There are multiple ways to achieve this.

• Function object

  Create a Function and set the body:

  var dynamicJS = "var x = 1+1;  return 2;"
  var DoSomething = new Function("value", dynamicJS );
  

• eval()

  While arguably more dangerous¹, eval() can be used.

  var dynamicJS = "function DoSomething(value){var x = 1+1  return 2;}"\
  eval(dynamicJS);
  

  Because you mentioned in a comment "it is intranet application and not going to outer world. no issues on this for this req.", this would likely be fine but please read the section below.

  Caution

  From the this section of the MDN documentation about eval():

  Don't use eval needlessly!

  _{eval() is a dangerous function, which executes the code it's passed with the privileges of the caller. If you run eval() with a string that could be affected by a malicious party, you may end up running malicious code on the user's machine with the permissions of your webpage / extension. More importantly, third party code can see the scope in which eval() was invoked, which can lead to possible attacks in ways to which the similar Function is not susceptible.}

  _{eval() is also generally slower than the alternatives, since it has to invoke the JS interpreter, while many other constructs are optimized by modern JS engines.}

  _{There are safer (and faster!) alternatives to eval() for common use-cases.} ²

See a demonstration of these methods utilized below.

var dynamicJS = "function DoSomething(value){var x = 1+1;  return 2;}"
var functionBody = "var x = 1+1;  return 2;";
document.addEventListener('DOMContentLoaded', function() {
  document.getElementById('eval').addEventListener('click', function() {
    eval(dynamicJS);
    console.log('DoSomething(3) -> ',DoSomething(3));
  });
  document.getElementById('function').addEventListener('click', function() {
    var dynamicFunction = new Function("value", functionBody);
    console.log('dynamic function(3) ->',dynamicFunction(3));
  });
});

<button id="eval">click to eval</button>
<button id="function">click to use Function</button>

¹_{http://stackoverflow.com/a/4599946/1575353}

²_{https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#Don't_use_eval_needlessly!}

I would believe the easier way will be to parse the String and then use the function constructor.

Something like this:

var DoSomething = new Function('value', 'var x = 1+1  return 2');

Perhaps try something like:

function myFunc(obj){
    var param = obj.hasOwnProperty('param') ? obj.param : undefined;
    console.log(param);
}

var funcString = "myFunc({ param: 'something' });",
    Construct = new Function(funcString);

Construct();

Haven't tested it to be honest ... but this way you avoid eval().

more info on Function object