this for function context

this is a reference to the context object that the function is operating on.

 
<!DOCTYPE html>
<html>
<head>
    <title>Example</title>
    <script type="text/javascript">
        window.color = "A"; 
        var o = { color: "B" }; 
        
        function sayColor(){ 
            document.writeln(this.color); 
        } 
        
        sayColor();  //A
        o.sayColor = sayColor; 
        o.sayColor(); //B
       
    </script>
</head>
<body>
</body>
</html>
Click to view the demo
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    Essential Types  

Function:
  1. The Function Type
  2. Function Declarations versus Function Expressions
  3. Functions as Values
  4. Returning a function from a function
  5. Function arguments
  6. this for function context
  7. Function caller
  8. Function length property
  9. Function apply()
  10. Function.call() method
  11. Function's bind() method
  12. Function toLocaleString() and toString()