Question

Reading Crockfords The Elements of JavaScript Style I notice he prefers defining variables like this:

var first='foo', second='bar', third='...';

What, if any benefit does that method provide over this:

var first='foo';
var second='bar';
var third='...';

Obviously the latter requires more typing but aside from aesthetics I’m wondering if there is a performance benefit gained by defining with the former style.

Answer 1

Aside of aesthetics, and download footprint, another reason could be that the var statement is subject to hoisting. This means that regardless of where a variable is placed within a function, it is moved to the top of the scope in which it is defined.

E.g:

var outside_scope = "outside scope";
function f1() {
    alert(outside_scope) ;
    var outside_scope = "inside scope";
}
f1();

Gets interpreted into:

var outside_scope = "outside scope";
function f1() {
    var outside_scope; // is undefined
    alert(outside_scope) ;
    outside_scope = "inside scope";
}
f1();

Because of that, and the function-scope only that JavaScript has, is why Crockford recommends to declare all the variables at the top of the function in a single var statement, to resemble what will really happen when the code is actually executed.