build: rebuild committed build outputs only at release

CONTRIBUTING.md

@@ -16,3 +16,7 @@ ci: add a dev container
 chore: declare the Node version for local development
 docs: trim local setup to the essentials
 ```
+
+## Build outputs
+
+`imagediff.js` and `imagediff.min.js` are generated by `npm run build` and committed for releases. Rebuild and commit them only when preparing a release; ordinary PRs change `js/imagediff.js` and leave the built files alone. `npm test` runs against the source, including the CLI, so nothing needs rebuilding to test a change.

README.md

@@ -88,11 +88,11 @@ npm test
 
 ### Scripts
 
-* `npm test` runs the specs in Node.js and then in headless Chromium, and fails if either fails.
+* `npm test` runs the specs in Node.js and then in headless Chromium, and fails if either fails. The specs and the CLI use the source in `js/`, so no build is needed first.
 * `npm run test:node` runs the specs, including a smoke test of the `imagediff` CLI, in Node.js against [node-canvas](https://github.com/Automattic/node-canvas) (the optional `canvas` dependency, which `npm ci` installs from prebuilt binaries on common platforms).
 * `npm run test:browser` runs the specs in headless Chromium with Playwright.
 * `npm run test:serve` serves the specs at http://localhost:8085/spec/ to watch them run in a browser.
-* `npm run build` (or `make imagediff`) writes `imagediff.js` (license header + `js/imagediff.js`) and `imagediff.min.js`, minified with [terser](https://terser.org/) to ES5 output.
+* `npm run build` (or `make imagediff`) writes `imagediff.js` (license header + `js/imagediff.js`) and `imagediff.min.js`, minified with [terser](https://terser.org/) to ES5 output. Run it only when preparing a release; see [CONTRIBUTING.md](CONTRIBUTING.md#build-outputs).
 
 ### Dev container
 

bin/imagediff

@@ -1,7 +1,7 @@
 #!/usr/bin/env node
 var
   fs = require('fs'),
-  imagediff = require('../imagediff.js'),
+  imagediff = require('../js/imagediff.js'),
   Canvas = require('canvas');
 
 commandLine(process.argv.slice(2));

spec/LoadSpec.js

@@ -1,12 +1,12 @@
-// Loads the built library in a fresh Node process, the way bundlers and
+// Loads the library source in a fresh Node process, the way bundlers and
 // installs without the optional `canvas` dependency see it.
 var
   childProcess = require('child_process'),
   path = require('path');
 
 describe('loading imagediff', function () {
 
-  var LIBRARY = path.join(__dirname, '..', 'imagediff.js');
+  var LIBRARY = path.join(__dirname, '..', 'js', 'imagediff.js');
 
   // Runs `body` in a child process with `canvas` unresolvable and calls back
   // with the JSON it prints.