addExplicitNewlines.js

function addExplicitNewlines(
  input,
  maxWidth,
  textCanvas,
  font,
  size
): string {
  const split = ['']

  const currentLine = () => split[split.length - 1]
  const append = (token) => (split[split.length - 1] += token)
  const carriageReturn = () => {
    // Remove trailing whitespace from the current line
    split.push(split.pop()!.trim())
    split.push('')
  }
  const width = (line) => {
    return textCanvas.textWidth(line)
  }

  // Separate words and whitespace into separate tokens
  const tokens = input
    // Split on whitespace, without getting rid of it
    .split(/(?=\s)/g)
    .flatMap((s) => {
      // Make the whitespace characters their own tokens
      const match = s.match(/^(\s)(.+)$/)
      if (match) {
        return [match[1], match[2]]
      } else {
        return s
      }
    })
    .reverse()

  textCanvas.push()
  if (font !== undefined) textCanvas.textFont(font)
  if (size !== undefined) textCanvas.textSize(size)

  while (tokens.length > 0) {
    const token = tokens.pop() as string
    if (token === '\n') {
      carriageReturn()
    } else if (width((currentLine() + token).trim()) <= maxWidth) {
      append(token)
    } else {
      if (currentLine().trim()) carriageReturn()
      append(token)
    }
  }
  if (!currentLine()) {
    split.pop()
  }

  textCanvas.pop()

  return split.join('\n')
}