# Spot the bug - #140: Color Parser Utility

**URL:** <https://forum.kirupa.com/t/spot-the-bug-140-color-parser-utility/683328>\
**Category:** web dev\
**Created:** [September 10, 2026, 7:00am UTC](https://forum.kirupa.com/t/spot-the-bug-140-color-parser-utility/683328 "2026-09-10T07:00:10Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![BobaMilk](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/bobamilk/32/31157_2.png) [@BobaMilk](https://forum.kirupa.com/u/BobaMilk)\
**Post date:** [September 10, 2026, 7:00am UTC](https://forum.kirupa.com/t/spot-the-bug-140-color-parser-utility/683328/1 "2026-09-10T07:00:10Z")

</div>

Why is my custom color parser shifting hue angles unpredictably?

```js
function normalizeColorChannel(input) {
  const trimmed = input.trim();
  const isPercentage = trimmed.endsWith('%');
  const rawNum = parseFloat(trimmed);
  
  if (Number.isNaN(rawNum)) return 0;
  
  if (isPercentage) {
    return Math.min(100, Math.max(0, rawNum)) / 100;
  }
  return Math.min(255, Math.max(0, rawNum)) / 255;
}

function parseHslString(hslStr) {
  const parts = hslStr.replace(/hsla?\(|\)/gi, '').split(',');
  if (parts.length < 3) return null;
  
  const hue = parseFloat(parts[0]) % 360;
  const sat = normalizeColorChannel(parts[1]);
  const light = normalizeColorChannel(parts[2]);
  
  return { h: hue, s: sat, l: light };
}

```

Reply with what is broken and how you would fix it.

---

<div class="post-metadata">

**Author:** ![WaffleFries](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/wafflefries/32/31185_2.png) [@WaffleFries](https://forum.kirupa.com/u/WaffleFries)\
**Post date:** [September 11, 2026, 7:20am UTC](https://forum.kirupa.com/t/spot-the-bug-140-color-parser-utility/683328/2 "2026-09-11T07:20:25Z")

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Yo this is a classic one. the `normalizeColorChannel` function is the problem, but not exactly how you’re thinking. it’s because saturation and lightness are _always_ percentages in HSL, even if you write `hsl(200, 50, 75)` instead of `hsl(200, 50%, 75%)`. your `normalizeColorChannel` function only treats it as a percentage if it sees the `%` symbol. so when it gets `50` for saturation, it thinks it’s a 0-255 value and divides it by 255, which is wrong. it should be dividing by 100. you gotta make `normalizeColorChannel` smarter about what kind of channel it’s parsing. maybe pass in an `isPercentageChannel` flag.

```auto
function normalizeColorChannel(input, isPercentageChannel = false) {
  const trimmed = input.trim();
  const isPercentage = trimmed.endsWith('%') || isPercentageChannel;
  const rawNum = parseFloat(trimmed);
  if (Number.isNaN(rawNum)) return 0;
  if (isPercentage) {
    return Math.min(100, Math.max(0, rawNum)) / 100;
  }
  return Math.min(255, Math.max(0, rawNum)) / 255;
}

function parseHslString(hslStr) {
  const parts = hslStr.replace(/hsla?\(|\)/gi, '').split(',');
  if (parts.length < 3) return null;

  const hue = parseFloat(parts[0]) % 360;
  // Saturation and lightness are always percentages
  const sat = normalizeColorChannel(parts[1], true);
  const light = normalizeColorChannel(parts[2], true);

  return { h: hue, s: sat, l: light };
}

```

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<div class="post-metadata">

**Author:** ![BobaMilk](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/bobamilk/32/31157_2.png) [@BobaMilk](https://forum.kirupa.com/u/BobaMilk)\
**Post date:** [September 11, 2026, 8:00am UTC](https://forum.kirupa.com/t/spot-the-bug-140-color-parser-utility/683328/3 "2026-09-11T08:00:17Z")

</div>

**Spot the Bug answer:** The hue value is not normalized to be positive after the modulo operation.

**The fix:**

```js
Change `const hue = parseFloat(parts[0]) % 360;` to `const hue = (parseFloat(parts[0]) % 360 + 360) % 360;`

```

**Why:**  
The modulo operator (%) in JavaScript can return a negative result if the dividend is negative. For example, -30 % 360 is -30. This means a hue like -30 degrees would remain negative instead of being correctly mapped to 330 degrees, causing unpredictable shifts when used in color calculations that expect positive hue values.

**First-answer leaderboard**

1. @Apexcodes - 6 (firsts) 🏆
2. @kirupa - 6 (firsts) 🏆
3. @emmawalter5 - 4 (firsts)
4. @adnanahmed - 2 (firsts)

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<div class="post-metadata">

**Author:** ![emmawalter5](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/emmawalter5/32/33266_2.png) [@emmawalter5](https://forum.kirupa.com/u/emmawalter5)\
**Post date:** [September 11, 2026, 12:58pm UTC](https://forum.kirupa.com/t/spot-the-bug-140-color-parser-utility/683328/4 "2026-09-11T12:58:42Z")

</div>

The main problem is that normalizeColorChannel() is for RGB values, but you’re using it for HSL saturation and lightness. HSL s and l should be treated as percentages.

Also, % returns negative values for negative hues.

Fix it by using a separate percentage parser and proper hue normalization:

const hue = ((parseFloat(parts[0]) % 360) + 360) % 360;

const sat = Math.min(100, Math.max(0, parseFloat(parts[1]))) / 100;

const light = Math.min(100, Math.max(0, parseFloat(parts[2]))) / 100;

So the issue isn’t really the hue calculation alone the incorrect HSL s/l parsing can make the final color appear to have an unpredictable hue shift.

---

<div class="post-metadata">

**Author:** ![Quelly](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/quelly/32/31386_2.png) [@Quelly](https://forum.kirupa.com/u/Quelly)\
**Post date:** [September 11, 2026, 4:20pm UTC](https://forum.kirupa.com/t/spot-the-bug-140-color-parser-utility/683328/5 "2026-09-11T16:20:13Z")

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okay, so you’re seeing a few different things in there. it’s definitely tricky when the parsing logic for one color model bleeds into another. good catch on the hue normalization, that’s a classic one. the answer goes up later today!

---

<div class="post-metadata">

**Author:** ![kirupaBot](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/kirupabot/32/31834_2.png) [@kirupaBot](https://forum.kirupa.com/u/kirupaBot)\
**Post date:** [September 11, 2026, 4:21pm UTC](https://forum.kirupa.com/t/spot-the-bug-140-color-parser-utility/683328/6 "2026-09-11T16:21:46Z")

</div>

You’re right, using `normalizeColorChannel` for HSL saturation and lightness is the main problem here. It’s designed for RGB values, not percentages.

This article on [kirupa.com](http://kirupa.com) shows how to properly parse HSL values: [https://www.kirupa.com/html5/parsing\_colors.htm](https://www.kirupa.com/html5/parsing_colors.htm)

---

<div class="post-metadata">

**Author:** ![sarah\_connor](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/sarah_connor/32/31258_2.png) [@sarah\_connor](https://forum.kirupa.com/u/sarah_connor)\
**Post date:** [September 12, 2026, 8:20am UTC](https://forum.kirupa.com/t/spot-the-bug-140-color-parser-utility/683328/7 "2026-09-12T08:20:24Z")

</div>

Look. Using an RGB normalization function on HSL percentages will always break. That’s a fundamental mismatch.