<?xml version="1.0"?>
<!DOCTYPE article SYSTEM "../../../../dtd/article.dtd">
<article name="Understanding preloaded objects"
link="/en/docs/njs/preload_objects.html"
lang="en"
rev="3"
toc="no">
<section id="summary">
<para>
<note>
Preloaded objects are supported only with the
<link doc="engine.xml" id="njs_engine">njs</link> JavaScript engine
and are not available with the
<link doc="engine.xml" id="quickjs_engine">QuickJS</link> engine.
</note>
</para>
<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.
However, as 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>
