I know this is very basic, but why does x return undefined in this code block? Is there a way to define a property and immediately use it to set another property?
var grid = { x : 75, y : 75, location : [grid.x, grid.y], init : function () { console.log(grid.location[0]); } } You can't use the variable to access properties of the object before the object has been assigned to the variable. While the object is created, the variable grid is still undefined. While the object is being created, you don't have any reference to it.
You can use the properies once the object has been assigned to the variable:
var grid = { x : 75, y : 75, init : function () { console.log(grid.location[0]); } } grid.location = [grid.x, grid.y]; You can also wrap this in a function expression to get code that returns the complete object:
var grid = (function(){ var obj = { x : 75, y : 75, init : function () { console.log(grid.location[0]); } }; obj.location = [obj.x, obj.y]; return obj; })(); 
this.location = [this.x, this.y]; into the .init() method instead of calling it manually from outside the object?
location property is only accessed inside the init method, or only accessed after init has been called. Also it would reassign the property if init was called more than once, and that might not be desirable..init()-method will act like some sort of constructor and will be called only once.
Is there a way to define a property and immediately use it to set another property?
No. However, you can use a getter, which is more-or-less syntactic sugar for a function:
var grid = { x : 75, y : 75, get location() { return [this.x, this.y]; }, init : function () { console.log(grid.location[0]); } } http://jsfiddle.net/mattball/erUJj
This answer sums up the various options: https://stackoverflow.com/a/15486618
location: function() { return [this.x, this.y] },. same for init. Then to call it outside of grid, you usegrid.location()