BIP-128: exact specification for the checksum calculation (#2121)

Author: oren-z0

bip-0128.mediawiki

@@ -156,13 +156,22 @@ The checksum is calculated by converting the top-level JSON object to an array o
 SHA256 hash of the result in lowercase hexadecimal format, and taking a prefix of at least 8
 characters.
 
-For example:
+"stringifying" in this context means converting the array to a JSON string following
+[https://262.ecma-international.org/16.0/index.html#sec-json.stringify ECMAScript's JSON.stringify function specification]
+(without additional <code>replacer</code> or <code>space</code> arguments).
+This specification covers special cases such as non-ascii characters and newlines.
+
+Optional fields without a value should not be included in the key-value pairs array.
+Also, there should not be a key-value pair for the "checksum" field itself.
+
+Code example:
 <source lang="javascript">
+delete recoveryPlanJson.checksum; // In case the checksum was calculated previously and the object was then modified
 const checksumData = new TextEncoder().encode(
   JSON.stringify(Object.entries(recoveryPlanJson).sort()),
 );
 const checksum = new Uint8Array(await crypto.subtle.digest('SHA-256', checksumData));
-const checksumHex = Array.from(checksum).map(b => b.toString(16).padStart(2, '0')).join('').slice(0, 8);
+recoveryPlanJson.checksum = Array.from(checksum).map(b => b.toString(16).padStart(2, '0')).join('').slice(0, 8);
 </source>
 
 Checksum hex string should be at least 8 characters long. Wallets may choose to use a longer