fix lone surrogate handling, add tests

Author: Rich-Harris

src/index.ts

@@ -1,7 +1,20 @@
 const chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_$';
 const reserved = /^(?:do|if|in|for|int|let|new|try|var|byte|case|char|else|enum|goto|long|this|void|with|await|break|catch|class|const|final|float|short|super|throw|while|yield|delete|double|export|import|native|return|switch|throws|typeof|boolean|default|extends|finally|package|private|abstract|continue|debugger|function|volatile|interface|protected|transient|implements|instanceof|synchronized)$/;
 const unsafe = /[<>\/\u2028\u2029]/g;
-const escaped: Record<string, string> = { '<': '\\u003C', '>' : '\\u003E', '/': '\\u002F', '\u2028': '\\u2028', '\u2029': '\\u2029' };
+const escaped: Record<string, string> = {
+	'<': '\\u003C',
+	'>' : '\\u003E',
+	'/': '\\u002F',
+	'\\': '\\\\',
+	'\b': '\\b',
+	'\f': '\\f',
+	'\n': '\\n',
+	'\r': '\\r',
+	'\t': '\\t',
+	'\0': '\\u0000',
+	'\u2028': '\\u2028',
+	'\u2029': '\\u2029'
+};
 const objectProtoOwnPropertyNames = Object.getOwnPropertyNames(Object.prototype).sort().join('\0');
 
 export default function devalue(value: any) {
@@ -226,16 +239,21 @@ function stringifyString(str: string) {
 	let result = '"';
 
 	for (let i = 0; i < str.length; i += 1) {
-		const char = str[i];
+		const char = str.charAt(i);
 		const code = char.charCodeAt(0);
 
 		if (char === '"') {
 			result += '\\"';
 		} else if (char in escaped) {
 			result += escaped[char];
-		} else if ((code >= 0xD800 && code <= 0xDBFF) && i < str.length - 1) {
-			// escape lone surrogates
-			result += `\\\\u${code.toString(16).toUpperCase()}`;
+		} else if ((code >= 0xD800 && code <= 0xDBFF)) {
+			const next = str.charCodeAt(i + 1);
+			if (next >= 0xDC00 && next <= 0xDFFF) {
+				result += char;
+			} else {
+				// lone surrogates
+				result += `\\u${code.toString(16).toUpperCase()}`;
+			}
 		} else {
 			result += char;
 		}

test/test.ts

@@ -35,6 +35,15 @@ describe('devalue', () => {
 		test('Map', new Map([['a', 'b']]), 'new Map([["a","b"]])');
 	});
 
+	describe('strings', () => {
+		test('newline', 'a\nb', JSON.stringify('a\nb'));
+		test('double quotes', '"yar"', JSON.stringify('"yar"'));
+		test('lone surrogate', "\uD800", '"\\uD800"');
+		test('surrogate pair', '𝌆', JSON.stringify('𝌆'));
+		test('nul', '\0', JSON.stringify('\0'));
+		test('backslash', '\\', JSON.stringify('\\'));
+	});
+
 	describe('cycles', () => {
 		let map = new Map();
 		map.set('self', map);