Docs: Add "Creating Tasks" documentation

docs/getting-started/3-creating-tasks.md

@@ -0,0 +1,191 @@
+<!-- front-matter
+id: creating-tasks
+title: Creating Tasks
+hide_title: true
+sidebar_label: Creating Tasks
+-->
+
+# Creating Tasks
+
+Each gulp task is an asynchronous JavaScript function - a function that accepts an error-first callback or returns a stream, promise, event emitter, child process, or observable ([more on that later][async-completion-docs]). Due to some platform limitations, synchronous tasks aren't supported, though there is a pretty nifty [alternative][using-async-await-docs].
+
+## Exporting
+
+Tasks can be considered **public** or **private**.
+
+* **Public tasks** are exported from your gulpfile, which allows them to be run by the `gulp` command.
+* **Private tasks** are made to be used internally, usually used as part of `series()` or `parallel()` composition.
+
+A private task looks and acts like any other task, but an end-user can't ever execute it independently. To register a task publicly, export it from your gulpfile.
+
+```js
+const { series } = require('gulp');
+
+// The `clean` function is not exported so it can be considered a private task.
+// It can still be used within the `series()` composition.
+function clean(cb) {
+  // body omitted
+  cb();
+}
+
+// The `build` function is exported so it is public and can be run with the `gulp` command.
+// It can also be used within the `series()` composition.
+function build(cb) {
+  // body omitted
+  cb();
+}
+
+exports.build = build;
+exports.default = series(clean, build);
+```
+
+![ALT TEXT MISSING][img-gulp-tasks-command]
+
+<small>In the past, `task()` was used to register your functions as tasks. While that API is still available, exporting should be the primary registration mechanism, except in edge cases where exports won't work.</small>
+
+## Compose tasks
+
+Gulp provides two powerful composition methods, `series()` and `parallel()`, allowing individual tasks to be composed into larger operations. Both methods accept any number of task functions or composed operations.  `series()` and `parallel()` can be nested within themselves or each other to any depth.
+
+To have your tasks execute in order, use the `series()` method.
+```js
+const { series } = require('gulp');
+
+function transpile(cb) {
+  // body omitted
+  cb();
+}
+
+function bundle(cb) {
+  // body omitted
+  cb();
+}
+
+exports.build = series(transpile, bundle);
+```
+
+For tasks to run at maximum concurrency, combine them with the `parallel()` method.
+```js
+const { parallel } = require('gulp');
+
+function javascript(cb) {
+  // body omitted
+  cb();
+}
+
+function css(cb) {
+  // body omitted
+  cb();
+}
+
+exports.build = parallel(javascript, css);
+```
+
+Tasks are composed immediately when either `series()` or `parallel()` is called.  This allows variation in the composition instead of conditional behavior inside individual tasks.
+
+```js
+const { series } = require('gulp');
+
+function minify(cb) {
+  // body omitted
+  cb();
+}
+
+
+function transpile(cb) {
+  // body omitted
+  cb();
+}
+
+function livereload(cb) {
+  // body omitted
+  cb();
+}
+
+if (process.env.NODE_ENV === 'production') {
+  exports.build = series(transpile, minify);
+} else {
+  exports.build = series(transpile, livereload);
+}
+```
+
+`series()` and `parallel()` can be nested to any arbitrary depth.
+
+```js
+const { series, parallel } = require('gulp');
+
+function clean(cb) {
+  // body omitted
+  cb();
+}
+
+function cssTranspile(cb) {
+  // body omitted
+  cb();
+}
+
+function cssMinify(cb) {
+  // body omitted
+  cb();
+}
+
+function jsTranspile(cb) {
+  // body omitted
+  cb();
+}
+
+function jsBundle(cb) {
+  // body omitted
+  cb();
+}
+
+function jsMinify(cb) {
+  // body omitted
+  cb();
+}
+
+function publish(cb) {
+  // body omitted
+  cb();
+}
+
+exports.build = series(
+  clean,
+  parallel(
+    cssTranspile,
+    series(jsTranspile, jsBundle)
+  ),
+  parallel(cssMinify, jsMinify),
+  publish
+);
+```
+
+When a composed operation is run, each task will be executed every time it was referenced.  For example, a `clean` task referenced before two different tasks would be run twice and lead to undesired results.  Tasks can be wrapped with the [async-once][async-once] module if this **(not recommended)** pattern is needed.
+
+```js
+// This pattern is NOT recommended but some edge cases might require it.
+const { series } = require('gulp');
+const once = require('async-once');
+
+const clean = once(function(cb) {
+  // body omitted
+  cb();
+});
+
+const css = series(clean, function(cb) {
+  // body omitted
+  cb();
+});
+
+const javascript = series(clean, function(cb) {
+  // body omitted
+  cb();
+})
+
+exports.build = series(css, javascript);
+```
+
+[async-completion-docs]: 4-async-completion.md
+[using-async-await-docs]: 4-async-completion.md#using-asyncawait
+[img-gulp-tasks-command]: https://gulpjs.com/img/docs-gulp-tasks-command.png
+[async-once]: https://github.com/gulpjs/async-once