What's the purpose of lonely code blocks in Javascript?

Question

In Javascript, you can have lonely code blocks, that is, code blocks without an if , function or something like that preceding them, placed arbitrarily in the code, like this:

var a = 3,
b = 4;
{
  a = 50;
  b = 50;
}
alert(a + b); //alerts 100

I know some other languages have this (I think C++ does), and they use it for scope control: variables declared inside the lonely code block cannot be accessed outside of it. But since Javascript has function scope rather than block scope, you have to use a self-executing function (function(){})() to acheive the same effect.

Is there a purpose for this construct? Have you ever seen anybody use it? Does the spec mention it? Is this just a side effect of the grammar having some sort of general rule about code blocks?

Answer 1

As I found out after googling my answer for How are labels used with statements that are not a loop? , the lonely code blocks can be related to labels:

myLabel:
{
    for(x in y) {
        break myLabel;
    }
}

Will break out not only of the loop, but of the outer code block as well.

Answer 2

Update (as this question shows up high on Google for "javascript code block"):

With the introduciton of the let keyword in ECMAScript 6, code blocks can also be used to limit scope of variables.

{
    let x = 20;
    alert(x) // 20
}
alert(x) // undefined

Answer 3

Only code readability, nothing more.

They do not even give you the advantage of closures:

var c = 'fail';
{
    var c = 123;
}
alert(c); //alerts 123

( see it live )

Answer 4

Only readability. Other than that, nothing. Do not do this as it's not future-proof. The following will happily break:

"use strict";

var a = 3,
b = 4;
{
  a = 50;
  b = 50;
}
alert(a + b); //alerts 100

I used to use it myself as it allowed logical separation of functionality, but no longer do due to the above.