"tsc --watch should clear screen on new compilation" Two: Electric Boogaloo

src/compiler/sys.ts

@@ -72,6 +72,7 @@ namespace ts {
         /*@internal*/ debugMode?: boolean;
         setTimeout?(callback: (...args: any[]) => void, ms: number, ...args: any[]): any;
         clearTimeout?(timeoutId: any): void;
+        clearScreen?(): void;
     }
 
     export interface FileWatcher {
@@ -436,6 +437,9 @@ namespace ts {
             }
 
             const nodeSystem: System = {
+                clearScreen: () => {
+                    process.stdout.write("\x1Bc");
+                },
                 args: process.argv.slice(2),
                 newLine: _os.EOL,
                 useCaseSensitiveFileNames,

src/compiler/watch.ts

@@ -493,6 +493,10 @@ namespace ts {
         }
 
         function updateProgram() {
+            if (watchingHost.system.clearScreen) {
+                watchingHost.system.clearScreen();
+            }
+
             timerToUpdateProgram = undefined;
             reportWatchDiagnostic(createCompilerDiagnostic(Diagnostics.File_change_detected_Starting_incremental_compilation));
 

src/harness/unittests/tscWatchMode.ts

@@ -2027,4 +2027,37 @@ declare module "fs" {
             assert.equal(host.readFile(outputFile1), file1.content + host.newLine);
         });
     });
+
+    describe("tsc-watch console clearing", () => {
+        it("doesn't clear the console when it starts", () => {
+            const file = {
+                path: "f.ts",
+                content: ""
+            };
+            const host = createWatchedSystem([file]);
+
+            createWatchModeWithoutConfigFile([file.path], host);
+            host.runQueuedTimeoutCallbacks();
+
+            host.checkScreenClears(0);
+        });
+
+        it("clears the console on recompile", () => {
+            const file = {
+                path: "f.ts",
+                content: ""
+            };
+            const host = createWatchedSystem([file]);
+            createWatchModeWithoutConfigFile([file.path], host);
+
+            const modifiedFile = {
+                ...file,
+                content: "//"
+            };
+            host.reloadFS([modifiedFile]);
+            host.runQueuedTimeoutCallbacks();
+
+            host.checkScreenClears(1);
+        });
+    });
 }

src/harness/virtualFileSystemWithWatch.ts

@@ -251,6 +251,7 @@ interface Array<T> {}`
         private toPath: (f: string) => Path;
         private timeoutCallbacks = new Callbacks();
         private immediateCallbacks = new Callbacks();
+        private screenClears = 0;
 
         readonly watchedDirectories = createMultiMap<TestDirectoryWatcher>();
         readonly watchedDirectoriesRecursive = createMultiMap<TestDirectoryWatcher>();
@@ -604,6 +605,10 @@ interface Array<T> {}`
             this.timeoutCallbacks.unregister(timeoutId);
         }
 
+        clearScreen(): void {
+            this.screenClears += 1;
+        }
+
         checkTimeoutQueueLengthAndRun(expected: number) {
             this.checkTimeoutQueueLength(expected);
             this.runQueuedTimeoutCallbacks();
@@ -638,6 +643,10 @@ interface Array<T> {}`
             this.immediateCallbacks.unregister(timeoutId);
         }
 
+        checkScreenClears(expected: number): void {
+            assert.equal(this.screenClears, expected);
+        }
+
         createDirectory(directoryName: string): void {
             const folder = this.toFolder(directoryName);
 

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -2762,6 +2762,7 @@ declare namespace ts {
         realpath?(path: string): string;
         setTimeout?(callback: (...args: any[]) => void, ms: number, ...args: any[]): any;
         clearTimeout?(timeoutId: any): void;
+        clearScreen?(): void;
     }
     interface FileWatcher {
         close(): void;

tests/baselines/reference/api/typescript.d.ts

@@ -2762,6 +2762,7 @@ declare namespace ts {
         realpath?(path: string): string;
         setTimeout?(callback: (...args: any[]) => void, ms: number, ...args: any[]): any;
         clearTimeout?(timeoutId: any): void;
+        clearScreen?(): void;
     }
     interface FileWatcher {
         close(): void;