Understanding preloaded objects
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Understanding preloaded objects
Preloaded objects are supported only with the
njs JavaScript engine
and are not available with the
QuickJS engine.
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.
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.
Working with preload objects
Here are some examples of how to work with a preload object in njs:
access properties by name:
preloaded_object.prop_name
preloaded_object[prop_name]
enumerate properties:
for (i in preloaded_object_name) {
...
}
apply non-modifying built-in methods using call():
Array.prototype.filter.call(preloaded_object_name, ...)