Use an array for building up mappings in the sourcemap generator.

DanielRosenwasser · web-flow · commit 4c86404b4149 · 2021-04-23T01:07:28.000Z
diff --git a/src/compiler/sourcemap.ts b/src/compiler/sourcemap.ts
@@ -17,7 +17,7 @@ namespace ts {
 
         const names: string[] = [];
         let nameToNameIndexMap: ESMap<string, number> | undefined;
-        let mappings = "";
+        let mappings: string[] = [];
 
         // Last recorded and encoded mappings
         let lastGeneratedLine = 0;
@@ -221,7 +221,7 @@ namespace ts {
             if (lastGeneratedLine < pendingGeneratedLine) {
                 // Emit line delimiters
                 do {
-                    mappings += ";";
+                    mappings.push(";");
                     lastGeneratedLine++;
                     lastGeneratedCharacter = 0;
                 }
@@ -231,30 +231,30 @@ namespace ts {
                 Debug.assertEqual(lastGeneratedLine, pendingGeneratedLine, "generatedLine cannot backtrack");
                 // Emit comma to separate the entry
                 if (hasLast) {
-                    mappings += ",";
+                    mappings.push(",");
                 }
             }
 
             // 1. Relative generated character
-            mappings += base64VLQFormatEncode(pendingGeneratedCharacter - lastGeneratedCharacter);
+            mappings.push(base64VLQFormatEncode(pendingGeneratedCharacter - lastGeneratedCharacter));
             lastGeneratedCharacter = pendingGeneratedCharacter;
 
             if (hasPendingSource) {
                 // 2. Relative sourceIndex
-                mappings += base64VLQFormatEncode(pendingSourceIndex - lastSourceIndex);
+                mappings.push(base64VLQFormatEncode(pendingSourceIndex - lastSourceIndex));
                 lastSourceIndex = pendingSourceIndex;
 
                 // 3. Relative source line
-                mappings += base64VLQFormatEncode(pendingSourceLine - lastSourceLine);
+                mappings.push(base64VLQFormatEncode(pendingSourceLine - lastSourceLine));
                 lastSourceLine = pendingSourceLine;
 
                 // 4. Relative source character
-                mappings += base64VLQFormatEncode(pendingSourceCharacter - lastSourceCharacter);
+                mappings.push(base64VLQFormatEncode(pendingSourceCharacter - lastSourceCharacter));
                 lastSourceCharacter = pendingSourceCharacter;
 
                 if (hasPendingName) {
                     // 5. Relative nameIndex
-                    mappings += base64VLQFormatEncode(pendingNameIndex - lastNameIndex);
+                    mappings.push(base64VLQFormatEncode(pendingNameIndex - lastNameIndex));
                     lastNameIndex = pendingNameIndex;
                 }
             }
@@ -271,7 +271,7 @@ namespace ts {
                 sourceRoot,
                 sources,
                 names,
-                mappings,
+                mappings: mappings.join(),
                 sourcesContent,
             };
         }
