Just had a quick question about why a certain order of assignment works and another doesn't.
I wanted to create a simple "inherit" / "copy" function (just for testing it) that copies properties from one object to another:
var cat = { tail:"yes", hairy:"yes, hairy" };
var dog = { sick:"extremely ill"};
function inherit(obj1, obj2) {
for (var p in obj1)
{
obj2[p] = obj1[p]; // this works, but "obj1[p] = obj2[p];" doesn't. Why??
}
}
inherit(cat, dog);
console.log(dog.tail);
You are looping over all the properties of obj1, so all those properties exist on obj1.
If you try to copy from obj2 then you are trying to copy properties that don't exist (on that object), so you cause an error.
You're reading the properties var p in obj1, so those indexes only necessarily exist in obj1. As such, trying to assign obj1[p] = obj2[p]; won't work as expected, since there is no guarantee (and in your particular example this is certainly the case) that obj[p] is defined. This assignment will simply assign undefined to indexes in obj1 that don't exist in obj2 and copy the values whose indexes do exist in obj2.
You need to loop over each object's properties separately (i.e. two loops), although this also isn't a good idea, since any values with the same index on both object will result in one being wiped out. What are you actually trying to achieve by this? It seems a very dangerous/volatile thing to do.
obj2[p] = obj2[p] || obj1[p]; (or some variant if you might be assigning falsy values). If it's just a programming exercise to learn about loops and assignment, it's a fair question, IMO.