Add colored stroke around objects in transparent (.png) image

package.json

{
  "dependencies": {
    "sharp": "^0.33.2",
    "ts-node": "^10.9.2"
  },
  "scripts": {
    "start": "ts-node script.ts"
  }
}

script.ts

// Add colored stroke around objects in transparent (.png) image

import sharp from 'sharp'
import type { Sharp } from 'sharp'

// Based on a code snippet from arunlodhi in the GitHub sharp repository
// https://github.com/lovell/sharp/issues/3759
const drawOutlineSharp = async (
  sharpInstance: Sharp,
  thickness: number = 10,
  blurSigma: number = 2,
  outlineColor = { r: 255, g: 0, b: 0, alpha: 1 }
): Promise<Sharp> => {
  const inputBuffer = await sharpInstance.toBuffer()
  const alphaMask = sharpInstance.clone().extractChannel("alpha").png()
  const negatedAlphaMask = sharp(
    await alphaMask.clone().blur(blurSigma).negate().toBuffer()
  )
  //negatedAlphaMask.toFile(("negatedAlphaMask.png"))
  const bgMask = negatedAlphaMask.blur(thickness / 2).unflatten()
  //bgMask.toFile(("bgMask.png"))
  let bg = bgMask.composite([
    {
      input: {
        create: {
          width: 1,
          height: 1,
          channels: 4,
          background: outlineColor,
        },
      },
      blend: "in",
      tile: true,
    },
  ])
  bg = sharp(await bg.toBuffer())
  bg = bg.composite([
    {
      input: inputBuffer,
      blend: "over",
    },
  ])
  //bg.toFile(("bg.png"))
  return sharp(await bg.toBuffer())
}

const drawOutline = async (
  inImgPath: string,
  outImgPath: string,
  thickness: number = 20,
  blurSigma: number = 10,
  outlineColor = { r: 255, g: 255, b: 255, alpha: 1 }
) => {
  drawOutlineSharp(sharp(inImgPath), thickness, blurSigma, outlineColor)
    .then((buf) => buf.toFile(outImgPath))
    .then((info) => console.info(`Exported image (${outImgPath}) information:`, info))
    .catch((err) => console.error(err))
}

// Demo:
drawOutline(
  "/home/niklas/OneDrive/Andere Dateien/osu/aubs-borgar.png",
  "/home/niklas/OneDrive/Andere Dateien/osu/aubs-borgar_edited.png"
)