Is Function object necessary

Question

It is common and easy to create function like:

var f = function(){
    alert("something");
};

So why is there Function object like:

var f = new Function("alert('something');");

The latter is hard to write/read. I can only come up with one situation in which someone typing some JS code to the webpage and running it. This can also be solved with eval.

Why Function object?

Answer 1

The Function object can be used to dynamically generate functions.

var some_fixed_value = 1;
var f = new Function("return " + some_fixed_value);

Opposed to:

var some_fixed_value = 1;
var f = function() {
    return some_fixed_value;
};
// Break the function logic:
some_fixed_value = 'not_fixed_any_more!';

or, using two function expressions:

var f = (function(copy_of_fixed_value) {
    return function() {
        return copy_of_fixed_value;
    };
})(some_fixed_value);

"This can also be solved with eval":

var func_func = new Function('x', 'return x');  //function anonymous(x){return x}
var evil_func = eval('function(x){return x}');  //SyntaxError: function statement
                                                // requires a name
var eval_func = eval('(function(x){return x})');//function(x){return x}

Answer 2

The Function (with capital F) is not a keyword, it's a base class for any other functions in JavaScript, given the fact that everything in JavaScript is an object, functions should/must inherit from a base class that represents base functionality of all other functions (methods/classes).

You can play a lot with function inheritance via Function's prototype:

here's an example to define a .run method for all functions in JS:

Function.prototype.run = function (delay) {
    setTimeout(this, delay);
};

test:

function test(){console.log("test")};
test.run(2000);

there's an undocumented __proto__ property that points to the base-class prototype, you can check it on each instance of function:

test.__proto__ == Function.prototype; // true