findIndex的实现``` var users = [ {id: 1, name: '张三'}, {id: 2, n


var users = [
  {id: 1, name: '张三'},
  {id: 2, name: '张三'},
  {id: 3, name: '张三'},
  {id: 4, name: '张三'}
]

Array.prototype.myFind = function (conditionFunc) {
  // var conditionFunc = function (item, index) { return item.id === 4 }
  for (var i = 0; i < this.length; i++) {
    if (conditionFunc(this[i], i)) {
      return i
    }
  }
}

var ret = users.myFind(function (item, index) {
  return item.id === 2
})

console.log(ret)