toSource() & toString() & eval() -- function to string & string to function

The toSource method returns the following values:
For the built-in Function object, toSource() returns the following string indicating that the source code is not available: 
function Function() { 
    [native code] 
} 
For custom functions, toSource() returns the JavaScript source that defines the object as a string.

@reference_1_mozilla

eval as a string defining function requires "(" and ")" as prefix and suffix

var fctStr1 = "function a() {}"
var fctStr2 = "(function a() {})"
var fct1 = eval(fctStr1)  // return undefined
var fct2 = eval(fctStr2)  // return a function

@reference_2_mozilla