Unminified JavaScript: the signals squirrel reads in a file

What minification removes

Minifying JavaScript strips whitespace and comments and shortens local identifiers, which is where most of the saving comes from: a bundle full of descriptive variable names compresses well but still parses every one of those characters.

Most bundlers do this in production mode, though esbuild needs it turned on explicitly. An unminified bundle in production is normally a build that ran in development mode, a minifier that was never configured, or a script that was written by hand and never went through the build at all.

What squirrel checks

The rule runs on every crawled page and looks at three sources. It emits one or two checks: either the pass, or one or both of the findings.

• unminified-js: warns with the message N JavaScript file(s) appear unminified, listing up to five sources with their size and estimated saving. The details carry the total saving and how many more there were.
• potentially-unminified-js: reports at info status with the message N script(s) may not be minified, listing up to five, with the note that these are files whose name has no .min in it.
• minified-js: passes with the message JavaScript appears to be minified when neither list has anything in it.

Content is analysed when the rule has it: inline <script> blocks of 2,048 characters or more, excluding any whose type mentions json or template, and external scripts the crawl fetched the body of, which are also skipped below 2,048 recorded bytes. Any run of leading /*! ... */ or //! ... licence banners is stripped first, and anything under 200 characters before or after that is treated as minified. A file counts as unminified when any of these fires: more than 0.5% newlines, more than three comments, more than one long variable declaration per 500 characters, more than five long function names, indentation on more than one line per 200 characters, or more than 5% of the file in the surplus characters of repeated whitespace runs, counting each run’s length minus one.

A script the rule has no content for is judged by filename. It is suspected when the name contains none of .min., -min. or .prod., has no dot-or-hyphen followed by eight or more hex characters and another dot, does not contain chunk, bundle or vendor, does not come from a URL containing cdn, unpkg or jsdelivr, and ends in .js or .mjs. The cloudflare exclusion applies only to scripts from the crawl’s own list, not to those found in the markup alone. A page with no parsed document emits nothing at all.

Elements inside <noscript> are skipped: a browser with scripting enabled reads <noscript> content as plain text, so nothing in it loads or renders. See content inside noscript.

How to fix it

Run the production build, and check that minification is turned on:

esbuild app.js --bundle --minify --outfile=dist/app.js

Vite and webpack minify in production mode by default; esbuild does not unless asked.

The common false positive is the filename half of the rule. A perfectly minified app.js is still reported at info status, because the rule read the name rather than the bytes. A content hash in the filename, which most bundlers add anyway, clears it.

Options

Configuration example

[rules."perf/unminified-js"]
min_size_bytes = 2048

Enable / disable

Disable this rule

[rules]
disable = ["perf/unminified-js"]

Disable all Performance rules

[rules]
disable = ["perf/*"]

Enable only this rule

[rules]
enable = ["perf/unminified-js"]

enable replaces the default ["*"], so this is the only rule selected. Leave disable out: a rule that matches any disable pattern is skipped even when enable names it. Only an enabled setting under a rule’s own [rule_options] table overrides both lists.

Related rules

• perf/js-file-size: the size those bytes add up to.
• perf/unminified-css: the same analysis on stylesheets.
• perf/source-maps: the other sign that a build shipped development output.
• perf/legacy-js: transpilation output and polyfills, another kind of bundle bloat.

Performance findings ship in every audit next to the SEO, security and agent experience rules. See Core Web Vitals for how an agent works through a report.

References

• Reduce network payloads using text compression, web.dev
• The script element, HTML Standard

Check your site

Run squirrel audit https://example.com and open the Performance section of the report. Each script is listed with the reason it was flagged. Signed-out local audits need no account and cost nothing. Signed-in audits spend credits.