Fire event on boolean change (JavaScript)

Question

I am creating an app where users earn trophies based on input. I want to alert the user when they earn a trophy. I'm uncertain how to write:

var trophy = false
if (userInput) {
    trophy = true
}

// alert user when boolean changes from false to true after userInput

All the solutions I come up with alert the user when the boolean is true (which is every instance after the change), but I just want to alert once on change. The boolean will only ever flip once; once it's true, it never goes back to false.

Thanks!

See this thread: http://stackoverflow.com/questions/1029241/object-watch-for-all-browsers — codechurn, Apr 13 '17 at 19:36
Possible duplicate of [Object.watch() for all browsers?](http://stackoverflow.com/questions/1029241/object-watch-for-all-browsers) — codechurn, Apr 13 '17 at 19:36

score 0 · Accepted Answer · answered Apr 13 '17 at 19:47

0

You can check the value just before you change it:

var trophy = false
if (userInput) {
    if (!trophy) {
        // alert user here
    }
    trophy = true
}

answered Apr 13 '17 at 19:47

thedude

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This is exactly what I was looking for. Thank you! – mkhira2 Apr 13 '17 at 20:12

Fire event on boolean change (JavaScript)

1 Answers1