# js相等比较算法

# 1.严格相等 （===）

比较 x === y:

参考链接

1. If Type(x) is different from Type(y), return false
2. If Type(x) is Undefined, return true
3. If Type(x) is Null, return true
4. If Type(x) is Number, then
   1. If x is NaN, return false
   2. If y is NaN, return false
   3. If x is the same Number value as y, return true
   4. If x is +0 and y is −0, return true
   5. If x is −0 and y is +0, return true
   6. Return false
5. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false
6. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false
7. Return true if x and y refer to the same object. Otherwise, return false

# 2. 非严格相等 （==）

比较 x == y:

1. If Type(x) is the same as Type(y) , then
   1. If Type(x) is Undefined, return true.
   2. If Type(x) is Null, return true.
   3. If Type(x) is Number, then
      1. If x is NaN, return false.
      2. If y is NaN, return false.
      3. If x is the same Number value as y, return true.
      4. If x is +0 and y is −0, return true.
      5. If x is −0 and y is +0, return true.
      6. Return false.
   4. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions). Otherwise, return false.
   5. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
   6. Return true if x and y refer to the same object. Otherwise, return false.
2. If x is null and y is undefined, return true.
3. If x is undefined and y is null, return true.
4. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).
5. If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.
6. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
7. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
8. If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
9. If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x) == y.
10. Return false.

这个算法中有个比较重要的求值算法 toPrimitive，这是一个内部方法，用来计算对象类型的原始值，也就是将object类型转换为非object类型，用法：toPrimitive(x,prefferedType?)，其中x为输入值，prefferedType为可选参数，表明想转换为的类型（string 或者 number）,在不传递此参数情况下，默认认为是需要转出number类型值。

求值过程如下：

1. prefferedType ?= number
   1. 尝试调用x.valueOf方法,如果该方法存在，且结果值是一个原始值，则返回该值
   2. 尝试调用x.toString方法，如果该方法存在，且结果值是一个原始值，则返回该值
   3. 抛出TypeError异常
2. prefferedType = string
   1. 尝试调用x.toString方法,如果该方法存在，且结果值是一个原始值，则返回该值
   2. 尝试调用x.valueOf方法，如果该方法存在，且结果值是一个原始值，则返回该值
   3. 抛出TypeError异常

# 3.同值相等（Object.is）

Object.is(x, y)比较两值是否相同

规则如下：

1. x, y类型都为undefined，true
2. x, y类型都为null，true
3. x, y类型都为string，且长度和字符以及顺序一致，true
4. x, y类型都为引用类型，并且引用都是内存中同一个对象，true
5. x, y类型都为number
   1. x, y都为 +0，true
   2. x, y都为 -0，true
   3. x, y都为 NaN, true
   4. x, y都为非零和非NaN的值，且值相同， true
6. false