How do I concatenate a string with a variable?

So I am trying to make a string out of a string and a passed variable(which is a number). How do I do that?

I have something like this:

function AddBorder(id){
    document.getElementById('horseThumb_'+id).className='hand positionLeft'
}

So how do I get that 'horseThumb' and an id into one string?

I tried all the various options, I also googled and besides learning that I can insert a variable in string like this getElementById("horseThumb_{$id}") <-- (didn't work for me, I don't know why) I found nothing useful. So any help would be very appreciated.

Solution 1:

Your code is correct. Perhaps your problem is that you are not passing an ID to the AddBorder function, or that an element with that ID does not exist. Or you might be running your function before the element in question is accessible through the browser's DOM.

Since ECMAScript 2015, you can also use template literals (aka template strings):

document.getElementById(`horseThumb_${id}`).className = "hand positionLeft";

To identify the first case or determine the cause of the second case, add these as the first lines inside the function:

alert('ID number: ' + id);
alert('Return value of gEBI: ' + document.getElementById('horseThumb_' + id));

That will open pop-up windows each time the function is called, with the value of id and the return value of document.getElementById. If you get undefined for the ID number pop-up, you are not passing an argument to the function. If the ID does not exist, you would get your (incorrect?) ID number in the first pop-up but get null in the second.

The third case would happen if your web page looks like this, trying to run AddBorder while the page is still loading:

<head>
<title>My Web Page</title>
<script>
    function AddBorder(id) {
        ...
    }
    AddBorder(42);    // Won't work; the page hasn't completely loaded yet!
</script>
</head>

To fix this, put all the code that uses AddBorder inside an onload event handler:

// Can only have one of these per page
window.onload = function() {
    ...
    AddBorder(42);
    ...
} 

// Or can have any number of these on a page
function doWhatever() {
   ...
   AddBorder(42);
   ...
}

if(window.addEventListener) window.addEventListener('load', doWhatever, false);
else window.attachEvent('onload', doWhatever);

Solution 2:

In javascript the "+" operator is used to add numbers or to concatenate strings. if one of the operands is a string "+" concatenates, and if it is only numbers it adds them.

example:

1+2+3 == 6
"1"+2+3 == "123"

Solution 3:

This can happen because java script allows white spaces sometimes if a string is concatenated with a number. try removing the spaces and create a string and then pass it into getElementById.

example:

var str = 'horseThumb_'+id;

str = str.replace(/^\s+|\s+$/g,"");

function AddBorder(id){

    document.getElementById(str).className='hand positionLeft'

}

Solution 4:

It's just like you did. And I'll give you a small tip for these kind of silly things: just use the browser url box to try js syntax. for example, write this: javascript:alert("test"+5) and you have your answer. The problem in your code is probably that this element does not exist in your document... maybe it's inside a form or something. You can test this too by writing in the url: javascript:alert(document.horseThumb_5) to check where your mistake is.

How do I concatenate a string with a variable?

Solution 1:

Solution 2:

Solution 3:

Solution 4:

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