What is the difference between new Function and eval?

eval

  

The eval () method evaluates JavaScript code represented as a string. MDN

Function

  

The Function constructor creates a new Function object. In JavaScript every function is actually a Function object. - MDN

Free translation:

  

The Function constructor creates a new anonymous function object. In Javascript all functions are currently a function object.

Documentation self-explanates, basically eval executes an expression, statement , and so on. The Function constructor creates a new function object entitled to all its prototype .

Examples:

var a = new Function('', 'return (1+1)');
var b = eval("2 + a()");
var c = b + a();
var d = new Function("p", "return a() + b + c + p");
var e = eval("d(10) - 20");

console.log(a())   // 2
console.log(b)     // 4
console.log(c)     // 6
console.log(d(10)) // 22
console.log(e);    // 2

On the W3Schools site has this example in the Function constructor session:

var myFunction = new Function("a", "b", "return a * b");
var x = myFunction(4, 3);

And then say the following followed by another example:

  

You actually do not have to use the constructor function. The above above is the same as writing:

var myFunction = function (a, b) {return a * b};
var x = myFunction(4, 3);

The phrase reads as follows (free translation):

  

You do not currently need to use the function constructor. The example below has the same effect.

They do not do the same thing:

eval() evaluates a string as a JavaScript expression within the current scope and can access local variables.

new Function parse the code for a function object, which can be called. It can not access local variables because the code runs in a separate scope.

What does this mean? Run this code:

function testEval() {
    var a = 'original';
    eval("(a = 'alterado')");
    console.log(a);
}

function testNewFunction() {
    var a = 'original';
    new Function("(a = 'alterado')")();
    console.log(a);
}

When we use eval the value of the variable a is changed. When we use new Function it remains unchanged.