JavaScript has functional scope. Meaning that if you (properly) define variables within functions, those variables remain accessible only inside the function.

Block scope, on the other hand, defines scope within a block of code, usually defined by braces. JavaScript now has block scope as of version 1.7, which means it’s available in these browsers:

Block scope is enabled in JavaScript with the use of “let”:

let(x=100) {
    alert(x);
};

It also works perfectly well on one line, without the use of braces:

let(x=100) alert(x);

Note that we can define global variables with the same name outside the block scope and the variables won’t interfere with each other:

x = 200;
let(x=100) alert(x);
alert(x);
// result: 100, 200

Note that there’s a slight caveat – not only is this not available in any version of IE, but it also requires a special script type declaration in order to work (at least for Firefox): type=”text/javascript;version=1.7″

References:
JavaScript Versions
Video: Best Practices in Javascript Library Design (John Resig)
New in JavaScript 1.7