Understanding preloaded objects

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, ...)