Question

Wondering why I can’t get document.getElementById("my_div").innerHTML to update the DOM when I re-assign the variable. For example:

<div id="my_div" onclick="clicky();">
    Bye.
</div>
<script type="text/javascript" charset="utf-8">
    function clicky() {
        var myDivValue = document.getElementById("my_div").innerHTML;
        myDivValue = "Hello";
        console.log(myDivValue);
    }
</script>

In the log I can see the variable get re-assigned when I click, but the innerHTML of my_div remains unchanged. Why is this?

**After several years more experience…**

For those just starting out (like I was) and found yourself asking the same thing, there are two important concepts that need to be understood:

1. Assign by reference: My mistake was that I thought var myDivValue = element.innerHTML would create a reference/address to innerHTML and every time I assigned a new value to that reference, I could just update content, but that’s not how it works.
2. Assign by value: Instead, I was simply making a copy of element.innerHTML value (which was blank) and assigning the value to myDivValue, then in myDivValue = "Hello";, I was assigning a new value to myDivValue, so of course it would never update element.innerHTML.

The confusing part: when using the assignment operator (=) in JS, it’s not explicit that you’re either assigning a reference or value ( var referenceName = reference_to_thing vs. var containerName = newValue),

As many have said in the answers, the right way to do this is to assign the document.getElementById("my_div") element to myDiv:

var myDiv = document.getElementById("my_div")

And now that I have a reference to the element called myDiv, I can update the innerHTML property whenever I like:

myDiv.innerHTML = "Hello" // innerHTML is now "Hello"
myDiv.innerHTML = "Goodbye" // Changed it to "Goodbye"

Answer 1

innerHTML evaluates to a string. I’m not sure why you would expect anything different. Consider this:

var a = 'foo'; // now a = 'foo'
var b = a; // now a = 'foo', b = 'foo'
b = 'bar'; // now a = 'foo', b = 'bar'

Re-assigning b doesn’t change a.

Edited to add: In case it’s not clear from the above, if you want to change innerHTML, you can just assign to it directly:

document.getElementById("my_div").innerHTML = "Hello";

You don’t need, and can’t use, an intermediary variable.