This is an archived page. It's not actively maintained.


This interface implements an array object.
Inherits from: nsISupports Last changed in Gecko 1.7

An indexed collection of elements. Provides basic functionality for retrieving elements at a specific position, searching for elements. Indexes are zero-based, such that the last element in the array is stored at the index length-1.

For an array which can be modified, see nsIMutableArray.

Neither interface makes any attempt to protect the individual elements from modification. The convention is that the elements of the array should not be modified. Documentation within a specific interface should describe variations from this convention.

It is also convention that if an interface provides access to an nsIArray, that the array should not be QueryInterfaced to an nsIMutableArray for modification. If the interface in question had intended the array to be modified, it would have returned an nsIMutableArray!

null is a valid entry in the array, and as such any nsISupports parameters may be null, except where noted.

Method overview

nsISimpleEnumerator enumerate();
unsigned long indexOf(in unsigned long startIndex, in nsISupports element);
void queryElementAt(in unsigned long index, in nsIIDRef uuid, [iid_is(uuid), retval] out nsQIResult result);


Attribute Type Description
length unsigned long The number of elements in the array. Read only.



Enumerate the array.

nsISimpleEnumerator enumerate();


Return value

A new enumerator positioned at the start of the array.

Exceptions thrown
If the array is empty (to make it easy to detect errors).
If there is not enough memory to complete the operation.


Get the position of a specific element. Note that since null is a valid input, exceptions are used to indicate that an element is not found.

unsigned long indexOf(
  in unsigned long startIndex,
  in nsISupports element
The initial element to search in the array. To start at the beginning, use 0 as the startIndex.
The element you are looking for.
Return value

A number >= startIndex which is the position of the element in the array.

Exceptions thrown
If the element was not in the array.


Retrieve a specific element of the array, and QueryInterface it to the specified interface. null is a valid result for this method, but exceptions are thrown in other circumstances.

void queryElementAt(
  in unsigned long index,
  in nsIIDRef uuid,
  [iid_is(uuid), retval] out nsQIResult result
Position of element.
The IID of the requested interface.
The object, QueryInterface'd to the requested interface.
Exceptions thrown
When index > length-1.

See also