Compiler supports configuring how to watch files and directories using compiler flags in TypeScript 3.8+, and environment variables before that.

Background

The --watch implementation of the compiler relies on using fs.watch and fs.watchFile which are provided by node, both of these methods have pros and cons.

fs.watch uses file system events to notify the changes in the file/directory. But this is OS dependent and the notification is not completely reliable and does not work as expected on many OS. Also there could be limit on number of watches that can be created, e.g. linux and we could exhaust it pretty quickly with programs that include large number of files. But because this uses file system events, there is not much CPU cycle involved. Compiler typically uses fs.watch to watch directories (e.g. source directories included by config file, directories in which module resolution failed etc) These can handle the missing precision in notifying about the changes. But recursive watching is supported on only Windows and OSX. That means we need something to replace the recursive nature on other OS.

fs.watchFile uses polling and thus involves CPU cycles. However, fs.watchFile is the most reliable mechanism to get the update on the status of file/directory. The compiler typically uses fs.watchFile to watch source files, config files and missing files (missing file references). This means the CPU usage when using fs.watchFile depends on number of files in the program.

Configuring file watching using a tsconfig.json

{
  // Some typical compiler options
  "compilerOptions": {
    "target": "es2020",
    "moduleResolution": "node"
    // ...
  },
  // NEW: Options for file/directory watching
  "watchOptions": {
    // Use native file system events for files and directories
    "watchFile": "useFsEvents",
    "watchDirectory": "useFsEvents",
    // Poll files for updates more frequently
    // when they're updated a lot.
    "fallbackPolling": "dynamicPriority",
    // Don't coalesce watch notification
    "synchronousWatchDirectory": true,
    // Finally, two additional settings for reducing the amount of possible
    // files to track  work from these directories
    "excludeDirectories": ["**/node_modules", "_build"],
    "excludeFiles": ["build/fileWhichChangesOften.ts"]
  }
}

You can read more about this in the release notes.

Configuring file watching using environment variable TSC_WATCHFILE

Configuring directory watching using environment variable TSC_WATCHDIRECTORY

The watching of directory on platforms that don’t support recursive directory watching natively in node, is supported through recursively creating directory watcher for the child directories using different options selected by TSC_WATCHDIRECTORY. Note that on platforms that support native recursive directory watching (e.g windows) the value of this environment variable is ignored.