如何让 JQuery 返回有效的 img src？

Question

So, I have images in a div that share the same class and one of them is selected at a time for a single view of that image. In this single view, I have arrows that I want to use to cycle between the images. The images sharing the same class have sequential alt values.

Right now, I can get the preceding alt value and set the src of the image in the single view to the source of that image. But, I can't seem to get the same effect with the following image.

    $("#leftButton").click(function(){
            var current= $('img[id="selected"]');
            var altVal=current.attr("alt").valueOf()-1;
            if(altVal>=0)
            {
                var next= $('img[alt='+altVal+']');
                current.attr("id", "");
                next.attr("id", "selected");
                $("#embiggenImage").attr("src", next.attr("src"));
            }
        }
    );

    $("#rightButton").click(function(){
            var gal=$('img[class="galleryImage"]');
            alert(gal.length);
            var current= $('img[id="selected"]');
            var altVal=current.attr("alt").valueOf()+1;
            if(altVal<gal.length)
            {
                alert("inside");
                var next= $('img[alt='+altVal+']');
                current.attr("id", "");
                next.attr("id", "selected");
                $("#embiggenImage").attr("src", next.attr("src"));
            }
        }
    );

As you can see, the code for changing the sources is exactly the same, and it reaches those two alerts fine, but returns a null source.

Edit: It seems that in the leftButton click function, alt is being properly decremented, but in the rightButton click function, when I am trying to increment alt, it instead, is appending the '1', such that the final value of altVal is '01'. This does not make any sense to me..

Answer 1

The problem is in the difference between these two lines:

var altVal=current.attr("alt").valueOf()-1;
var altVal=current.attr("alt").valueOf()+1;

- only has one meaning: subtract. If you call it on a string, it will be converted into a number before the operation.

+ has two meanings: add and concatenate. If you call it on a number, it adds. If you call it on a string, it concatenates. attr returns a string (as does valueOf -- I'm not sure what the point of that is), so +1 means "put add 1 to the end of the string".

You need to convert the value to a number instead. I'd use the unary + operator :

var altVal = +current.attr('alt') + 1;

Answer 2

If the final value in the rightButton click event for "alt" is "01", try using parseInt() before adding to it, like so:

parseInt(current.attr("alt").valueOf())+1;