syntax error in javascript callback demo

Question

The jsfiddle.net code below produces Uncaught SyntaxError: Unexpected token ( and the text "Here", but does not produce the result I wish to study, which will help me understand callbacks in functions and how their return value, true/false in my case, can be used to determine which of two things are to be computed.

I would like help removing the syntax error so I can see the result.

function checkPos(input, callback) {
    if (input > 0)
    function () {
        callback(true);
    } else
    function () {
        callback(false);
    }
}

function dosomething(number) {
    var diff = 10 - number;
    checkPos(diff, function (e) {
        return e;
    });
}

var count = 1;
var multiplier = 6;
var result;
if (dosomething(11)) result = 5 + count
else result = 5 - count

document.getElementById("demo").innerHTML = String(result*multiplier);

The html code is simply as follows.

<body>Here
    <p id="demo"></p>
</body>

What are these `function` keywords doing there in `checkPos`? I mean, what are they *supposed* to do there? — Bergi, Jul 20 '14 at 20:03
The `return` value of a callback function is returned to where it was *called*, not where the function was defined. Your `doSomething` won't work. — Bergi, Jul 20 '14 at 20:04
Yes, as you can see, I don't know how to fix that. Do you know how? — zerowords, Jul 20 '14 at 21:31
Give `doSomething` a callback parameter as well, which you call from the other callback. See also http://stackoverflow.com/q/14220321/1048572 and possibly http://stackoverflow.com/q/23667086/1048572. If `checkPos` is not asynchronous, remove the callback style altogether and just `return`. — Bergi, Jul 20 '14 at 21:49
You are right in suggesting to remove the callback-style at all. But you are wrong in assuming, some method has to be asynchronous to use callbacks; although it makes more sense, if it were. — Thomas Junk, Jul 20 '14 at 22:29
My purpose in asking this question was to learn to use an asynchronous callback utility that I thought I would need to use to learn when a styled map marker system is actually dropped because it is now "deprecated" by google. However, upon further research, I was able to get myself on a relevant google forum mailing list with automatic mail updates. So I think I can omit the synch utility which would have automatically checked for system failure and wait instead for an email warning. — zerowords, Jul 21 '14 at 15:51

score 3 · Accepted Answer · answered Jul 20 '14 at 19:59

3

What you wrote is

function checkPos(input, callback) {
    if (input > 0)
    function () {
        callback(true);
    } else
    function () {
        callback(false);
    }
}

That is a Syntax Error. You could not define a function in the midst of an if-statement

But you can call your callback:

function checkPos(input, callback) {
    if (input > 0)
        callback(true);
    else
        callback(false);
}

Here is the updated Fiddle

answered Jul 20 '14 at 19:59

Thomas Junk

5,588
2
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Something is apparently wrong. I always get the result 24, and never 36, which would be the result if `10-number` were positive, for example, if number were -3. What's wrong? – zerowords Jul 20 '14 at 20:42
1

Your function doSomething is wrong. Especially this line: checkPos(diff, function (e) { return e; }); – Thomas Junk Jul 20 '14 at 20:46
Right. So how do I get the true/false result back where I need it? – zerowords Jul 20 '14 at 21:01
Maybe try returning something ... return checkPos(diff, function (e) { return e; }); – Thomas Junk Jul 20 '14 at 21:05
@ThomasJunk: That's what he does, and which does obviously not work. – Bergi Jul 20 '14 at 21:47
@Bergi Where and what does the `doSomething` function return? I only see calling a function with an anonymous callback function, which returns to the scope to 'doSomething` and nothing is done. – Thomas Junk Jul 20 '14 at 22:25
Yes, that's the problem, so `return e` is not useful. And `checkPos()` doesn't return anything either. – Bergi Jul 20 '14 at 22:40
1

Oh, you pointed to the right point:return checkPos(diff, function (e) { return e; }); is one half and `checkPos` should return the result of the callback. That should do the trick. http://jsfiddle.net/LgcaX/4/ – Thomas Junk Jul 20 '14 at 22:45
I just saw this successful implementation. I could not get it before from your comment, but maybe I did it wrong. – zerowords Jul 21 '14 at 17:08

score 0 · Answer 2 · answered Jul 20 '14 at 20:01

0

You need to name your functions if they are not attached to a variable ex.

function()
//not okay
var abc=function()
//acceptable

answered Jul 20 '14 at 20:01

B_Troutman

273
1
3
4

That is not the problem here: defining an anonymous function in an if-statement. – Thomas Junk Jul 20 '14 at 20:02

score 0 · Answer 3 · edited Jun 20 '20 at 09:12

0

if/if-else statement in javascript/ecmascript has the form:

if ( Expression ) Statement else Statement

if ( Expression ) Statement

You used function expression in statement's location, which according to the grammar isn't correct and hence the error.

edited Jun 20 '20 at 09:12

Community

1
1

answered Jul 20 '14 at 20:07

soulcheck

36,297
6
91
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syntax error in javascript callback demo

3 Answers3