2.1.46 [ECMA-262/9:2018] Section 21.2.5.2.3 AdvanceStringIndex ( S, index, unicode )

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V0173: AdvanceStringIndex advances the index by 1, not 2, when the unicode flag is specified

The specification states:

 21.2.5.2.3 AdvanceStringIndex ( S, index, unicode )
  
     The abstract operation AdvanceStringIndex with arguments S, index, and unicode 
     performs the following steps:
  
         1.  Assert: Type(S) is String.
         2.  Assert: index is an integer such that 0≤index≤2^53-1.
         3.  Assert: Type(unicode) is Boolean.
         4.  If unicode is false, return index+1.
         5.  Let length be the number of code units in S.
         6.  If index+1 ≥ length, return index+1.
         7.  Let first be the code unit value at index index in S.
         8.  If first < 0xD800 or first > 0xDBFF, return index+1.
         9.  Let second be the numeric value of the code unit ["code unit value" prior to 
             2018] at index index+1 in S.
         10. If second < 0xDC00 or second > 0xDFFF, return index+1.
         11. Return index+2.

EdgeHTML Mode

AdvanceStringIndex advances the index by 1, not 2 when the unicode flag is specified. For example, the following should hold:

    /\udf06/u.exec('\ud834\udf06') == null

Instead exec returns \udf06; that is:

    /\udf06/u.exec('\ud834\udf06') == '\udf06'