gulpfile.js

const gulp = require("gulp");
const del = require("del");
const rollupConfig = require("./rollup.config");
const rollup = require("rollup");
const ts = require("gulp-typescript");

gulp.task("clean-lib", function(callback) {
  del("lib/**");
  callback();
});

gulp.task("copy-files", function() {
  return gulp
    .src(["package.json", "README.md", "LICENSE"])
    .pipe(gulp.dest("lib/"));
});

gulp.task("ts", function(callback) {
  const tsProject = ts.createProject("tsconfig.json");
  return tsProject
    .src()
    .pipe(tsProject())
    .pipe(gulp.dest("lib"));
});

gulp.task(
  "rollup",
  gulp.series(
    function() {
      const tsProject = ts.createProject("tsconfig.json", {
        module: "esnext"
      });
      return tsProject
        .src()
        .pipe(tsProject())
        .pipe(gulp.dest("lib/es"));
    },
    async function() {
      const bundle = await rollup.rollup(rollupConfig);
      return await bundle.write(rollupConfig.output);
    }
  )
);

gulp.task("build", gulp.series("clean-lib", "copy-files", "ts", "rollup"));