use-isInstance.rst

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use-isInstance

==============

Prefer ``.isInstance()`` in chrome scripts over the standard ``instanceof``

operator for DOM interfaces, since the latter will return false when the object

is created from a different context.

These files are covered:

- ``*.sys.mjs``

- ``*.jsm``

- ``*.xhtml`` with ``there.is.only.xul``

- ``*.js`` with a heuristic

Since there is no straightforward way to detect chrome scripts, currently the

linter assumes that any script including the following words are chrome

privileged. This of course may not be sufficient and is open for change:

- ``ChromeUtils``, but not ``SpecialPowers.ChromeUtils``

- ``BrowserTestUtils``, ``PlacesUtils``

- ``document.createXULElement``

- ``loader.lazyRequireGetter``

- ``Services.foo``, but not ``SpecialPowers.Services.foo``

Examples of incorrect code for this rule:

-----------------------------------------

.. code-block:: js

    node instanceof Node

    text instanceof win.Text

    target instanceof this.contentWindow.HTMLAudioElement

Examples of correct code for this rule:

---------------------------------------

.. code-block:: js

    Node.isInstance(node)

    win.Text.isInstance(text)

    this.contentWindow.HTMLAudioElement.isInstance(target)