Cleanup getDiagnosticsForProject (#18151)

Author: Andy

src/server/session.ts

@@ -1609,7 +1609,10 @@ namespace ts.server {
             }
 
             // No need to analyze lib.d.ts
-            let fileNamesInProject = fileNames.filter(value => value.indexOf("lib.d.ts") < 0);
+            const fileNamesInProject = fileNames.filter(value => value.indexOf("lib.d.ts") < 0);
+            if (fileNamesInProject.length === 0) {
+                return;
+            }
 
             // Sort the file name list to make the recently touched files come first
             const highPriorityFiles: NormalizedPath[] = [];
@@ -1625,7 +1628,7 @@ namespace ts.server {
                 else {
                     const info = this.projectService.getScriptInfo(fileNameInProject);
                     if (!info.isScriptOpen()) {
-                        if (fileNameInProject.indexOf(Extension.Dts) > 0) {
+                        if (fileExtensionIs(fileNameInProject, Extension.Dts)) {
                             veryLowPriorityFiles.push(fileNameInProject);
                         }
                         else {
@@ -1638,14 +1641,11 @@ namespace ts.server {
                 }
             }
 
-            fileNamesInProject = highPriorityFiles.concat(mediumPriorityFiles).concat(lowPriorityFiles).concat(veryLowPriorityFiles);
-
-            if (fileNamesInProject.length > 0) {
-                const checkList = fileNamesInProject.map(fileName => ({ fileName, project }));
-                // Project level error analysis runs on background files too, therefore
-                // doesn't require the file to be opened
-                this.updateErrorCheck(next, checkList, delay, /*requireOpen*/ false);
-            }
+            const sortedFiles = [...highPriorityFiles, ...mediumPriorityFiles, ...lowPriorityFiles, ...veryLowPriorityFiles];
+            const checkList = sortedFiles.map(fileName => ({ fileName, project }));
+            // Project level error analysis runs on background files too, therefore
+            // doesn't require the file to be opened
+            this.updateErrorCheck(next, checkList, delay, /*requireOpen*/ false);
         }
 
         getCanonicalFileName(fileName: string) {