Question

The javascript for keyword will iterate over all properties of an object. If the object is modified within the loop body, what happens?

For example, is the following code OK?

for(var key in obj)
    if (whatever(obj[key]))
        delete obj[key];

OK would be if this code works in a deterministic fashion and preferably that all keys in obj are tested exactly once. By contrast, in .NET or Java similar constructs will typically throw an exception.

Answer 1

I think it works. Just be careful to ask for hasOwnProperty(key) – because for will also happily iterate over inherited properties (and methods, which are just properties with function values).

Also: http://www.w3schools.com/js/js_loop_for_in.asp says:

Note: The code in the body of the for…in loop is executed once for each property.

Also: https://developer.mozilla.org/en/JavaScript/Reference/Statements/for…in says:

A for…in loop iterates over the properties of an object in an arbitrary order (see the delete operator for more on why one cannot depend on the seeming orderliness of iteration, at least in a cross-browser setting). If a property is modified in one iteration and then visited at a later time, the value exposed by the loop will be its value at that later time. A property which is deleted before it has been visited will not then be visited later. Properties added to the object over which iteration is occurring may either be visited or omitted from iteration. In general it is best not to add, modify, or remove properties from the object during iteration, other than the property currently being visited; there is no guarantee whether or not an added property will be visited, whether a modified property will be visited before or after it is modified, or whether a deleted property will be visited before it is deleted.

What I read from this is – if you’re modifying values other than the current one, the nondeterminism might bite you in the ass. However, modifying the current one should be okay.