Add documentation to js.Build (#1175)

Fixes #1171

content/en/hugo-pipes/babel.md

@@ -9,9 +9,9 @@ keywords: []
 menu:
   docs:
     parent: "pipes"
-    weight: 49
-weight: 49
-sections_weight: 49
+    weight: 48
+weight: 48
+sections_weight: 48
 draft: false
 ---
 

content/en/hugo-pipes/js.md

@@ -0,0 +1,63 @@
+---
+title: JavaScript Building
+description: Hugo Pipes can process JavaScript files with [ESBuild](https://github.com/evanw/esbuild).
+date: 2020-07-20
+publishdate: 2020-07-20
+lastmod: 2020-07-20
+categories: [asset management]
+keywords: []
+menu:
+  docs:
+    parent: "pipes"
+    weight: 45
+weight: 45
+sections_weight: 45
+draft: false
+---
+
+Any JavaScript resource file can be transpiled and "tree shaken" using `js.Build` which takes for argument either a string for the filepath or a dict of options listed below.
+
+### Options
+
+targetPath [string]
+: If not set, the source path will be used as the base target path. 
+Note that the target path's extension may change if the target MIME type is different, e.g. when the source is TypeScript.
+
+minify [bool]
+: Let `js.Build` handle the minification.
+
+target [string]
+: The language target.
+  One of: `es2015`, `es2016`, `es2017`, `es2018`, `es2019`, `es2020` or `esnext`.
+  Default is `esnext`.
+
+externals [slice]
+: External dependencies. If a dependency should not be included in the bundle (Ex. library loaded from a CDN.), it should be listed here.
+
+```go-html-template
+{{ $externals := slice "react" "react-dom" }}
+```
+
+defines [map]
+: Allow to define a set of string replacement to be performed when building. Should be a map where each key is to be replaced by its value.
+
+```go-html-template
+{{ $defines := dict "process.env.NODE_ENV" `"development"` }}
+```
+
+### Examples
+
+```go-html-template
+{{ $built := resources.Get "js/index.js" | js.Build "main.js" }}
+```
+
+Or with options:
+
+```go-html-template
+{{ $externals := slice "react" "react-dom" }}
+{{ $defines := dict "process.env.NODE_ENV" `"development"` }}
+
+{{ $opts := dict "targetPath" "main.js" "externals" $externals "defines" $defines }}
+{{ $built := resources.Get "scripts/main.js" | js.Build $opts }}
+<script type="text/javascript" src="{{ $built.RelPermalink }}" defer></script>
+```