Question

This is the code I get, but I don’t know this two lines means:

o[arr[i]] = o[arr[i]] || {};
o = o[arr[i]]; 

Full code:

var GLOBAL={};
GLOBAL.namespace=function(str){
    var arr = str.split("."), o=GLOBAL;

    for(i=(arr[0]=="GLOBAL") ? 1 : 0; i<arr.length; i++){
        o[arr[i]] = o[arr[i]] || {};
        o = o[arr[i]];
    }
};

GLOBAL.namespace("A.Dog");
// GLOBAL.A = {};
// GLOBAL.A.Dog = {};
GLOBAL.A.Dog.name = "diudiu";
alert(GLOBAL.A.Dog.name)

Answer 1

Break it down step-by-step to make is clearer:

o[arr[i]] = o[arr[i]] || {};

is the same as:

var x = arr[i];
o[x] = o[x] || {};

is the same as:

var x = arr[i];
if (o[x] == undefined) {
  o[x] = {};
}

The pattern A = B || DEFAULT_VALUE is an idiom which uses the short-circuiting nature of the || operator. In javascript, the || operator does not return true or false but instead it returns the first non-false value or false. So if the first variable is not falsy it evaluates to the value of the first variable otherwise it evaluates to the value of the second variable.