util: escape lone surrogate code points using .inspect()

lib/internal/util/inspect.js

@@ -176,10 +176,10 @@ const kArrayType = 1;
 const kArrayExtrasType = 2;
 
 /* eslint-disable no-control-regex */
-const strEscapeSequencesRegExp = /[\x00-\x1f\x27\x5c\x7f-\x9f]/;
-const strEscapeSequencesReplacer = /[\x00-\x1f\x27\x5c\x7f-\x9f]/g;
-const strEscapeSequencesRegExpSingle = /[\x00-\x1f\x5c\x7f-\x9f]/;
-const strEscapeSequencesReplacerSingle = /[\x00-\x1f\x5c\x7f-\x9f]/g;
+const strEscapeSequencesRegExp = /[\x00-\x1f\x27\x5c\x7f-\x9f]|[\ud800-\udbff](?![\udc00-\udfff])|(?<![\ud800-\udbff])[\udc00-\udfff]/;
+const strEscapeSequencesReplacer = /[\x00-\x1f\x27\x5c\x7f-\x9f]|[\ud800-\udbff](?![\udc00-\udfff])|(?<![\ud800-\udbff])[\udc00-\udfff]/g;
+const strEscapeSequencesRegExpSingle = /[\x00-\x1f\x5c\x7f-\x9f]|[\ud800-\udbff](?![\udc00-\udfff])|(?<![\ud800-\udbff])[\udc00-\udfff]/;
+const strEscapeSequencesReplacerSingle = /[\x00-\x1f\x5c\x7f-\x9f]|[\ud800-\udbff](?![\udc00-\udfff])|(?<![\ud800-\udbff])[\udc00-\udfff]/g;
 /* eslint-enable no-control-regex */
 
 const keyStrRegExp = /^[a-zA-Z_][a-zA-Z_0-9]*$/;
@@ -458,7 +458,10 @@ function addQuotes(str, quotes) {
   return `'${str}'`;
 }
 
-const escapeFn = (str) => meta[StringPrototypeCharCodeAt(str)];
+function escapeFn(str) {
+  const charCode = StringPrototypeCharCodeAt(str);
+  return meta.length > charCode ? meta[charCode] : `\\u${charCode.toString(16)}`;
+}
 
 // Escape control characters, single quotes and the backslash.
 // This is similar to JSON stringify escaping.
@@ -496,8 +499,7 @@ function strEscape(str) {
 
   let result = '';
   let last = 0;
-  const lastIndex = str.length;
-  for (let i = 0; i < lastIndex; i++) {
+  for (let i = 0; i < str.length; i++) {
     const point = StringPrototypeCharCodeAt(str, i);
     if (point === singleQuote ||
         point === 92 ||
@@ -509,10 +511,20 @@ function strEscape(str) {
         result += `${StringPrototypeSlice(str, last, i)}${meta[point]}`;
       }
       last = i + 1;
+    } else if (point >= 0xd800 && point <= 0xdfff) {
+      if (point <= 0xdbff && i + 1 < str.length) {
+        const point = StringPrototypeCharCodeAt(str, i + 1);
+        if (point >= 0xdc00 && point <= 0xdfff) {
+          i++;
+          continue;
+        }
+      }
+      result += `${StringPrototypeSlice(str, last, i)}${`\\u${point.toString(16)}`}`;
+      last = i + 1;
     }
   }
 
-  if (last !== lastIndex) {
+  if (last !== str.length) {
     result += StringPrototypeSlice(str, last);
   }
   return addQuotes(result, singleQuote);

test/parallel/test-util-inspect.js

@@ -837,6 +837,48 @@ assert.strictEqual(util.inspect(Object.create(Date.prototype)), 'Date {}');
   );
 }
 
+// Escape unpaired surrogate pairs.
+{
+  const edgeChar = String.fromCharCode(0xd799);
+
+  for (let charCode = 0xD800; charCode < 0xDFFF; charCode++) {
+    const surrogate = String.fromCharCode(charCode);
+
+    assert.strictEqual(
+      util.inspect(surrogate),
+      `'\\u${charCode.toString(16)}'`
+    );
+    assert.strictEqual(
+      util.inspect(`${'a'.repeat(200)}${surrogate}`),
+      `'${'a'.repeat(200)}\\u${charCode.toString(16)}'`
+    );
+    assert.strictEqual(
+      util.inspect(`${surrogate}${'a'.repeat(200)}`),
+      `'\\u${charCode.toString(16)}${'a'.repeat(200)}'`
+    );
+    if (charCode < 0xdc00) {
+      const highSurrogate = surrogate;
+      const lowSurrogate = String.fromCharCode(charCode + 1024);
+      assert(
+        !util.inspect(
+          `${edgeChar}${highSurrogate}${lowSurrogate}${edgeChar}`
+        ).includes('\\u')
+      );
+      assert.strictEqual(
+        (util.inspect(
+          `${highSurrogate}${highSurrogate}${lowSurrogate}`
+        ).match(/\\u/g) ?? []).length,
+        1
+      );
+    } else {
+      assert.strictEqual(
+        util.inspect(`${edgeChar}${surrogate}${edgeChar}`),
+        `'${edgeChar}\\u${charCode.toString(16)}${edgeChar}'`
+      );
+    }
+  }
+}
+
 // Test util.inspect.styles and util.inspect.colors.
 {
   function testColorStyle(style, input, implicit) {