gulpfile.js

const gulp = require("gulp"); 
const webpack = require("webpack-stream");
const sass = require('gulp-sass')(require('sass'));

const dist = "C:\\MAMP\\htdocs\\react_admin\\admin"; 

gulp.task("copy-html", () => {
  return gulp.src("./app/src/index.html") 
              .pipe(gulp.dest(dist))
});

gulp.task("build-js", () => {
  return gulp.src("./app/src/main.js")
              .pipe(webpack({ 
                mode: 'development', 
                output: { 
                  filename: 'script.js'
                },
                watch: false,
                devtool: "source-map", 
                module: { 
                  rules: [
                    {
                      test: /\.(?:js|mjs|cjs)$/,
                      exclude: /node_modules/,
                      use: {
                        loader: 'babel-loader',
                        options: {
                          presets: [
                            ['@babel/preset-env', { 
                              debug: true, 
                              corejs: 3, 
                              useBuiltIns: "usage",  
                              targets: "defaults" 
                            }],
                            "@babel/react"
                          ]
                        }
                      }
                    }
                  ]
                }
              }))
              .pipe(gulp.dest(dist))
});

gulp.task("build-sass", () => {
  return gulp.src("./app/src/scss/style.scss")
              .pipe(sass().on('error', sass.logError))
              .pipe(gulp.dest(dist))
});

gulp.task("copy-api", () => {
  return gulp.src("./app/api/**/*.*")
              .pipe(gulp.dest(dist + "/api"))
});

gulp.task("copy-assets", () => {
  return gulp.src("./app/assets/**/*.*")
              .pipe(gulp.dest(dist + "/assets"))
});

gulp.task("watch", () => {
  gulp.watch("./app/src/index.html", gulp.parallel("copy-html"));
  gulp.watch("./app/assets/**/*.*", gulp.parallel("copy-assets"));
  gulp.watch("./app/api/**/*.*", gulp.parallel("copy-api"));
  gulp.watch("./app/src/scss/**/*.scss", gulp.parallel("build-sass"));
  gulp.watch("./app/src/**/*.js", gulp.parallel("build-js"));
});

gulp.task("build", gulp.parallel("copy-html", "copy-assets", "copy-api", "build-sass", "build-js"));

gulp.task("default", gulp.parallel("watch", "build"));