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javascript

An expression in a function is not evaluating to true when it should. How can I fix this?

发布于 2020-03-27 10:32:18

I am making a function to wait until a condition is true, then execute a function. The expression inputted is a == 5. a starts equal to 0, and every time the function is executed it goes up by 1. when it is equal to five, the function still evaluates to false.

my .js file:

var _ = {
    waitUntil: function(duringExecution, expression, onCompletion) {
        test();
        function test(){
            console.log(a + ' ' + expression);
            if((expression) == true) {
                onCompletion();
            } else {
                duringExecution();
                setTimeout(test, 200);
            }
        }
    }
}

a = 0;
_.waitUntil(function() {
    a++;
}, a == 5, function() {
    console.log('yay');
});

My HTML file has nothing in it, except for the script tag.

<!DOCTYPE html>
<html>
    <head>
        <title>example lul</title>
    </head>
    <body>
        <script src='under.js'></script>
    </body>
</html>

Every time the function is run, it prints the value of a and the results of the expression. The console just prints:

1 false
2 false
3 false
4 false
5 false
6 false
7 false
...

It just goes forever. When a == 5, it should print a true and stop, but it doesn't.

Questioner
Daniel Reynolds
Viewed
44
分享
Maheer Ali 2019-07-04 00:05

Whenever you write an expression its always evaluated to its value. For example when you pass a == 5 to function it return false and false is passed to the function.

You need to wrap a == 5 in a function. And call it when you want to check the condition.

var _ = {
    waitUntil: function(duringExecution, expression, onCompletion) {
        test();
        function test(){
            console.log(a + ' ' + expression());
            if(expression() == true) {
                onCompletion();
            } else {
                duringExecution();
                setTimeout(test, 500);
            }
        }
    }
}

a = 0;
_.waitUntil(function() {
    a++;
},function(){ 
    return a == 5
}, function() {
    console.log('yay');
});
Daniel Reynolds 2019-07-04 00:05:20

Thank you! This solves my problem, but it would be nice if you didn't have to. Is there any way to have _.waitUntil turn a == 5 into a function or not? (I don't know what I'm really talking about so please let me know if i'm wrong.)

Daniel Reynolds 2019-07-04 00:06:20

@MarkC.That is strange. for me it seems to work fine.

Maheer Ali 2019-07-04 00:06:21

@MarkC. Sorry Its fixed now. I was missing return

Maheer Ali 2019-07-04 00:09:32

@DanielReynolds You will need to wrap a == 5 in a function. If you want to pass a == 5 only and change it to function dynamically inside _.waitUntil then its not possible.

Daniel Reynolds 2019-07-04 00:10:43

@MarkC. ok. Thanks!

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