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<article name="Understanding preloaded objects"
        link="/en/docs/njs/preload_objects.html"
        lang="en"
        rev="1"
        toc="no">

<section id="summary">

<para>
For each incoming request njs creates a separate virtual machine.
This brings a lot of benefits such as predictable memory consumption
or requests isolation.
</para>

<para>
However, it also has its own downsides.
Since all requests are isolated,
if a request handler needs to access some data,
it has to read it by itself.
This is not efficient especially when the amount of data is large.
</para>

<para>
To address this limitation,
a preloaded shared object was introduced.
Such objects are created immutable and do not have prototype chains:
their values cannot be changed, properties cannot be added or removed.
</para>

</section>


<section id="working_with_preload_objects"
       name="Working with preload objects">

<para>
Here are some examples of how to work with a preload object in njs:

<list type="bullet">

<listitem>
access properties by name:
<programlisting>
preloaded_object.prop_name
preloaded_object[prop_name]
</programlisting>
</listitem>

<listitem>
enumerate properties:
<programlisting>
for (i in preloaded_object_name) {
    ...
}
</programlisting>
</listitem>

<listitem>
apply non-modifying built-in methods using <literal>call()</literal>:
<programlisting>
Array.prototype.filter.call(preloaded_object_name, ...)
</programlisting>
</listitem>

</list>
</para>

</section>

</article>