2.1.42 [ECMA-262/9:2018] Section String.prototype.toUpperCase ( )

V0185: Only characters in the Basic Multilingual Plane are converted to uppercase

The specification states:

 ... String.prototype.toUpperCase ( )
     This function interprets a String value as a sequence of UTF-16 encoded code points, 
     as described in 6.1.4.
     This function behaves in exactly the same way as String.prototype.toLowerCase, except 
     that the String is mapped using the toUppercase algorithm of the Unicode Default Case 

EdgeHTML Mode

Only those characters in the Basic Multilingual Plane (values no greater than 0xFFFF) are converted to uppercase. Others are left unchanged.