Is jest fully scanning node_modules and then ignoring its content by default ?

⚡ Performance improvement ?

Jest has been slow to start on my computer, which has a relatively slow filesystem (windows ntfs and a few layers of encryption).

Reading the default configuration, I’m wondering whether jest scans every file in the <rootDir> and then ignores the ones in node_modules. I replaced my roots with <rootDir>/src/ instead of <rootDir> and it feels faster.

To Reproduce

• Develop a javascript project with a lot of npm dependencies with many files on a slow filesystem.
• Run jest with no config.
• Run jest with roots: ['<rootDir>/src/'] in its config.

Expected behavior

Jest should scan the files to find the tests relatively fast. But by default I’m not sure it’s the case.

envinfo

System:
    OS: Windows 10 10.0.18363
    CPU: (8) x64 Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
  Binaries:
    Node: 12.13.0 - C:\Program Files\nodejs\node.EXE
    npm: 6.12.0 - C:\Program Files\nodejs\npm.CMD
  npmPackages:
    jest: ^25.4.0 => 25.4.0

[ustun]

See also this config change for watchman so that it ignores node_modules. That speeds up the initial run significantly: https://github.com/facebook/jest/issues/10833#issuecomment-741748347

[lexaurin]

Also worth mentioning that last saved index must be available in jest cache and cache key must be valid otherwise it builds index again.

And completely agree that it should be evident from CLI that indexing is happening and in documentation I think it should be stated how to avoid it.

From jest-haste-map code:

 * The HasteMap is created as follows:
 *  1. read data from the cache or create an empty structure.
 *
 *  2. crawl the file system.
 *     * empty cache: crawl the entire file system.
 *     * cache available:
 *       * if watchman is available: get file system delta changes.
 *       * if watchman is unavailable: crawl the entire file system.