Configuring Watch

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


The --watch implementation of the compiler relies on using and fs.watchFile which are provided by node, both of these methods have pros and cons. 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, eg. 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 to watch directories (eg. 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. But this is the most reliable mechanism to get the update on the status of file/directory. Compiler typically uses fs.watchFile to watch source files, config files and missing files (missing file references) that means the CPU usage 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"

You can read more about this in the release notes.

Configuring file watching using environment variable TSC_WATCHFILE

Option Description
PriorityPollingInterval Use fs.watchFile but use different polling intervals for source files, config files and missing files
DynamicPriorityPolling Use a dynamic queue where in the frequently modified files will be polled at shorter interval and the files unchanged will be polled less frequently
UseFsEvents Use which uses file system events (but might not be accurate on different OS) to get the notifications for the file changes/creation/deletion. Note that few OS eg. linux has limit on number of watches and failing to create watcher using will result it in creating using fs.watchFile
UseFsEventsWithFallbackDynamicPolling This option is similar to UseFsEvents except on failing to create watch using, the fallback watching happens through dynamic polling queues (as explained in DynamicPriorityPolling)
UseFsEventsOnParentDirectory This option watches parent directory of the file with (using file system events) thus being low on CPU but can compromise accuracy.
default (no value specified) If environment variable TSC_NONPOLLING_WATCHER is set to true, watches parent directory of files (just like UseFsEventsOnParentDirectory). Otherwise watch files using fs.watchFile with 250ms as the timeout for any file

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.

Option Description
RecursiveDirectoryUsingFsWatchFile Use fs.watchFile to watch the directories and child directories which is a polling watch (consuming CPU cycles)
RecursiveDirectoryUsingDynamicPriorityPolling Use dynamic polling queue to poll changes to the directory and child directories.
default (no value specified) Use to watch directories and child directories