Spot the bug - #166: Gamer Tag Tagger

programming web dev
1

Why is this username padder producing weirdly short results?

function padTag(tag, length = 8) {
  const cleanTag = tag.trimStart();
  return cleanTag.padStart(length - cleanTag.length, "~");
}

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

2

Yo this is a classic. the trimStart() is definitely the culprit here.

function padTag(tag, length = 8) {
  const originalLeadingSpaces = tag.length - tag.trimStart().length;
  const cleanTag = tag.trimStart();
  return cleanTag.padStart(length - cleanTag.length - originalLeadingSpaces, "~");
}

That’s one way to fix it, by accounting for those trimmed spaces in the padding calculation. or you could just pad first and then trim if the goal is to always hit length characters after cleaning. depends on the exact spec.

3

Spot the Bug answer: The padStart method is used incorrectly, causing the string to be padded by the difference between the desired length and the current length, rather than to the desired length.

The fix:

return cleanTag.padStart(length, "~");

Why:
The padStart method expects the first argument to be the final desired length of the string. By passing length - cleanTag.length, the function calculates the number of characters to add, but padStart interprets this as the target length, leading to a shorter string than intended. The correct usage is to provide the total target length.

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4

That’s a good explanation of the padStart behavior. I’ve seen similar issues when people try to manually calculate padding instead of letting the method handle the target length directly.

5

You’re right, padStart is definitely the way to go here. The explanation for why it was acting weird is that the length argument needs to be the final desired string length, not just the padding amount.

6

The issue is that padStart() uses length - cleanTag.length as the target length. That means it subtracts the characters twice, so the result can be shorter than expected.

Use length directly as the target length:

function padTag(tag, length = 8) {
  const cleanTag = tag.trimStart();
  return cleanTag.padStart(length, "~");
}

Now the tag will be padded with ~ on the left until it reaches the requested length. If the tag is already long enough, it stays unchanged.

7

lol nice catch, you’re on fire! the full answer goes up later today, so no spoilers from me :slight_smile: