Example multi-file gulp setup

gulpfile.js

export lint from './task-lint';
export test from './task-test';
export build from './task-build';
export dev from './task-dev';

export default dev;

task-build.js

import {src, dest, series} from 'gulp';
import buildPlugin from 'gulp-build-plugin';
import {SRC_DIRECTORY, DIST_DIRECTORY} from './task-constants';
import test from './task-test';

const buildFiles = () => src(SRC_DIRECTORY)
  .pipe(buildPlugin())
  .pipe(dest(DIST_DIRECTORY));

export default series(test, buildFiles);

task-constants.js

export const SRC_DIRECTORY = './src';
export const TEST_DIRECTORY = './test';
export const DIST_DIRECTORY = './dist';

task-dev.js

import {watch, parallel} from 'gulp';
import {SRC_DIRECTORY, TEST_DIRECTORY} from './task-constants';
import test from './task-test';
import build from './task-build';

export default () => {
  watch(SRC_DIRECTORY, parallel(test, build));
  watch(TEST_DIRECTORY, test);
};

task-lint.js

import {src, parallel} from 'gulp';
import lintPlugin from 'gulp-lint-plugin';
import {SRC_DIRECTORY, TEST_DIRECTORY} from './task-constants';

const lintDir = dir => src(dir)
  .pipe(lintPlugin());

const lintSrc = () => lintDir(SRC_DIRECTORY);
const lintTest = () => lintDir(TEST_DIRECTORY);
const lintFiles = parallel(lintSrc, lintTest);

export const reportLint = () => lintPlugin.report();

export default series(lintFiles, reportLint);

task-test.js

import {src, parallel, series} from 'gulp';
import testPlugin from 'gulp-test-plugin';
import {TEST_DIRECTORY} from './task-constants';
import {reportLint}, lint from './task-lint';

const testFiles = () => src(TEST_DIRECTORY)
  .pipe(testPlugin());

const reportTest = () => testPlugin.report();
const reportLintAndTest = series(reportLint, reportTest);

export default series(parallel(lint, testFiles), reportLintAnTest);