Describe limitations and add example use case

Author: styd

README.md

@@ -32,14 +32,19 @@ And then execute:
 ```ruby
 {
   speed: function(val) {
-           return parseFloat(val).toLocaleString();
+           return '$' + parseFloat(val).toLocaleString();
          }
 }.to_json
-#=> "{\"speed\":function(val) { return parseFloat(val).toLocaleString(); }}"
+#=> "{\"speed\":function(val) { return '$' + parseFloat(val).toLocaleString(); }}"
 ```
 
-But, don't go wild just yet and try not to put multiple functions on the same
-line.
+But, don't go wild just yet because there are limitations:
+- You cannot use `if` keyword because the way JavaScript uses it is different from
+  ruby. It is meant to be used for a simple function. One use case I'm targetting
+  is for formatting options to be used by JavaScript libraries.
+- Also, don't put multiple functions on the same line. You'll get unexpected result
+  if you do.
+- You should end statement with `;`.
 
 If you try the above example on your REPL (e.g. `irb` or `pry`) it won't
 work since the gem will try to find the file that stores the code. So, you
@@ -49,7 +54,7 @@ need to put it on a file and run it.
 
 Well, technically, it's not a JavaScript function. It's a Ruby method named
 `function` that's available in any object. In JavaScript, a form like that is
-a _function declaration_, but in Ruby it's a method invocation. So, in that case,
+a _function declaration_, but in Ruby it's a _method invocation_. So, in that case,
 `val` needs to already exist before you call it, and it is. Both `function` and
 `val` are already a method of an object. If you want to use arguments with
 another name, you need to declare them as variable first.
@@ -61,7 +66,17 @@ arg1, arg2 = nil
               return arg1 + ' == ' + arg2
             }
 }.to_json
-#=> "{\"equality\":function(arg1, arg2) { arg1 + ' == ' + arg2 }}"
+#=> "{\"equality\":function(arg1, arg2) { return arg1 + ' == ' + arg2 }}"
+```
+
+### Use Case
+
+One example use case is [ApexCharts.RB](https://github.com/styd/apexcharts.rb)
+formatter. You can add tooltip formatter like this:
+
+```erb
+<%= line_chart data, tooltip: {y: {formatter: function(val) { return '$' + parseFloat(val).toLocaleString(); }}} %>
+
 ```