How do I say 'not in' in Javascript?

Question

I have Javascript code as follows:

addEventListener("keydown", function (e) {
    keysDown[e.keyCode] = true;
}, false);
addEventListener("keyup", function (e) {
    delete keysDown[e.keyCode];
}, false);    
if (81 in keysDown){
    doThisFunction() //q
}

doThisFunction will be called when I press the q button on the keyboard. I found the line "81 in keysDown" to be quite interesting.

I was wondering if there was a way to say "if 81 is NOT in keysDown" so that I can detect a moment when q has been let go or when q is not pressed.

Thanks in advance!

`!` is "not", `in` is "in", `!(... in ...)` is "not in". Is nesting so hard? — Siguza, Jun 14 '17 at 20:11
In this particular case it looks like that you should do just `if(keysDown['81'])` and `if(!keysDown['81'])`. — JJJ, Jun 14 '17 at 20:12

score 5 · Answer 1 · answered Jun 14 '17 at 20:09

5

Just use the negation operator:

if (!(81 in keydown)) …

Notice you need the additional grouping parenthesis to get the desired operator precedence.

answered Jun 14 '17 at 20:09

Bergi

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score 1 · Answer 2 · answered Jun 14 '17 at 20:09

1

Just get the index:

if(keysDown.indexOf(81) < 0){
    //code
}

Or if it is an object:

if (!(num in keysDown)) { ... }

answered Jun 14 '17 at 20:09

A.J. Uppal

18,339
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score 1 · Answer 3 · answered Jun 14 '17 at 20:11

1

Yes you can by wrapping the whole conditional in parentheses and negating it, like so:

if(!(81 in keysDown)) {
    doThisFunction();
}

answered Jun 14 '17 at 20:11

Thomas Maddocks

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How do I say 'not in' in Javascript?

3 Answers3