API. CreateWatchProgram emits all files even when they have not changed

[Yegorich555]

TypeScript Version: 3.7.4

Search Terms:
ts.createWatchProgram is writing to disk too often; ts.createWatchProgram writes even no changes
Code

const ts = require("typescript");

// creating hooks
const sysConfig = { ...ts.sys };

sysConfig.writeFile = function writeFileWrapper(path) {
  console.warn("write", path);
  return ts.sys.writeFile(...arguments);
};

sysConfig.readFile = function readFile(path) {
  console.warn("read", path);
  return ts.sys.readFile(...arguments);
};

const host = ts.createWatchCompilerHost(
  ["./src/mockServer.ts"],
  {
    esModuleInterop: true,
    allowJs: true,
    skipLibCheck: true,
    outDir: "./dist",
    module: ts.ModuleKind.CommonJS
    // todo wait for transpileOnly option: https://github.com/microsoft/TypeScript/issues/29651
  },
  sysConfig,
  ts.createSemanticDiagnosticsBuilderProgram
);
ts.createWatchProgram(host);

Expected behavior:
One file changed - one writing on disk
One file re-saved but not changed - no writing on disk

Actual behavior:
One file changed - every file is written on disk

Playground Link:

1. clone the repo: https://github.com/Yegorich555/webpack-mock-server/tree/tsError
2. npm i
3. npm run serve
4. try to re-save any /src/*.ts file
5. take a look at console under write tags

Related Issues:
#29927

Notes:

1. tsc --watch works as expected, but ts.createWatchProgram doesn't work as expected
2. screenshot with explanation:
   

Additional question

1. How can I detect that compilation is done? Does compilationCallback exist? diagnostic callback looks like has a different meaning.

[sheetalkamat]

You are using createSemanticDiagnosticsBuilderProgram which is meant only to get errors efficiently for changed files. Emit of files is not tracked.. To support emit you need to use createEmitAndSemanticDiagnosticsBuilderProgram

[Yegorich555]

Oh, thanks a lot. I spent a some days on investigation and missed this. createEmitAndSemanticDiagnosticsBuilderProgram really works fine now.

One question I have, how can I detect in a perfect way that was changes and compilation done: it should be a callback for restarting compiled files.
Right now I detect ts.sys.writeFile and in diagnosticReport restart child_process, but it looks not pretty.
Could you suggest a smarter way?

[sheetalkamat]

https://github.com/microsoft/TypeScript/wiki/Using-the-Compiler-API#writing-an-incremental-program-watcher shows how to use the API. reportWatchStatusChanged is called with diagnostics for starting compilation and watch complete diagnostics.

[Yegorich555]

Thanks a lot. I use this as I mentioned upper