Es6源码--问题记录--关于includes、indexOf

问题一：
这是一个乌龙问题。

function(){
    console.log([].includes(NaN));  //true 实验结果
    console.log([].includes(3));    //false 实验结果
}(1,2,4,5,NaN)

相当纳闷，参数是怎么传出去的。。。是否存在我还不知道的潜规则？？
as we know
function 有一个属性——arguments：包含所有传进方法的参数
一开始我想源码里是不是这样写了

Array.prototype.includes = function(){
    //value:调用此方法对象
    var value = this || !function(){
        var tempArr = []
        for(var i =0;i<arguments.length,i++){
            tempArr.push(arguments[i]);
        }
        return tempArr
    }()
    ...
}

本着格物致知的原则，确定一下，没找到源码。

回顾看了一下

https://developer.mozilla.org/

---

示例代码

(function() {
  console.log([].includes.call(arguments, 'a')); // true
  console.log([].includes.call(arguments, 'd')); // false
})('a','b','c');

原来用了call

那么问题又来了
includes()源码到底怎么写的。
伪代码如下

Let O be ? ToObject(this value).
Let len be ? ToLength(? Get(O, "length")).
If len is 0, return false.
Let n be ? ToInteger(fromIndex). (If fromIndex is undefined, this step produces the value 0.)
If n ≥ 0, then 
    Let k be n.
Else n < 0,
    Let k be len + n.
    If k < 0, let k be 0.
Repeat, while k < len
    Let elementK be the result of ? Get(O, ! ToString(k)).
    If SameValueZero(searchElement, elementK) is true, return true.
    Increase k by 1.
Return false.

查询一下SameValueZero
是Es6的方法

SameValueZero: used by %TypedArray% and ArrayBuffer constructors, as well as Map and Set operations, and also String.prototype.includes and Array.prototype.includes since ES2016

what’s it?

http://www.ecma-international.org/ecma-262/7.0/

---

The internal comparison abstract operation SameValueZero(x, y), where x and y are ECMAScript language values, produces true or false. Such a comparison is performed as follows:

If Type(x) is different from Type(y), return false.
If Type(x) is Number, then
    If x is NaN and y is NaN, return true.
    If x is +0 and y is -0, return true.
    If x is -0 and y is +0, return true.
    If x is the same Number value as y, return true.
    Return false.
Return SameValueNonNumber(x, y).

NOTE [笔记]
SameValueZero differs from SameValue only in its treatment of +0 and -0.

欧克
这就可以解决了——为什么includes() 可以判断NaN是否在数组里

REMAKE [备注]

→7.2.11SameValueNonNumber (x, y)

The internal comparison abstract operation SameValueNonNumber(x, y), where neither x nor y are Number values, produces true or false. Such a comparison is performed as follows:

Assert: Type(x) is not Number.
Assert: Type(x) is the same as Type(y).
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is String, then
    If x and y are exactly the same sequence of code units (same length and same code units at corresponding indices), return true; otherwise, return false.
If Type(x) is Boolean, then

    If x and y are both true or both false, return true; otherwise, return false.
If Type(x) is Symbol, then

    If x and y are both the same Symbol value, return true; otherwise, return false.

Return true if x and y are the same Object value. Otherwise, return false.

人不装B伪少年，by h.k

2017年12月29日
看源码是爱好、解决问题是职业——h.k