Merge pull request #65 from gtm-nayan/perf-stringify

perf: speed up stringify_string

Author: Rich-Harris

src/uneval.js

@@ -8,7 +8,7 @@ import {
 } from './utils.js';
 
 const chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_$';
-const unsafe_chars = /[<>\b\f\n\r\t\0\u2028\u2029]/g;
+const unsafe_chars = /[<\b\f\n\r\t\0\u2028\u2029]/g;
 const reserved =
 	/^(?:do|if|in|for|int|let|new|try|var|byte|case|char|else|enum|goto|long|this|void|with|await|break|catch|class|const|final|float|short|super|throw|while|yield|delete|double|export|import|native|return|switch|throws|typeof|boolean|default|extends|finally|package|private|abstract|continue|debugger|function|volatile|interface|protected|transient|implements|instanceof|synchronized)$/;
 

src/utils.js

@@ -1,15 +1,12 @@
 /** @type {Record<string, string>} */
 export const escaped = {
 	'<': '\\u003C',
-	'>': '\\u003E',
-	'/': '\\u002F',
 	'\\': '\\\\',
 	'\b': '\\b',
 	'\f': '\\f',
 	'\n': '\\n',
 	'\r': '\\r',
 	'\t': '\\t',
-	'\0': '\\u0000',
 	'\u2028': '\\u2028',
 	'\u2029': '\\u2029'
 };
@@ -31,7 +28,9 @@ export function is_primitive(thing) {
 	return Object(thing) !== thing;
 }
 
-const object_proto_names = Object.getOwnPropertyNames(Object.prototype)
+const object_proto_names = /* @__PURE__ */ Object.getOwnPropertyNames(
+	Object.prototype
+)
 	.sort()
 	.join('\0');
 
@@ -51,35 +50,50 @@ export function get_type(thing) {
 	return Object.prototype.toString.call(thing).slice(8, -1);
 }
 
+/** @param {string} char */
+function get_escaped_char(char) {
+	switch (char) {
+		case '"':
+			return '\\"';
+		case '<':
+			return '\\u003C';
+		case '\\':
+			return '\\\\';
+		case '\n':
+			return '\\n';
+		case '\r':
+			return '\\r';
+		case '\t':
+			return '\\t';
+		case '\b':
+			return '\\b';
+		case '\f':
+			return '\\f';
+		case '\u2028':
+			return '\\u2028';
+		case '\u2029':
+			return '\\u2029';
+		default:
+			return char < ' '
+				? `\\u${char.charCodeAt(0).toString(16).padStart(4, '0')}`
+				: '';
+	}
+}
+
 /** @param {string} str */
 export function stringify_string(str) {
-	let result = '"';
-
-	for (let i = 0; i < str.length; i += 1) {
-		const char = str.charAt(i);
-		const code = char.charCodeAt(0);
-
-		if (char === '"') {
-			result += '\\"';
-		} else if (char in escaped) {
-			result += escaped[char];
-		} else if (code <= 0x001F) {
-			result += `\\u${code.toString(16).toUpperCase().padStart(4, "0")}`
-		} else if (code >= 0xd800 && code <= 0xdfff) {
-			const next = str.charCodeAt(i + 1);
+	let result = '';
+	let last_pos = 0;
+	const len = str.length;
 
-			// If this is the beginning of a [high, low] surrogate pair,
-			// add the next two characters, otherwise escape
-			if (code <= 0xdbff && next >= 0xdc00 && next <= 0xdfff) {
-				result += char + str[++i];
-			} else {
-				result += `\\u${code.toString(16).toUpperCase()}`;
-			}
-		} else {
-			result += char;
+	for (let i = 0; i < len; i += 1) {
+		const char = str[i];
+		const replacement = get_escaped_char(char);
+		if (replacement) {
+			result += str.slice(last_pos, i) + replacement;
+			last_pos = i + 1;
 		}
 	}
 
