Template tag for preserving indents

indented.ts

function getIndentation(lines: string[] | TemplateStringsArray): string {
  let firstIndent: string

  if (lines[0] === '') {
    firstIndent = lines[1]
  } else {
    firstIndent = lines[0]
  }

  const matched = firstIndent.match(/^(\s+).*$/)
  return matched ? matched[1] : ''
}

function indented(strings: TemplateStringsArray, ...values: string[]) {
  const indentLevel = getIndentation(strings[0].split('\n'))

  const joined = strings
    .map((str, i) => {
      const inner = values[i]
      const lines = str.split('\n')
      const innerIndent = lines[lines.length - 1]
      if (inner) {
        // don't reindent the first line of inner, since that's where the indent level comes from
        const reindentedInner = inner
          .split('\n')
          .map((v, j) => (j > 0 ? innerIndent + v : v))
          .join('\n')
        return `${str}${reindentedInner}`
      } else {
        return str
      }
    })
    .join('')
    .trimStart()
    .trimEnd() // remove extra empty strings at start and end

  return joined
    .split('\n')
    .map((line) => line.replace(new RegExp(`^${indentLevel}`), ''))
    .join('\n')
}