Object.is()与===
有一篇面经中提到面试官提问,如何判断两个变量相等?
在ES6之前,我们使用===来判断两个变量是否相等。ES6中增加了一个方法Object.is(val1,val2)来比较两个变量。那么两者有什么不同之处呢
ECMA规范中的Object.is()执行过程如下:
/* Object.is(x,y) 1、If Type(x) is different from Type(y), return false. //比较类型,不同返回false。typeof x/y 2、If Type(x) is Number, then a、If x is NaN and y is NaN, return true. //NaN与NaN相等 b、If x is +0 and y is -0, return false. //+0与-0不相等 c、If x is -0 and y is +0, return false. //-0与+0不相等 d、If x is the same Number value as y, return true. //数字类型,值不同则不相等 e、Return false. 3、Return SameValueNonNumber(x, y). */ /* sameValueNonNumber(x,y) 1、Assert: Type(x) is not Number. //x不是数字 2、Assert: Type(x) is the same as Type(y). //x与y类型相同 3、If Type(x) is Undefined, return true. //如果typeof x是undefined,则y也是undefined,两者相等 4、If Type(x) is Null, return true. //typeof x是Null,则y也是Null,则两者相等 5、If Type(x) is String, then //字符类型的,值相同则相等 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. 6、If Type(x) is Boolean, then //boolean,值相同则相等 If x and y are both true or both false, return true; otherwise, return false. 7、If Type(x) is Symbol, then //symbol类型的,值相同则相等 If x and y are both the same Symbol value, return true; otherwise, return false. 8、If x and y are the same Object value, return true. Otherwise, return false. //object类型的,同一个对象则相等 */
===运算符的规范如下:
/* 1、If Type(x) is different from Type(y), return false. 2、If Type(x) is Number, then a、If x is NaN, return false. b、If y is NaN, return false. c、If x is the same Number value as y, return true. d、If x is +0 and y is -0, return true. e、If x is -0 and y is +0, return true. f、Return false. 3、Return SameValueNonNumber(x, y). */
===与Object.is()主要差别是NaN、+0、-0这几个特殊的数字类型上。如果是需要判断这几个值,可以选择Object.is(),其他情况下,依然推荐使用===