What does "unexpected identifier" mean in JavaScript, and how can I fix it?

General Discussions
, ,
1

I’m trying to compress my JavaScript code for better performance and reduced traffic on my site. However, after turning my ajax function into a one-liner, I encountered an issue. Here’s the code that’s causing the error:

function(){if(xmlhttp.readyState==4&&xmlhttp.status==200){document.getElementById("content").innerHTML=xmlhttp.responseText;}}xmlhttp.open("GET","data/"+id+".html",true);xmlhttp.send();

Chrome’s console is showing an “unexpected identifier” error, while Firefox points out that there’s a missing semicolon. I can’t figure out what’s wrong. Could anyone help me understand what might be causing the unexpected identifier in JavaScript?

2

Been through this so many times. If you’re getting an unexpected identifier JavaScript error, the first thing I’d check is whether you’ve declared your function properly. Often, folks forget that a function needs a correct declaration before calling methods like xmlhttp.open and xmlhttp.send. Here’s a fix I usually go for:

(function(){
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        document.getElementById("content").innerHTML = xmlhttp.responseText;
    }
})();
xmlhttp.open("GET", "data/" + id + ".html", true);
xmlhttp.send();

This IIFE (Immediately Invoked Function Expression) structure prevents the unexpected identifier JavaScript error because it wraps the logic properly inside a function that’s executed right away.

3

Jumping in — I agree with @raimavaswani and just adding from my experience: fixing the function declaration is step one, but you might still see the unexpected identifier JavaScript error if your code structure is off. Specifically, if your xmlhttp.open and xmlhttp.send calls are placed outside or awkwardly after the function without proper flow, it can throw errors.

What I recommend (and what has saved me a ton of late-night debugging) is to move your AJAX request inside the function, like this:

(function(){
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        document.getElementById("content").innerHTML = xmlhttp.responseText;
    }
    xmlhttp.open("GET", "data/" + id + ".html", true);
    xmlhttp.send();
})();

This way, everything is contained nicely, and you seriously minimize the chances of hitting an unexpected identifier JavaScript problem due to broken execution flow.

4

Both nailed it! Just layering one more thing from my experience — even after handling declarations and structure, sometimes small things like missing semicolons cause the unexpected identifier JavaScript issue.

JavaScript has something called ASI (Automatic Semicolon Insertion), but it’s not foolproof. I always recommend adding semicolons explicitly, like so:

(function(){
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        document.getElementById("content").innerHTML = xmlhttp.responseText;
    };
    xmlhttp.open("GET", "data/" + id + ".html", true);
    xmlhttp.send();
})();

A tiny missing semicolon at the wrong place can trip up the parser and throw an unexpected identifier JavaScript error when you least expect it. Better safe than sorry!