Issues regarding the charcodeat function in JavaScript, specifically focusing on the hex values 0xd7ff, 0xe000, and 0

Can you explain the significance of the values 0xd7ff,0xe000,and 

(code << 10) + next - 0x35fdc00
?

const nonASCIIwhitespace = /[\u1680\u180e\u2000-\u200a\u202f\u205f\u3000\ufeff]/

        const fullCharCodeAtPos = (input: string, pos: number) => {
            if (String.fromCharCode) return input.codePointAt(pos)
            const code = input.charCodeAt(pos)
            if (code <= 0xd7ff || code >= 0xe000) return code

            const next = input.charCodeAt(pos + 1)
            return (code << 10) + next - 0x35fdc00
        }
javascripttypescriptunicode

Answer №1

When the condition 

(code <= 0xd7ff || code >= 0xe000)
is met, it signifies that the code is not a surrogate, as detailed in UnicodeData.txt:

…
D800;<Non Private Use High Surrogate, First>;Cs;0;L;;;;;N;;;;;
DB7F;<Non Private Use High Surrogate, Last>;Cs;0;L;;;;;N;;;;;
DB80;<Private Use High Surrogate, First>;Cs;0;L;;;;;N;;;;;
DBFF;<Private Use High Surrogate, Last>;Cs;0;L;;;;;N;;;;;
DC00;<Low Surrogate, First>;Cs;0;L;;;;;N;;;;;
DFFF;<Low Surrogate, Last>;Cs;0;L;;;;;N;;;;;
…

The range for surrogates lies between 0xD800 (== 0xd7ff + 1) and 0xDFFF (== 0xe000 - 1).

To decode UTF-16 surrogate pair code and next into a code point (beyond Basic Multilingual Plane), the formula used is 

(code << 10) + next - 0x35fdc00
.

This formula represents another version of a more easily understandable expression:

0x10000 + ((code - 0xD800) << 10) + (next - 0xDC00)

(as discussed in the informative article on The WTF-8 encoding site).

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