gulpfile.js

"use strict";
var gulp = require("gulp"),
    $ = require("gulp-load-plugins")(),
    source = require("vinyl-source-stream"),
    browserify = require("browserify"),
    watchify = require("watchify"),
    path = require("path"),
    fs = require("fs"),
    babelify = require("babelify");

gulp.task("scripts:server", () => {
    return gulp.src("./src-server/**/*.js")
        .pipe($.cached("server")) 
        .pipe($.babel())
        .pipe(gulp.dest("./build"));
})

gulp.task("watch:scripts:server", gulp.series("scripts:server", () => {
    return gulp.watch("./src-server/**/*.js", gulp.series("scripts:server"))
}));

gulp.task("watch:scripts:client", () => {
    const files = fs.readdirSync("./src-client");
    for (let i = 0; i < files.length; i++) {
        const file = files[i];
        if(path.extname(file) !== ".js") continue;

        initBundlerWatch(path.join("src-client", file));
    }

    return gulp.watch("./src-client/**/*.js")
        .on("change", initBundlerWatch);
})

gulp.task("watch:scripts", gulp.parallel(
    "watch:scripts:client",
    "watch:scripts:server"));

let bundlers = {};
function initBundlerWatch(file){
    if(bundlers.hasOwnProperty(file)) return;

    const bundler = createBundler(file);
    bundlers[file] = bundler;
    const watcher = watchify(bundler);
    const filename = path.basename(file);

    function bundle(){
        return bundler
            .bundle()
            .on("error", error => console.error(error))
            .pipe(source(filename))
            .pipe(gulp.dest("./public/build"));
    }

    watcher.on("update", bundle);
    watcher.on("time", time => console.log(`B:C:${time}ms`));

    bundle();
}

function createBundler(file) {
    const bundler = browserify(file);
    bundler.transform(babelify);
    return bundler;
}