[question] format: 'esm' generates var instead of let or const

[BePo65]

If I run esbuild to generate esm output with the following options:

  platform: 'browser',
  format: 'esm',
  bundle: true,
  target: 'es2020'

the created output contains var xx = class ... for every class defined in the source file in addition to the expected export {...}.

Question: is the use of var by intention (e.g. for use in "classical" html pages) or should it better be const to avoid cluttering the global context / window object?

[nettybun]

It's intentional because let/const introduces a "TDZ" Temporal Dead Zone which is fine in theory but in practice caused extreme performance issues in Safari. #478. There was some talk with the WebKit team I think. Ultimately switched everything to var :)

ESM is for modules so the code will run as an ESM import or as a <script type=module ... block, both of which do not clutter the global context/window - you need to explicitly say window.x = to do that.

If you're running the ESM output code in a normal <script> block and are worried about the global context then use format IIFE.

[BePo65]

oh, so much for my knowledge about using var in a browser environment ;-)

Thank you for your very detailed answer.