fix: escape trailing whitespace before newlines in template literals …

js/src/javascript/tokenizer.js

@@ -361,6 +361,32 @@ Tokenizer.prototype._read_string = function(c) {
 
     resulting_string = resulting_string.replace(acorn.allLineBreaks, '\n');
 
+    if (c === '`') {
+      // Escape trailing whitespace characters that appear before a newline
+      // inside a template literal. Without this, editors that strip trailing
+      // whitespace (e.g. VS Code) would silently corrupt the template string's
+      // value.  The opening and closing backticks are at positions 0 and last,
+      // so we only touch the content between them.
+      //
+      // Example (issue #2390): a packer may encode \n as a real newline:
+      //   `[ \t<NL>\f\r]`  →  `[ \t\n\f\r]`   (semantically identical)
+      resulting_string = resulting_string.replace(
+        /([^\S\n]+)(\n)/g,
+        function(match, trailing_ws, newline) {
+          // Convert each trailing whitespace character to its escape sequence
+          // so that editors (e.g. VS Code) that strip trailing whitespace on
+          // save will not corrupt the template string's value.
+          var escaped = trailing_ws
+            .replace(/\t/g, '\\t')
+            .replace(/\f/g, '\\f')
+            .replace(/\v/g, '\\v')
+            .replace(/\u00a0/g, '\\u00a0')
+            .replace(/ /g, '\\x20');
+          return escaped + newline;
+        }
+      );
+    }
+
     return this._create_token(TOKEN.STRING, resulting_string);
   }
 

python/jsbeautifier/javascript/tokenizer.py

@@ -329,6 +329,22 @@ def _read_string(self, c):
                 resulting_string += self._input.next()
 
             resulting_string = re.sub(self.acorn.allLineBreaks, "\n", resulting_string)
+            if c == "`":
+
+                def escape_ws(match):
+                    ws = match.group(1)
+                    ws = (
+                        ws.replace("\t", "\\t")
+                        .replace("\f", "\\f")
+                        .replace("\v", "\\v")
+                        .replace("\u00a0", "\\u00a0")
+                        .replace(" ", "\\x20")
+                    )
+                    return ws + match.group(2)
+
+                resulting_string = re.sub(
+                    r"([^\S\n]+)(\n)", escape_ws, resulting_string
+                )
 
             return self._create_token(TOKEN.STRING, resulting_string)
 

test/data/javascript/tests.js

@@ -162,6 +162,19 @@ exports.test_data = {
         { unchanged: 'a = "This is a continuation\\\\\nstring."' },
         { unchanged: '`SELECT\n  nextval(\\\'${this.options.schema ? `${this.options.schema}.` : \\\'\\\'}"${this.tableName}_${this.autoIncrementField}_seq"\\\'::regclass\n  ) nextval;`' },
         {
+          comment: 'Issue #2390 - escape trailing whitespace before real newlines in template literals',
+          fragment: true,
+          input: 'let pattern=`[ \t\n\\\\f\\\\r]`',
+          output: 'let pattern = `[\\\\x20\\\\t\n\\\\f\\\\r]`'
+        }, {
+          comment: 'Issue #2390 - tab-only trailing whitespace is escaped',
+          fragment: true,
+          input: '`foo\t\nbar`',
+          output: '`foo\\\\t\nbar`'
+        }, {
+          comment: 'Issue #2390 - template literal with newlines but no trailing whitespace should be unchanged',
+          unchanged: '`line1\nline2\nline3`'
+        }, {
           comment: 'Tests for #1030',
           unchanged: [
             'const composeUrl = (host) => {',