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Add parentheses to factory function of UMD bundles to avoid lazy parsing #3183

merged 1 commit into from Oct 23, 2019

Add parentheses to factory function of UMD bundles to avoid lazy parsing #3183

merged 1 commit into from Oct 23, 2019


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@ajihyf ajihyf commented Oct 22, 2019

This PR contains:

  • bugfix
  • feature
  • refactor
  • documentation
  • other

Are tests included?

  • yes (bugfixes and features will not be merged without tests)
  • no

Breaking Changes?

  • yes (breaking changes will not be merged unless absolutely necessary)
  • no

List any relevant issue numbers:


Most modern javascript engines use lazy parsing technique to boost startup performance, which means function will not be fully parsed until it is executed. For IIFE or soon-be-called callback function, we should use parentheses to wrap it to avoid double parsing(lazy parsing + full parsing).

Tested using the previous benchmark, unwrapped bundles are a lot slower to load. You can also use this gist to test locally using node.

Unwrapped Module

Wrapped Module

The benchmarks above are all wrong. We should use eval. Example gist:

There was already #774 to solve this issue. However, the latest Rollup lost this functionality. This PR just adds parentheses back and some comments to prevent regression.

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codecov bot commented Oct 22, 2019

Codecov Report

Merging #3183 into master will not change coverage.
The diff coverage is 100%.

Impacted file tree graph

@@           Coverage Diff           @@
##           master    #3183   +/-   ##
  Coverage   90.19%   90.19%           
  Files         165      165           
  Lines        5905     5905           
  Branches     1797     1797           
  Hits         5326     5326           
  Misses        350      350           
  Partials      229      229
Impacted Files Coverage Δ
src/finalisers/umd.ts 94.91% <100%> (ø) ⬆️

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@lukastaegert lukastaegert left a comment

Thanks a lot! Seeing that I am most probably the responsible person, I am really sorry I did not see the context and just thought this was unneeded noise. To my defence, #774 was more than a year before my time at Rollup. So the comments are really needed, as they would have given me pause when I removed the parentheses. Your perf benchmark is very convincing to underline this is definitely a thing!

@lukastaegert lukastaegert merged commit dae509d into rollup:master Oct 23, 2019
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@ajihyf ajihyf deleted the fix_lazy_parsing_regression branch Oct 23, 2019
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2 participants