SOURCE

String.prototype.trim = function () {
  return /^\s*(.*?)\s*$/.exec(this)[1]
}

String.prototype.trim2 = function () {
  return this.replace(/^\s*|\s*$/g, '')
}

String.prototype.trim3 = function () {
  let len = this.length
  let startIndex = -1
  let endIndex = -1
  let i = 0
  while(startIndex === -1) {
    if (this[i] !== ' ') {
      startIndex = i
    }
    i++
  }
  i = this.length - 1
  while(endIndex === -1) {
    if (this[i] !== ' ') {
      endIndex = i
    }
    i--
  }
  console.log(startIndex, endIndex)
  return this.slice(startIndex, endIndex + 1)
}

let str = '  zhang    '
console.log('--' + str.trim() + '--')
console.log('--' + str.trim2() + '--')
console.log('--' + str.trim3() + '--')