javascript best practice define variable (namespace) check is not already defined

Question

Wanted to get opinions from the experts, I am declaring a variable that will act as the namespace for my application javascript code, but i want to check it is not already defined.

this code is terse and 'seems' to work - any reason I should avoid this and use typeof 'undef' checking instead?

var MY_NAMESPACE = MY_NAMESPACE || {};

Thanks

Answer 1

This is the standard way of doing it. See Matt Snider's analysis of the YUI YAHOO.namespace function which uses this same check (Also for a look at how to make it easy to create namespaces).

Matt's code, which he adapted from YUI to namespace off the window object instead of the YAHOO object:

window.object_mes_namespace = function() {   
    var a = arguments,    
        o = window,   
        i = 0,   
        j = 0,   
        tok = null,   
        name = null;   

    // iterate on the arguments   
    for (i = 0; i < a.length; i = i + 1) {   
        tok = a[i].split(".");   

        // iterate on the object tokens   
        for (j = 0; j < tok.length; j = j + 1) {   
            name = tok[j];             
            o[name] = o[name] || {};   
            o = o[name];   
        }   
    }   

    return o;   
}  

Note the o[name] = o[name] || {}; line, which parallels your example.