Question

I’ve just started out with Javascript and I’m coming across declarations of the following type quite often:
var var_name = window.var_name = window.var_name || {};
Can someone explain what is the significance of such a declaration?

Answer 1

This expression:

window.var_name || {}

…simply returns the value of window.var_name unless that value is falsy (false, 0, '', undefined, null, or NaN), in which case it evaluates to an empty object ({}). This is used to provide a default value when window.var_name does not exist.

The resulting value is then assigned back to window.var_name:

window.var_name = window.var_name || {};

Which is then assigned to a (possibly) new variable named var_name:

var var_name = window.var_name = window.var_name || {};

In the global scope var_name is equal to window.var_name. Inside a function, var_name will refer to a new local variable.