[ 源代码： node-defined ]
软件包：node-defined（1.0.0-2）
return the first argument that is `!== undefined`
Most of the time when you chain together ||s, you actually just want the first item that is not undefined, not the first non-falsy item.
This module is like the defined-or (//) operator in perl 5.10+.
Node.js is an event-based server-side JavaScript engine.