Syntax doubt

[asd123ea]

I'm reading the 10 chapter, Building Microservice systems.
And I came accross to this syntax below at page 506 (Standardizing service boilerplate)

function createClient (ns, opts) {
  return createClient[ns] || (createClient[ns] = nos(net.connect(opts)))
}

Didn't quite understand the createClient[ns] = nos(net.connect(opts)) part, What it means? Why should I use it?

Is this syntax documented anywhere?