Is rewrite_javascript enabled by default?

Yes. rewrite_javascript is in CoreFilters, the default RewriteLevel, so it runs unless you turn it off with DisableFilters. It is also part of the OptimizeForBandwidth level.

How do I turn off rewrite_javascript?

Add ModPagespeedDisableFilters rewrite_javascript on Apache or pagespeed DisableFilters rewrite_javascript; on nginx. On IIS, add pagespeed DisableFilters rewrite_javascript to pagespeed.config.

Minify JavaScript (rewrite_javascript)

Category:: JavaScript
CoreFilters:: Yes
OptimizeForBandwidth:: Yes
Risk:: Generally safe

The three filters on this section share one minifier. It removes comments and collapses whitespace; identifiers, string literals, regular expressions and property names are emitted unchanged. It keeps a line break wherever automatic semicolon insertion could otherwise merge two statements into one.

What it switches on

rewrite_javascript is a compound name. Enabling it enables these 2 filters; each can be disabled on its own.

rewrite_javascript_external — Minify external JavaScript
rewrite_javascript_inline — Minify inline JavaScript

How it works

How safe JavaScript minification handles automatic semicolon insertion has the details.

rewrite_javascript is the compound CoreFilter for JavaScript minification: enabling it switches on the external and the inline sub-filter together, so every script on the page is minified wherever it lives. It is also part of OptimizeForBandwidth, the level that minifies resources in place without changing their URLs. To minify only one scope, enable rewrite_javascript and disable the other sub-filter by name. Under CoreFilters the compound is on already. A script delivered with an integrity attribute is left untouched by the whole family, because subresource integrity pins the file’s bytes. Since v1.15.0+r21 the tokenizer-based minifier is the only one behind rewrite_javascript, and files that use template literals (backtick strings) minify normally; IE conditional-compilation comments (/*@ ... @*/) and a leading #! line are the only comments it keeps. Live demo: rewrite_javascript.

rewrite_javascript_external handles the external half. Each <script src> file on a domain the module is authorized to fetch is minified and served from a rewritten .pagespeed.jm. URL with a long cache lifetime, so repeat visitors download the minified file once and keep it; the original file on disk is never modified. In OptimizeForBandwidth mode the minified bytes replace the original response in place and the URL stays as authored. Minification saves the most on hand-formatted source; a file a bundler already minified gains nothing and only costs rewrite time. rewrite_javascript_external runs as part of the compound and can also be enabled on its own.

Enabled on its own, rewrite_javascript_external leaves inline <script> blocks exactly as authored; minifying those is the inline sub-filter’s job. Files on domains the configuration does not authorize keep their original URLs as well, so a page that mixes own and third-party scripts sees only the own files rewritten.

rewrite_javascript_inline minifies the contents of <script> blocks in the HTML itself, in place. No URL changes and nothing new is cached; the smaller script simply rides along inside every page view. Blocks whose type does not denote executable JavaScript, such as JSON-LD data islands and HTML templates, are left as they are. Inline scripts tend to be short, so the saving per block is small; the filter’s job inside the compound is to leave no script unminified. Like the external half, rewrite_javascript_inline is in CoreFilters and in OptimizeForBandwidth, and it can be enabled by itself.

rewrite_javascript_inline never moves a block or changes when it runs; it only shrinks the text between <script> and </script>. On pages whose HTML is served many times between deploys that small saving repeats on every serve, which is where the filter earns its place.

When to use it

In CoreFilters, the default RewriteLevel: it runs unless you disable it.
Part of OptimizeForBandwidth, the level that optimizes resources without changing their URLs.
Risk rating on these docs: Generally safe.

Risks

The minifier never renames identifiers; the historical failure mode is a script that inspects its own source text, for example through Function.prototype.toString(), and reacts to the changed formatting.
Verify with the X-Mod-Pagespeed response header and a ?PageSpeedFilters=-rewrite_javascript comparison; Is it working? has the steps.
rewrite_javascript ignores the deprecated UseExperimentalJsMinifier directive, which is accepted for compatibility and logs a warning at configuration load (ModPagespeedUseExperimentalJsMinifier on Apache, pagespeed UseExperimentalJsMinifier on nginx). Remove it from your configuration.

Configuration

Enable it in the module configuration, at server, virtual-host or location scope:

Apache

ModPagespeedEnableFilters rewrite_javascript

nginx

pagespeed EnableFilters rewrite_javascript;

IIS (pagespeed.config)

pagespeed EnableFilters rewrite_javascript

It is on by default under CoreFilters; to turn it off:

ModPagespeedDisableFilters rewrite_javascript   # Apache
pagespeed DisableFilters rewrite_javascript;   # nginx
pagespeed DisableFilters rewrite_javascript   # IIS

/* before: sum a shopping cart */
function cartTotal(cart) {
  var total = 0;  // running sum
  for (var i = 0; i < cart.items.length; i++) {
    total = total + cart.items[i].price;
  }
  return total;
}

/* after */
function cartTotal(cart){var total=0;for(var i=0;i<cart.items.length;i++){total=total+cart.items[i].price;}return total;}

On the worker

The worker runs its own pipeline, configured by flags. Its equivalent of this filter is the js minification transform. Always-on under pagespeed on;; use --disable-js to skip JS rewriting entirely.

Scoping, ForbidFilters and the thresholds filters read: Choosing filters .

Live example

Strips comments and whitespace from JavaScript.

Measured on the demo page, original against optimized:

Total bytes

1.8 KB → 1.3 KB −26%
HTML size

1.6 KB → 1.3 KB −17%

See the before and after, with the source diff →

Frequently asked questions

What does the rewrite_javascript filter do?: The three filters on this section share one minifier. It removes comments and collapses whitespace; identifiers, string literals, regular expressions and property names are emitted unchanged. It keeps a line break wherever automatic semicolon insertion could otherwise merge two statements into one.
Is rewrite_javascript enabled by default?: Yes. rewrite_javascript is in CoreFilters, the default RewriteLevel, so it runs unless you turn it off with DisableFilters. It is also part of the OptimizeForBandwidth level.
How do I turn off rewrite_javascript?: Add ModPagespeedDisableFilters rewrite_javascript on Apache or pagespeed DisableFilters rewrite_javascript; on nginx. On IIS, add pagespeed DisableFilters rewrite_javascript to pagespeed.config.

combine_javascript — Combine JavaScript
inline_javascript — Inline JavaScript
defer_javascript — Defer JavaScript
rewrite_javascript_external — Minify external JavaScript
rewrite_javascript_inline — Minify inline JavaScript
extend_cache_scripts — Extend cache for scripts
include_js_source_maps — Include JS source maps
make_show_ads_async — Async AdSense

This page is drawn from the rewrite_javascript entry on JavaScript filters. Every filter in one table: PageSpeed filters.