Use single replacer for string escaping

src/compiler/utilities.ts

@@ -2466,7 +2466,6 @@ namespace ts {
     const singleQuoteEscapedCharsRegExp = /[\\\'\u0000-\u001f\t\v\f\b\r\n\u2028\u2029\u0085]/g;
     const backtickQuoteEscapedCharsRegExp = /[\\\`\u0000-\u001f\t\v\f\b\r\n\u2028\u2029\u0085]/g;
     const escapedCharsMap = createMapFromTemplate({
-        "\0": "\\0",
         "\t": "\\t",
         "\v": "\\v",
         "\f": "\\f",
@@ -2481,7 +2480,6 @@ namespace ts {
         "\u2029": "\\u2029", // paragraphSeparator
         "\u0085": "\\u0085"  // nextLine
     });
-    const escapedNullRegExp = /\\0[0-9]/g;
 
     /**
      * Based heavily on the abstract 'Quote'/'QuoteJSONString' operation from ECMA-262 (24.3.2.2),
@@ -2493,14 +2491,19 @@ namespace ts {
             quoteChar === CharacterCodes.backtick ? backtickQuoteEscapedCharsRegExp :
             quoteChar === CharacterCodes.singleQuote ? singleQuoteEscapedCharsRegExp :
             doubleQuoteEscapedCharsRegExp;
-        return s.replace(escapedCharsRegExp, getReplacement).replace(escapedNullRegExp, nullReplacement);
+        return s.replace(escapedCharsRegExp, getReplacement);
     }
 
-    function nullReplacement(c: string) {
-        return "\\x00" + c.charAt(c.length - 1);
-    }
-
-    function getReplacement(c: string) {
+    function getReplacement(c: string, offset: number, input: string) {
+        if (c.charCodeAt(0) === CharacterCodes.nullCharacter) {
+            const lookAhead = input.charCodeAt(offset + c.length);
+            if (lookAhead >= CharacterCodes._0 && lookAhead <= CharacterCodes._9) {
+                // If the null character is followed by digits, print as a hex escape to prevent the result from parsing as an octal (which is forbidden in strict mode)
+                return "\\x00";
+            }
+            // Otherwise, keep printing a literal \0 for the null character
+            return "\\0";
+        }
         return escapedCharsMap.get(c) || get16BitUnicodeEscapeSequence(c.charCodeAt(0));
     }
 

tests/baselines/reference/nonstrictTemplateWithNotOctalPrintsAsIs.js

@@ -0,0 +1,8 @@
+//// [nonstrictTemplateWithNotOctalPrintsAsIs.ts]
+// https://github.com/Microsoft/TypeScript/issues/21828
+const d2 = `\\0041`;
+
+
+//// [nonstrictTemplateWithNotOctalPrintsAsIs.js]
+// https://github.com/Microsoft/TypeScript/issues/21828
+var d2 = "\\0041";

tests/baselines/reference/nonstrictTemplateWithNotOctalPrintsAsIs.symbols

@@ -0,0 +1,5 @@
+=== tests/cases/compiler/nonstrictTemplateWithNotOctalPrintsAsIs.ts ===
+// https://github.com/Microsoft/TypeScript/issues/21828
+const d2 = `\\0041`;
+>d2 : Symbol(d2, Decl(nonstrictTemplateWithNotOctalPrintsAsIs.ts, 1, 5))
+

tests/baselines/reference/nonstrictTemplateWithNotOctalPrintsAsIs.types

@@ -0,0 +1,6 @@
+=== tests/cases/compiler/nonstrictTemplateWithNotOctalPrintsAsIs.ts ===
+// https://github.com/Microsoft/TypeScript/issues/21828
+const d2 = `\\0041`;
+>d2 : "\\0041"
+>`\\0041` : "\\0041"
+

tests/cases/compiler/nonstrictTemplateWithNotOctalPrintsAsIs.ts

@@ -0,0 +1,2 @@
+// https://github.com/Microsoft/TypeScript/issues/21828
+const d2 = `\\0041`;