What's the meaning of var variableName = variableName || {};

Question

I'm new to javascript from C++ background. I always see code like

var variableName = variableName || {};

It seems quite strange to me. Can somebody explain the meaning of the code? Thanks a lot.

Answer 1

It means if that variable isn't set, make it to be a new empty object and if it is set, use the value of that variable. It equals to code:

var variableName;
if(!variableName){
    variableName = {};
}

var a = a || {}; // {}
var b = {"key":"value"};
var b = b || {}; // {"key":"value"}

Normally it's used to create a namespace to organize the code. For example,

var myNameSpace = myNameSpace || {};
myNameSpace.print= function(msg) {
    console.log("message is: "+msg);
}
myNameSpace.bar = function(){
    //do some thing
}

Answer 2

That means, that if variableName is set, variableName should stay that value. Else, if it's not set, it should become an object (in this case an empty one).