make suggested changes

Author: Rich-Harris

src/index.ts

@@ -11,7 +11,6 @@ const escaped: Record<string, string> = {
 	'\n': '\\n',
 	'\r': '\\r',
 	'\t': '\\t',
-	'\0': '\\u0000',
 	'\u2028': '\\u2028',
 	'\u2029': '\\u2029'
 };
@@ -246,10 +245,10 @@ function stringifyString(str: string) {
 			result += '\\"';
 		} else if (char in escaped) {
 			result += escaped[char];
-		} else if ((code >= 0xD800 && code <= 0xDBFF)) {
+		} else if (code >= 0xD800 && code <= 0xDBFF) {
 			const next = str.charCodeAt(i + 1);
 			if (next >= 0xDC00 && next <= 0xDFFF) {
-				result += char;
+				result += char + str[++i];
 			} else {
 				// lone surrogates
 				result += `\\u${code.toString(16).toUpperCase()}`;

test/test.ts

@@ -38,9 +38,13 @@ describe('devalue', () => {
 	describe('strings', () => {
 		test('newline', 'a\nb', JSON.stringify('a\nb'));
 		test('double quotes', '"yar"', JSON.stringify('"yar"'));
-		test('lone surrogate', "\uD800", '"\\uD800"');
+		test('lone low surrogate', "a\uDC00b", '"a\\uDC00b"');
+		test('lone high surrogate', "a\uD800b", '"a\\uD800b"');
+		test('two low surrogates', "a\uDC00\uDC00b", '"a\\uDC00\\uDC00b"');
+		test('two high surrogates', "a\uD800\uD800b", '"a\\uD800\\uD800b"');
 		test('surrogate pair', '𝌆', JSON.stringify('𝌆'));
-		test('nul', '\0', JSON.stringify('\0'));
+		test('surrogate pair in wrong order', 'a\uDC00\uD800b', '"a\uDC00\uD800b"');
+		test('nul', '\0', '"\0"');
 		test('backslash', '\\', JSON.stringify('\\'));
 	});