Q:

Unicode composition in javascript

I am looking for a way to count ligatures as single units as they are displayed to user, e.g. https://www.compart.com/en/unicode/U+FEFB.  
When this character is typed (type G on Arabic keyboard), it's inserted in decomposition form, i.e. U+0644 U+0627.  
I'm able to decompose U+FEFB by
escape(String.fromCodePoint(0xFEFB).normalize("NFKD")) // '%u0644%u0627'

Is there a way to compose U+0644 U+0627 into 0xFEFB? 
Why this does work? 
escape(String.fromCodePoint(0x0644, 0x0627).normalize("NFKC"))

The only idea I has was to iterate over unicode ranges I'm interested in, decompose and create a map, but I'm hoping there's a better way. 

A:

Given that the ES2019 spec requires the implementation to:

Let ns be the String value that is the result of normalizing S into the normalization form named by f as specified in https://unicode.org/reports/tr15/.

and given that https://www.unicode.org/Public/12.1.0/ucd/NormalizationTest.txt describes that character as
FEFB;FEFB;FEFB;0644 0627;0644 0627; # (ﻻ; ﻻ; ﻻ; لا; لا; ) ARABIC LIGATURE LAM WITH ALEF ISOLATED FORM

it is the compliant behaviour.  See
# 1.  The following invariants must be true for all conformant implementations
#
#    NFC
#      c2 ==  toNFC(c1) ==  toNFC(c2) ==  toNFC(c3)
#      c4 ==  toNFC(c4) ==  toNFC(c5)
#
#    NFD
#      c3 ==  toNFD(c1) ==  toNFD(c2) ==  toNFD(c3)
#      c5 ==  toNFD(c4) ==  toNFD(c5)
#
#    NFKC
#      c4 == toNFKC(c1) == toNFKC(c2) == toNFKC(c3) == toNFKC(c4) == toNFKC(c5)
#
#    NFKD
#      c5 == toNFKD(c1) == toNFKD(c2) == toNFKD(c3) == toNFKD(c4) == toNFKD(c5)

No normalisation converts either c4 or c5 form back to c1, or c2, or c3. 
So to my unicode-amateur opinion there is no standard-compliant way to normalise U+0644 U+0627 back to U+FEFB.