// Declare array with no element at index 2, 3 and 4
var a = [0,1,,,,5,6];

// Shows all indexes, not just those that have been assigned values
a.find(function(value, index) {
  console.log(''Visited index '' + index + '' with value '' + value); 
});

// Shows all indexes, including deleted
a.find(function(value, index) {

  // Delete element 5 on first iteration
  if (index == 0) {
    console.log(''Deleting a[5] with value '' + a[5]);
    delete a[5];  // 注：这里只是将a[5]设置为undefined，可以试试用a.pop()删除最后一项，依然会遍历到被删的那一项
  }
  // Element 5 is still visited even though deleted
  console.log(''Visited index '' + index + '' with value '' + value); 
});

// https://tc39.github.io/ecma262/#sec-array.prototype.find
if (!Array.prototype.find) {
    Object.defineProperty(Array.prototype, ''find'', {
      value: function(predicate) {//遍历的处理函数
       // 1. Let O be ? ToObject(this value).
        if (this == null) {//数组为null
          throw new TypeError(''"this" is null or not defined'');
        }
  
        var o = Object(this);//数组
  
        // 2. Let len be ? ToLength(? Get(O, "length")).
        var len = o.length >>> 0;//数组长度
  
        // 3. If IsCallable(predicate) is false, throw a TypeError exception.
        if (typeof predicate !== ''function'') {//第一个参数必须是函数
          throw new TypeError(''predicate must be a function'');
        }
  
        // 4. If thisArg was supplied, let T be thisArg; else let T be undefined.
        var thisArg = arguments[1];//第二个参数指定this
  
        // 5. Let k be 0.
        var k = 0;//遍历索引
  
        // 6. Repeat, while k < len
        while (k < len) {
          // a. Let Pk be ! ToString(k).
          // b. Let kValue be ? Get(O, Pk).
          // c. Let testResult be ToBoolean(? Call(predicate, T, « kValue, k, O »)).
          // d. If testResult is true, return kValue.
          var kValue = o[k];
          if (predicate.call(thisArg, kValue, k, o)) {//如果当前值经过处理函数后返回了true，就返回当前值
            return kValue;
          }
          // e. Increase k by 1.
          k++;
        }
  
        // 7. Return undefined.
        return undefined;//未找到就返回undefined
      }
    });
  }

// https://tc39.github.io/ecma262/#sec-array.prototype.findIndex
if (!Array.prototype.findIndex) {
    Object.defineProperty(Array.prototype, ''findIndex'', {
      value: function(predicate) {//第一个参数处理函数
       // 1. Let O be ? ToObject(this value).
        if (this == null) {//数组为null抛错误
          throw new TypeError(''"this" is null or not defined'');
        }
  
        var o = Object(this);//数组
  
        // 2. Let len be ? ToLength(? Get(O, "length")).
        var len = o.length >>> 0;//数组长度
  
        // 3. If IsCallable(predicate) is false, throw a TypeError exception.
        if (typeof predicate !== ''function'') {//第一个参数不是函数抛错误
          throw new TypeError(''predicate must be a function'');
        }
  
        // 4. If thisArg was supplied, let T be thisArg; else let T be undefined.
        var thisArg = arguments[1];//第二个参数是指定this值
  
        // 5. Let k be 0.
        var k = 0;//遍历索引
  
        // 6. Repeat, while k < len
        while (k < len) {
          // a. Let Pk be ! ToString(k).
          // b. Let kValue be ? Get(O, Pk).
          // c. Let testResult be ToBoolean(? Call(predicate, T, « kValue, k, O »)).
          // d. If testResult is true, return k.
          var kValue = o[k];
          if (predicate.call(thisArg, kValue, k, o)) {//如果当前值符合要求就返回当前值的索引
            return k;
          }
          // e. Increase k by 1.
          k++;
        }
  
        // 7. Return -1.
        return -1;//找不到符合要求的值就返回-1
      }
    });
  }