses a keyboard -- they work. 

But if their device doesn't have it -- then there should be a way to live without modifier keys.
```

## Coordinates: clientX/Y, pageX/Y

All mouse events provide coordinates in two flavours:

1. Window-relative: `clientX` and `clientY`.
2. Document-relative: `pageX` and `pageY`.

We already covered the difference between them in the chapter <info:coordinates>.

In short, document-relative coordinates `pageX/Y` are counted from the left-upper corner of the document, and do not change when the page is scrolled, while `clientX/Y` are counted from the current window left-upper corner. When the page is scrolled, they change.

For instance, if we have a window of the size 500x500, and the mouse is in the left-upper corner, then `clientX` and `clientY` are `0`, no matter how the page is scrolled. 

And if the mouse is in the center, then `clientX` and `clientY` are `250`, no matter what place in the document it is. They are similar to `position:fixed` in that aspect.

````online
Move the mouse over the input field to see `clientX/clientY` (the example is in the `iframe`, so coordinates are relative to that `iframe`):

```html autorun height=50
<input onmousemove="this.value=event.clientX+':'+event.clientY" value="Mouse over me">
```
````

## Preventing selection on mousedown

Double mouse click has a side-effect that may be disturbing in some interfaces: it selects text.

For instance, double-clicking on the text below selects it in addition to our handler:

```html autorun height=50
<span ondblclick="alert('dblclick')">Double-click me</span>
```

If one presses the left mouse button and, without releasing it, moves the mouse, that also makes the selection, often unwanted.

There are multiple ways to prevent the selection, that you can read in the chapter <info:selection-range>.

In this particular case the most reasonable way is to prevent the browser action on `mousedown`. It prevents both these selections:

```html autorun height=50
Before...
<b ondblclick="alert('Click!')" *!*onmousedown="return false"*/!*>
  Double-click me
</b>
...After
```

Now the bold element is not selected on double clicks, and pressing the left button on it won't start the selection.

Please note: the text inside it is still selectable. However, the selection should start not on the text itself, but before or after it. Usually that's fine for users.

````smart header="Preventing copying"
If we want to disable selection to protect our page content from copy-pasting, then we can use another event: `oncopy`.

```html autorun height=80 no-beautify
<div *!*oncopy="alert('Copying forbidden!');return false"*/!*>
  Dear user,
  The copying is forbidden for you.
  If you know JS or HTML, then you can get everything from the page source though.
</div>
```
If you try to copy a piece of text in the `<div>`, that won't work, because the default action `oncopy` is prevented.

Surely the user has access to HTML-source of the page, and can take the content from there, but not everyone knows how to do it.
````

## Summary

Mouse events have the following properties:

- Button: `button`.
- Modifier keys (`true` if pressed): `altKey`, `ctrlKey`, `shiftKey` and `metaKey` (Mac).
  - If you want to handle `key:Ctrl`, then don't forget Mac users, they usually use `key:Cmd`, so it's better to check `if (e.metaKey || e.ctrlKey)`.

- Window-relative coordinates: `clientX/clientY`.
- Document-relative coordinates: `pageX/pageY`.

The default browser action of `mousedown` is text selection, if it's not good for the interface, then it should be prevented.

In the next chapter we'll see more details about events that follow pointer movement and how to track element changes under it.