removeHeadingWhitespaces.js

const removeHeadingWhitespaces = str => {
  
    const lines = str.split('\n')

    while (lines.length > 0 && /^\s*$/.test(lines[0])) {
        lines.shift()
    }

    while (lines.length > 0 && /^\s*$/.test(lines[lines.length - 1])) {
        lines.pop()
    }

    const length = lines[0].match(/^\s*/)[0].length
    const re = new RegExp(`^\\s{0,${length}}`)

    return lines.map(line => line.replace(re, '')).join('\n')
}

export { removeHeadingWhitespaces }
export default removeHeadingWhitespaces

from (any string beginnings with whitespaces)

      #foo {
          color: red;
      }

returns

#foo {
    color: red;
}