eval in javascript

//eval() is a global function in JavaScript that evaluates a specified string as JavaScript 
//code and executes it.
eval("alert('this is executed by eval()')");
					//OR
var x = 10;
var y = 20;
var a = eval("x * y") + "<br>";
var res =  a;  var x = 10;
var y = 20;
var a = eval("x * y") + "<br>";
var b =  eval("2 + 2") + "<br>";
var c = eval("x + 17") + "<br>";

var res =  a + b + c;