feat(build): added check-mtime option to build.js to speed up generation

During development, when the developer modifies a couple of files, generating
js files only for the edited files can take just a second while generating the
whole tree can take 10 seconds or more (depending on hardware of course).

build.js

@@ -1,5 +1,15 @@
 'use strict';
 
+/**
+  * The script that generates all the code to the dist folder from the source js
+  * and ts files in the lib folder.
+  *
+  * Parameters:
+  *   --check-mtimes   Do only a partial generation of the files missing in
+  *                    dist or which source files are newer than their generated
+  *                    counterparts. This is faster than a full build.
+  */
+
 const glob = require('fast-glob');
 const { promisify } = require('util');
 const { build } = require('esbuild');
@@ -32,16 +42,60 @@ async function rmDistDir() {
   }
 }
 
-async function main() {
-  console.info('Compiling sequelize...');
-  const [declarationFiles, filesToCompile] = await Promise.all([
+function sourcePathToDestPath(source) {
+  return path.resolve(outdir, source).replace(/\.ts$/, '.js');
+}
+
+async function ageCheckFiles(paths) {
+  return (await Promise.all(
+    paths.map(source => new Promise(resolve => {
+      const dest = sourcePathToDestPath(source);
+      const sourceStat = fs.statSync(source, { throwIfNoEntry: false });
+      const destStat = fs.statSync(dest, { throwIfNoEntry: false });
+      const ok = sourceStat && (!destStat || destStat.mtimeMs < sourceStat.mtimeMs);
+      resolve(ok ? source : null);
+    })),
+  )).filter(Boolean);
+}
+
+async function listFilesToGenerate() {
+  let toCompile;
+  let rmPromise;
+
+  const toCompilePromise = glob('./lib/**/*.{js,ts,mjs,mts,cjs,cts}', { onlyFiles: true, absolute: false });
+
+  if (process.argv.includes('--check-mtime')) {
+    toCompile = await ageCheckFiles(await toCompilePromise);
+    if (toCompile.length === 0) {
+      console.log('Nothing to do');
+
+      return [[], []];
+    }
+
+    console.log(`Generating ${toCompile.length} files`);
+  } else {
+    toCompile = toCompilePromise;
+    rmPromise = rmDistDir();
+  }
+
+  return await Promise.all([
     // Find all .d.ts files from types/
     glob('./types/**/*.d.ts', { onlyFiles: true, absolute: false }),
     // Find all .js and .ts files from lib/
-    glob('./lib/**/*.[tj]s', { onlyFiles: true, absolute: false }),
+    toCompile,
     // Delete dist/ for a full rebuild.
-    rmDistDir(),
+    rmPromise,
   ]);
+}
+
+async function main() {
+  console.info('Compiling sequelize...');
+
+  const [declarationFiles, filesToCompile] = await listFilesToGenerate();
+
+  if (filesToCompile.length === 0) {
+    return;
+  }
 
   // copy .d.ts files prior to generating them from the .ts files
   // so the .ts files in lib/ will take priority..

package.json

@@ -279,6 +279,7 @@
     "sscce-db2": "cross-env DIALECT=db2 node sscce.js",
     "prepare": "npm run build && husky install",
     "build": "node ./build.js",
+    "build-incremental": "node ./build.js --check-mtime",
     "---------------------------------------------------------------------------------------------------": ""
   },
   "support": true