Performance: Cache regular expression instead of creating anew for every file in ScriptTransformer.

[scotthovestadt]

Summary

This PR improves performance by caching the regular expression string -> RegExp conversion.

Benchmarks on cached regular expression vs. creating new each time:

  new regex x   596 ops/sec ±0.95% (97 runs sampled)
  cached regex  x 1,668 ops/sec ±0.61% (97 runs sampled)

For large projects, Jest may process a LOT of files. The more workers in use, the higher chance that the same file is processed multiple times. Additionally, each time it's hit multiple regular expressions may be tested against.

The benchmark I used:

suite.add('new regex', function() {
  for (var i = 0; i < 10000; i++) {
    new RegExp('^.+\\.[jt]sx?$').test(
      'jest/packages/jest-haste-map/src/__tests__/haste_impl.js'
    );
  }
});

suite.add('cached regex', function() {
  const regex = new RegExp('^.+\\.[jt]sx?$');
  for (var i = 0; i < 10000; i++) {
    regex.test(
      'jest/packages/jest-haste-map/src/__tests__/haste_impl.js'
    );
  }
});

Test plan

• All tests pass.
• Performance benefit verified.

[codecov-io]

Codecov Report

Merging #8235 into master will increase coverage by 0.01%.
The diff coverage is 80.95%.

@@            Coverage Diff             @@
##           master    #8235      +/-   ##
==========================================
+ Coverage   62.34%   62.36%   +0.01%     
==========================================
  Files         265      265              
  Lines       10559    10569      +10     
  Branches     2568     2570       +2     
==========================================
+ Hits         6583     6591       +8     
- Misses       3387     3388       +1     
- Partials      589      590       +1

Impacted Files	Coverage Δ
packages/jest-transform/src/ScriptTransformer.ts	80.76% <80.95%> (-0.05%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 384a0d9...6107b50. Read the comment docs.

[thymikee]

Looks good!

[SimenB]

When can this be falsy?

[scotthovestadt]

When the array passed from config with transforms is 0-length.

[SimenB]

Ah, transform: {}? Gotcha

[scotthovestadt]

calcTransformRegExp returns undefined if config.transform.length === 0.

I try not to return empty arrays to loop through when the function should actually be telling you "nothing to do here". I personally find it makes the code more readable and sometimes make optimizations in the future possible.

[SimenB]

this changed introduced a lint warning

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.
Please note this issue tracker is not a help forum. We recommend using StackOverflow or our discord channel for questions.