What's the hardest problem you've had to debug recently, and what did you learn from it?

_bigblind profile image Frederik 👨‍💻➡️🌐 Creemers ・2 min read

Mine was a problem with a React app. I was loading an SVG icon sheet using XHR, and dynamically prepend it to the <body> tag's content. This was done lazily, the first time an Icon was rendered.

I was trying to put a grid of actions, each of which has an icon, into an Accordion component. The accordion would work with any other content, but when I added the icon grid, it would just not respond. I first started looking for the cause by inserting an SVG in the accordion directly, without the <use> SVG tag, and that worked. Then I loaded my sprite sheet, and inserted a <svg><use ... /></svg> tag, and it stopped working. I was tearing my head out, until at one point while having lunch, the cause popped into my head. Here's how I was fetching and inserting my sprite sheet:

fetch(location.origin + "/api/icons", {/* ... options ...*/}).then((resp) => {
    return resp.text();
}).then((svg) => {
    document.body.innerHTML = svg + document.body.innerHTML;

The problem was that doing this removes all the event listeners from the page, as you're basically recreating the entire DOM from a string.

Here's what my code looks like now:

fetch(location.origin + "/api/icons", {/* ... */}).then((resp) => {
    return resp.text()
}).then((txt) => {
    let div = document.createElement("div");
    div.innerHTML = txt;
    document.body.insertBefore(div.firstChild, document.body.firstChild)

So I'm inserting the SVG I get from the server as text into a newly created div element. Then, I insert the contents of that div, right before the first tag in the body. The important thing, is that I'm actually inserting the SVG element as a DOM node, not as text. This way, the DOM stays intact, and the event handlers still work. hurray! 🎉

Now it's time to tell your stories from the debugging battlefield.

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_bigblind profile

Frederik 👨‍💻➡️🌐 Creemers


I'm never sure what to put in a bio. If there's anything you want to know, don't be afraid to ask!


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Background: I've been a C programmer since almost the year it was invented. That matters because of the problem that I found today. You'll see it immediately.

int startPosition = 0;

for (int startPosition = 0; startPosition<max; startPosition++) {
// Search for something in a buffer. Break when found.
if (buffer[startPosition] == SEARCHED_VALUE) {

buffer[startPosition] should contain the value that I was looking for. It took me several hours to figure out why startPosition was always 0!

Part of the problem was the difficult debug environment that involved downloading code to an engine computer and running the diesel engine.