JavaScript不清不楚之includes、indexOf、lastIndexOf

Array.prototype.includes

以下代码均来自：MDN

1. includes判断一个数组是否包含某个元素，找到返回true，否则返回false，默认从0开始查找，如果传入的索引值大于等于数组长度则不会遍历数组，直接返回false
2. IE系列不支持

// https://tc39.github.io/ecma262/#sec-array.prototype.includes
if (!Array.prototype.includes) {
  Object.defineProperty(Array.prototype, 'includes', {
    value: function(searchElement, fromIndex) {

      // 1. Let O be ? ToObject(this value).
      if (this == null) {
        throw new TypeError('"this" is null or not defined');
      }

      var o = Object(this);

      // 2. Let len be ? ToLength(? Get(O, "length")).
      var len = o.length >>> 0;

      // 3. If len is 0, return false.
      //空数组直接返回false
      if (len === 0) {
        return false;
      }

      // 4. Let n be ? ToInteger(fromIndex).
      //    (If fromIndex is undefined, this step produces the value 0.)
      // undefined null NaN和0 | 0 都会输出0
      var n = fromIndex | 0; 

      // 5. If n ≥ 0, then
      //  a. Let k be n.
      // 6. Else n < 0,
      //  a. Let k be len + n.
      //  b. If k < 0, let k be 0.
      //k值会被扶正，k >= 0 ,如果k >= len 不会循环
      var k = Math.max(n >= 0 ? n : len - Math.abs(n), 0);

      // 7. Repeat, while k < len
      while (k < len) {
        // a. Let elementK be the result of ? Get(O, ! ToString(k)).
        // b. If SameValueZero(searchElement, elementK) is true, return true.
        // c. Increase k by 1.
        // NOTE: === provides the correct "SameValueZero" comparison needed here.
        if (o[k] === searchElement) {
          return true;
        }
        k++;
      }

      // 8. Return false
      return false;
    }
  });
}

//也可以用在非数组
(function() {
  console.log([].includes.call(arguments, 'a')); // true
  console.log([].includes.call(arguments, 'd')); // false
})('a','b','c');

Array.prototype.indexOf

1. 从数组中找到给定的元素的索引，找到了返回该索引，找不到返回-1
2. IE9+支持此方法
3.

// Production steps of ECMA-262, Edition 5, 15.4.4.14
// Reference: http://es5.github.io/#x15.4.4.14
if (!Array.prototype.indexOf) {
  Array.prototype.indexOf = function(searchElement, fromIndex) {
    var k;

    // 1. Let O be the result of calling ToObject passing
    //    the this value as the argument.
    if (this == null) {
      throw new TypeError('"this" is null or not defined');
    }

    var O = Object(this);

    // 2. Let lenValue be the result of calling the Get
    //    internal method of O with the argument "length".
    // 3. Let len be ToUint32(lenValue).
    var len = O.length >>> 0;

    // 4. If len is 0, return -1.
    //空数组直接返回false
    if (len === 0) {
      return -1;
    }

    // 5. If argument fromIndex was passed let n be
    //    ToInteger(fromIndex); else let n be 0.
    var n = +fromIndex | 0;

    //n为无穷则赋值为0
    if (Math.abs(n) === Infinity) {
      n = 0;
    }

    //如果n大于数组长度，直接返回-1
    // 6. If n >= len, return -1.
    if (n >= len) {
      return -1;
    }

    // 7. If n >= 0, then Let k be n.
    // 8. Else, n<0, Let k be len - abs(n).
    //    If k is less than 0, then let k be 0.
    k = Math.max(n >= 0 ? n : len - Math.abs(n), 0);

    // 9. Repeat, while k < len
    while (k < len) {
      // a. Let Pk be ToString(k).
      //   This is implicit for LHS operands of the in operator
      // b. Let kPresent be the result of calling the
      //    HasProperty internal method of O with argument Pk.
      //   This step can be combined with c
      // c. If kPresent is true, then
      //    i.  Let elementK be the result of calling the Get
      //        internal method of O with the argument ToString(k).
      //   ii.  Let same be the result of applying the
      //        Strict Equality Comparison Algorithm to
      //        searchElement and elementK.
      //  iii.  If same is true, return k.
      if (k in O && O[k] === searchElement) {
        return k;
      }
      k++;
    }
    return -1;
  };
}

//查找引用类型的值
var a = [5];
[1,2,3,4,a,6].indexOf(a);
// 4

[1,2,3,4,[5]].indexOf([5]);
// -1

//检测某个元素出现的次数
var indices = [];
var array = ['a', 'b', 'a', 'c', 'a', 'd'];
var element = 'a';
var idx = array.indexOf(element);
while (idx != -1) {
  indices.push(idx);
  //如果找到了，就从此次的idx + 1开始继续搜索
  idx = array.indexOf(element, idx + 1);
}
console.log(indices);
// [0, 2, 4]

Array.prototype.lastIndexOf

1. 从传入的索引处开始，从数组的后面向前查找相同元素，找到则返回元素项的索引，未找到则返回-1
2. 空数组直接返回-1，如果传入的索引值是负数，绝对值大于数组长度，则直接返回-1

var indices = [];
var array = ['a', 'b', 'a', 'c', 'a', 'd'];
var element = 'a';
var idx = array.lastIndexOf(element);

while (idx != -1) {
  indices.push(idx);
  //当索引值等于0时便不用在循环查找了
  idx = (idx > 0 ? array.lastIndexOf(element, idx - 1) : -1);
}

console.log(indices);
// [4, 2, 0];