In addRange, don't check for undefined values; do support slice

src/compiler/core.ts

@@ -755,15 +755,16 @@ namespace ts {
     /**
      * Appends a range of value to an array, returning the array.
      *
-     * @param to The array to which `value` is to be appended. If `to` is `undefined`, a new array
-     * is created if `value` was appended.
-     * @param from The values to append to the array. If `from` is `undefined`, nothing is
-     * appended. If an element of `from` is `undefined`, that element is not appended.
+     * @param to The array to which values are to be appended. If `to` is `undefined` and `from` is non-empty, a new array will be created.
+     * @param from The values to append to the array. If `from` is `undefined`, nothing is appended.
+     * @param sliceStart Index of the first element in `from` to append. (inclusive)
+     * @param sliceEnd Index after the last element in `from` to append. (exclusive)
      */
-    export function addRange<T>(to: T[] | undefined, from: T[] | undefined): T[] | undefined {
-        if (from === undefined) return to;
-        for (const v of from) {
-            to = append(to, v);
+    export function addRange<T>(to: T[] | undefined, from: T[] | undefined, sliceStart = 0, sliceEnd = from && from.length): T[] | undefined {
+        if (from === undefined || from.length === 0) return to;
+        to = to || [];
+        for (let i = sliceStart; i < sliceEnd; i++) {
+            to.push(from[i]);
         }
         return to;
     }

src/compiler/factory.ts

@@ -2485,8 +2485,8 @@ namespace ts {
             helpers
         } = sourceEmitNode;
         if (!destEmitNode) destEmitNode = {};
-        if (leadingComments) destEmitNode.leadingComments = addRange(leadingComments.slice(), destEmitNode.leadingComments);
-        if (trailingComments) destEmitNode.trailingComments = addRange(trailingComments.slice(), destEmitNode.trailingComments);
+        if (leadingComments) destEmitNode.leadingComments = concatenate(leadingComments, destEmitNode.leadingComments);
+        if (trailingComments) destEmitNode.trailingComments = concatenate(trailingComments, destEmitNode.trailingComments);
         if (flags) destEmitNode.flags = flags;
         if (commentRange) destEmitNode.commentRange = commentRange;
         if (sourceMapRange) destEmitNode.sourceMapRange = sourceMapRange;

src/compiler/transformers/esnext.ts

@@ -667,7 +667,7 @@ namespace ts {
             const trailingStatements = endLexicalEnvironment();
             if (statementOffset > 0 || some(statements) || some(trailingStatements)) {
                 const block = convertToFunctionBody(body, /*multiLine*/ true);
-                addRange(statements, block.statements.slice(statementOffset));
+                addRange(statements, block.statements, statementOffset);
                 addRange(statements, trailingStatements);
                 return updateBlock(block, setTextRange(createNodeArray(statements), block.statements));
             }