-	result += '"';
-	return result;
+	return `"${last_pos === 0 ? str : result + str.slice(last_pos)}"`;
 }

test/test.js

@@ -167,26 +167,26 @@ const fixtures = {
 		{
 			name: 'lone low surrogate',
 			value: 'a\uDC00b',
-			js: '"a\\uDC00b"',
-			json: '["a\\uDC00b"]'
+			js: '"a\uDC00b"',
+			json: '["a\uDC00b"]'
 		},
 		{
 			name: 'lone high surrogate',
 			value: 'a\uD800b',
-			js: '"a\\uD800b"',
-			json: '["a\\uD800b"]'
+			js: '"a\uD800b"',
+			json: '["a\uD800b"]'
 		},
 		{
 			name: 'two low surrogates',
 			value: 'a\uDC00\uDC00b',
-			js: '"a\\uDC00\\uDC00b"',
-			json: '["a\\uDC00\\uDC00b"]'
+			js: '"a\uDC00\uDC00b"',
+			json: '["a\uDC00\uDC00b"]'
 		},
 		{
 			name: 'two high surrogates',
 			value: 'a\uD800\uD800b',
-			js: '"a\\uD800\\uD800b"',
-			json: '["a\\uD800\\uD800b"]'
+			js: '"a\uD800\uD800b"',
+			json: '["a\uD800\uD800b"]'
 		},
 		{
 			name: 'surrogate pair',
@@ -197,8 +197,8 @@ const fixtures = {
 		{
 			name: 'surrogate pair in wrong order',
 			value: 'a\uDC00\uD800b',
-			js: '"a\\uDC00\\uD800b"',
-			json: '["a\\uDC00\\uD800b"]'
+			js: '"a\uDC00\uD800b"',
+			json: '["a\uDC00\uD800b"]'
 		},
 		{
 			name: 'nul',
@@ -215,8 +215,8 @@ const fixtures = {
 		{
 			name: 'control character extremum',
 			value: '\u001F',
-			js: '"\\u001F"',
-			json: '["\\u001F"]'
+			js: '"\\u001f"',
+			json: '["\\u001f"]'
 		},
 		{
 			name: 'backslash',
@@ -342,20 +342,20 @@ const fixtures = {
 		{
 			name: 'Dangerous string',
 			value: `</script><script src='https://evil.com/script.js'>alert('pwned')</script><script>`,
-			js: `"\\u003C\\u002Fscript\\u003E\\u003Cscript src='https:\\u002F\\u002Fevil.com\\u002Fscript.js'\\u003Ealert('pwned')\\u003C\\u002Fscript\\u003E\\u003Cscript\\u003E"`,
-			json: `["\\u003C\\u002Fscript\\u003E\\u003Cscript src='https:\\u002F\\u002Fevil.com\\u002Fscript.js'\\u003Ealert('pwned')\\u003C\\u002Fscript\\u003E\\u003Cscript\\u003E"]`
+			js: `"\\u003C/script>\\u003Cscript src='https://evil.com/script.js'>alert('pwned')\\u003C/script>\\u003Cscript>"`,
+			json: `["\\u003C/script>\\u003Cscript src='https://evil.com/script.js'>alert('pwned')\\u003C/script>\\u003Cscript>"]`
 		},
 		{
 			name: 'Dangerous key',
 			value: { '<svg onload=alert("xss_works")>': 'bar' },
-			js: '{"\\u003Csvg onload=alert(\\"xss_works\\")\\u003E":"bar"}',
-			json: '[{"\\u003Csvg onload=alert(\\"xss_works\\")\\u003E":1},"bar"]'
+			js: '{"\\u003Csvg onload=alert(\\"xss_works\\")>":"bar"}',
+			json: '[{"\\u003Csvg onload=alert(\\"xss_works\\")>":1},"bar"]'
 		},
 		{
 			name: 'Dangerous regex',
 			value: /[</script><script>alert('xss')//]/,
-			js: `new RegExp("[\\u003C\\u002Fscript\\u003E\\u003Cscript\\u003Ealert('xss')\\u002F\\u002F]", "")`,
-			json: `[["RegExp","[\\u003C\\u002Fscript\\u003E\\u003Cscript\\u003Ealert('xss')\\u002F\\u002F]"]]`
+			js: `new RegExp("[\\u003C/script>\\u003Cscript>alert('xss')//]", "")`,
+			json: `[["RegExp","[\\u003C/script>\\u003Cscript>alert('xss')//]"]]`
 		}
 	],