Conditionally apply a ! to a method in JavaScript

Question

Not sure how better to word this (and, consequently, couldn't find previous answers, though I expect this has been answered before), but I'm interested in whether there's a way to turn code like this:

if ( this.props.mailboxFilter == 'sent' ) {
  return this.currentUser.canActOnBehalfOf( m.senderID );
} else {
  return !this.currentUser.canActOnBehalfOf( m.senderID );
}

To something like the below (not sure how better to express it):

var bangOrNot = this.props.mailboxFilter == 'sent ? '!' : '';
bangOrNot( this.currentUser.canActOnBehalfOf( m.senderID ) );

As in, is there a way to avoid the extended if/else syntax and all the repetition by choosing whether or not to call the return line with a bang?

The solution might be elsewhere in your logic. Do you really want to send the inverse of the permissions? That seems weird. — isherwood, Dec 15 '15 at 22:15

score 9 · Accepted Answer · answered Dec 15 '15 at 22:14

You could simplify it in other ways:

var response = this.currentUser.canActOnBehalfOf(m.senderID);
return this.props.mailboxFilter == 'sent' ? response : !response;

In general, you would want to avoid this method if your function changes any state. However, since in this case you are calling it regardless, there's no harm in caching its value first.

score 5 · Answer 2 · edited May 23 '17 at 11:44

5

If this.currentUser.canActOnBehalfOf( m.senderID ) allways returns a boolean value, you could do XOR.

return (this.props.mailboxFilter != 'sent') ^ this.currentUser.canActOnBehalfOf(m.senderID);

XOR a boolean value with true makes a NOT. And XOR with false makes a buffer which maintains the same value.

Note that I changed the == to !=.

And, just make sure you put comments on this part of the code if you are going to use it actually. It is not that easy to read this code after a while.

Update

As Bergi suggested, it is better to use Boolean XOR != to return a boolean value.

return (this.props.mailboxFilter != 'sent') != this.currentUser.canActOnBehalfOf(m.senderID);

And of course if it can be used with boolean XNOR as well, just replace both !=s with == in the code above.

edited May 23 '17 at 11:44

Community

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answered Dec 15 '15 at 22:20

Aᴍɪʀ

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This is the most efficient way. – TbWill4321 Dec 15 '15 at 22:21
I like this... but your future self won't! – Xophmeister Dec 15 '15 at 22:23
@Xophmeister Of course :) One can use this to maximize his job security! No one else can read this code! – Aᴍɪʀ Dec 15 '15 at 22:24
This returns a number, not a boolean. – Bergi Dec 15 '15 at 22:56
I was going to answer it this way too, until @Bergi had the obvious and elegant equivalence operator. If it was done this way with a bit-wise XOR, it just needs to be cast to a bool before being returned wrap in brackets and prepend '!!' – James Wakefield Dec 15 '15 at 23:28
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@JamesWakefield: If it was done with XOR, you just should use the boolean XOR: `!=` :-) – Bergi Dec 15 '15 at 23:29

score 3 · Answer 3 · answered Dec 15 '15 at 22:56

3

You want to use the boolean XNOR operator here - also known as equivalence:

return this.currentUser.canActOnBehalfOf(m.senderID) == (this.props.mailboxFilter == 'sent');

answered Dec 15 '15 at 22:56

Bergi

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score 0 · Answer 4 · answered Dec 15 '15 at 23:13

There are many good comments regarding why or why NOT to do such thing. I'm assuming the OP has good reason to want to apply the bang (!) in front of the result of the function call. My answer is more of a technical solution to the OP's question.

function bang(r) {
    return !r;
}
function notBang(r) {
    return r;
}
function canActOnBehalf() {
    return true;
}
var filter = 'notsent';
var f = (filter == 'sent' ? bang : notBang);
f(canActOnBehalf('whatever'));

score -4 · Answer 5 · answered Dec 15 '15 at 22:14

-4

Assuming you're using javascript, you can use the eval method to evaluate a string of javascript code. You can conditionally apply the bang to the front of the string and then return the result of eval.

answered Dec 15 '15 at 22:14

William Rosenbloom

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Why not? This is not to challenge you I'm seriously curious. – William Rosenbloom Dec 15 '15 at 22:15
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Neither comment is constructive. Here: http://stackoverflow.com/questions/86513/why-is-using-the-javascript-eval-function-a-bad-idea – isherwood Dec 15 '15 at 22:16
There's lots of articles on it from people way more knowledgeable than me. It's one of those things that's drilled in as bad practice in almost any case. @isherwood's link is good. – Lucas Dec 15 '15 at 22:17
Ok I understand there is a risk of injection but in this case he would not be allowing the user to input any of the code used in `eval`. I don't see the risk. – William Rosenbloom Dec 15 '15 at 22:17
http://archive.oreilly.com/pub/a/javascript/excerpts/javascript-good-parts/bad-parts.html – Lucas Dec 15 '15 at 22:19
I think I'll go with Xophmeister's answer, which makes sense and isn't "evil", but I really don't see what's wrong with William Rosenbloom's `eval` version, or get why it needs to be down-voted as opposed to just not used/upvoted. Just my 2 cents. It may not be a blanket "best solution", but it doesn't open any of the "bad" doors mentioned in the links pasted, apart from the slight speed issue. It's using `eval` for a pretty unique purpose. At any rate, thanks for all the thoughts/feedback. – Sasha Dec 15 '15 at 22:24

Conditionally apply a ! to a method in JavaScript

5 Answers5