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
– Bergi, Commented Jul 20, 2014 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
– Bergi, Commented Jul 20, 2014 at 20:04
Yes, as you can see, I don't know how to fix that. Do you know how? — zerowords
– zerowords, Commented Jul 20, 2014 at 21:31
Give doSomething a callback parameter as well, which you call from the other callback. See also stackoverflow.com/q/14220321/1048572 and possibly stackoverflow.com/q/23667086/1048572. If checkPos is not asynchronous, remove the callback style altogether and just return. — Bergi
– Bergi, Commented Jul 20, 2014 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
– Thomas Junk, Commented Jul 20, 2014 at 22:29

Thomas Junk · Accepted Answer · 2014-07-20 19:59:22Z

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, 2014 at 19:59

Thomas Junk

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9 Comments

zerowords Over a year ago

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?

Thomas Junk Over a year ago

Your function doSomething is wrong. Especially this line: checkPos(diff, function (e) { return e; });

zerowords Over a year ago

Right. So how do I get the true/false result back where I need it?

Thomas Junk Over a year ago

Maybe try returning something ... return checkPos(diff, function (e) { return e; });

Thomas Junk Over a year ago

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. jsfiddle.net/LgcaX/4

|

B_Troutman · Accepted Answer · 2014-07-20 20:01:06Z

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, 2014 at 20:01

B_Troutman

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1 Comment

Thomas Junk Over a year ago

That is not the problem here: defining an anonymous function in an if-statement.

Community · Accepted Answer · 2020-06-20 09:12:55Z

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, 2020 at 9:12

CommunityBot

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answered Jul 20, 2014 at 20:07

soulcheck

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