Retain trailing comments in array expressions (babel#10369)

* Retain trailing comments in array expressions

This is a proposed fix for babel#10368
with a simple test.

* Move lastElement in the block where it's used

* Test trailing comment after array expression

* Don't move comments after the array expression

* Retain trailing comment after the array expression

packages/babel-parser/src/parser/comments.js

@@ -86,7 +86,7 @@ export default class CommentsParser extends BaseParser {
 
     // Attach comments that follow a trailing comma on the last
     // property in an object literal or a trailing comma in function arguments
-    // as trailing comments
+    // or a trailing comma in array expressions as trailing comments
     if (firstChild && this.state.leadingComments.length > 0) {
       const lastComment = last(this.state.leadingComments);
 
@@ -137,6 +137,32 @@ export default class CommentsParser extends BaseParser {
             }
           }
         }
+      } else if (node.type === "ArrayExpression" && node.elements.length > 0) {
+        if (this.state.commentPreviousNode) {
+          for (j = 0; j < this.state.leadingComments.length; j++) {
+            if (
+              this.state.leadingComments[j].end <
+              this.state.commentPreviousNode.end
+            ) {
+              this.state.leadingComments.splice(j, 1);
+              j--;
+            }
+          }
+
+          const lastElement = last(node.elements);
+          lastElement.trailingComments = [];
+          while (this.state.leadingComments.length) {
+            const leadingComment = this.state.leadingComments.shift();
+            if (leadingComment.end < node.end) {
+              lastElement.trailingComments.push(leadingComment);
+            } else {
+              if (node.trailingComments === undefined) {
+                node.trailingComments = [];
+              }
+              node.trailingComments.push(leadingComment);
+            }
+          }
+        }
       }
     }
 

packages/babel-parser/test/fixtures/comments/basic/array-expression-trailing-comma/input.js

@@ -0,0 +1,18 @@
+const nonTrailing = [
+  "One", // One
+   // Two
+  "Two" // Three
+  // Four
+]
+
+const trailingAfterComma = [
+  "One", // One
+   // Two
+  "Two", // Three
+  // Four
+]
+
+const trailingAfterArray = [
+  "One", // One
+   // Two
+] // Three