Callback Support (alternative implementation)

README.md

@@ -58,8 +58,8 @@ Most of the options are disabled by default.
 | `includeAutoGeneratedTags`     | Insert tags generated by HTML parser | `true` |
 | `keepClosingSlash`             | Keep the trailing slash on singleton elements | `false` |
 | `maxLineLength`                | Specify a maximum line length. Compressed output will be split by newlines at valid HTML split-points |
-| `minifyCSS`                    | Minify CSS in style elements and style attributes (uses [clean-css](https://github.com/jakubpawlowicz/clean-css)) | `false` (could be `true`, `Object`, `Function(text, type)`) |
-| `minifyJS`                     | Minify JavaScript in script elements and event attributes (uses [UglifyJS](https://github.com/mishoo/UglifyJS2)) | `false` (could be `true`, `Object`, `Function(text, inline)`) |
+| `minifyCSS`                    | Minify CSS in style elements and style attributes (uses [clean-css](https://github.com/jakubpawlowicz/clean-css)) | `false` (could be `true`, `Object`, `Function(text, type[, cb])`) |
+| `minifyJS`                     | Minify JavaScript in script elements and event attributes (uses [UglifyJS](https://github.com/mishoo/UglifyJS2)) | `false` (could be `true`, `Object`, `Function(text, inline[, cb])`) |
 | `minifyURLs`                   | Minify URLs in various attributes (uses [relateurl](https://github.com/stevenvachon/relateurl)) | `false` (could be `String`, `Object`, `Function(text)`) |
 | `preserveLineBreaks`           | Always collapse to 1 line break (never remove it entirely) when whitespace between tags include a line break. Must be used in conjunction with `collapseWhitespace=true` | `false` |
 | `preventAttributesEscaping`    | Prevents the escaping of the values of attributes | `false` |
@@ -142,6 +142,8 @@ For command line usage please see `html-minifier --help`
 
 ### Node.js
 
+#### Synchronous Usage
+
 ```js
 var minify = require('html-minifier').minify;
 var result = minify('<p title="blah" id="moo">foo</p>', {
@@ -150,6 +152,62 @@ var result = minify('<p title="blah" id="moo">foo</p>', {
 result; // '<p title=blah id=moo>foo</p>'
 ```
 
+#### Asynchronous Usage
+
+`minify` can take an optional callback function that will be invoked once the
+minification is complete (or an error occurs).
+
+This allows for the following functions to complete asynchronously:
+
+* minifyCSS
+* minifyJS
+
+```js
+var minify = require('html-minifier').minify;
+
+minify(
+  '<style>body { margin: 0; display: flex; }</style>',
+  {
+    minifyCSS: function(css, cb) {
+      doSomethingAsync(css, function(updatedCss) {
+        cb(updatedCss);
+      });
+    },
+  },
+  function(error, result) {
+    if (error) {
+      // Handle error.
+    } else {
+      // Do something with the result.
+    }
+  }
+);
+```
+
+##### Promisify
+
+`minify` is compatable with NodeJS's [`util.promisify`](https://nodejs.org/dist/latest-v8.x/docs/api/util.html#util_util_promisify_original)
+function added in v8.0.0
+
+```js
+const util = require('util');
+const minify = util.promisify(require('html-minifier').minify);
+
+minify('<style>body { margin: 0; display: flex; }</style>', {
+  minifyCSS: (css, cb) => {
+    doSomethingAsync(css, updatedCss => {
+      cb(updatedCss);
+    });
+  }
+})
+  .then(result => {
+    // Do something with the result.
+  })
+  .catch(error => {
+    // Handle error.
+  });
+```
+
 ## Running benchmarks
 
 Benchmarks for minified HTML: