数组方法 ​

if (!Array.of) {
  Array.of = function () {
    return Array.prototype.slice.call(arguments)
    // return [].slice.call(arguments)
  }
}

if (!Array.isArray) {
  Array.isArray = function (arg) {
    return Object.prototype.toString.call(arg) === '[object Array]'
    // return ({}).toString.call(arg) === '[object Array]'
  }
}

if (!Array.prototype.concat) {
  Array.prototype.concat = function (arr1, ...arrs) {
    let ret = Array.isArray(arr1) ? arr1.slice() : Array.of(arr1)
    ret.push.apply(ret, arrs)
    // ret.push.call(ret, ...arrs)
    // ret.push.bind(ret, ...arrs)()
    return ret
  }
}

if (!Array.prototype.flat) {
  Array.prototype.flat = function flat(arr, dep = 1) {
    let _dep = 1
    function _flat(arr, dep, _dep) {
      return arr.reduce(function (acc, cur) {
        return acc.concat(Array.isArray(cur) && (dep === Infinity || _dep < dep)
          ? flat(cur, dep, _dep + 1)
          : cur)
      }, [])
    }
    return _flat(arr, dep, _dep)
  }
}

if (!Array.prototype.pop) {
  Array.prototype.pop = function () {
    return this.splice(this.length - 1, 1)[0]
  }
}

if (!Array.prototype.shift) {
  Array.prototype.shift = function () {
    return this.splice(0, 1)[0]
  }
}

if (!Array.prototype.push) {
  Array.prototype.push = function () {
    this.splice.apply(this, [this.length, 0].concat([].slice.apply(arguments)))
    return this.length
  }
}

if (!Array.prototype.unshift) {
  Array.prototype.unshift = function () {
    this.splice.apply(this, [0, 0].concat([].slice.apply(arguments)))
    return this.length
  }
}