Check if it's != null to avoid exception:

> a=null
null
> a.length
TypeError: Cannot read property 'length' of null
    at repl:1:3
    at REPLServer.self.eval (repl.js:109:21)
    at Interface.<anonymous> (repl.js:248:12)
    at Interface.EventEmitter.emit (events.js:96:17)
    at Interface._onLine (readline.js:200:10)
    at Interface._line (readline.js:518:8)
    at Interface._ttyWrite (readline.js:736:14)
    at ReadStream.onkeypress (readline.js:97:10)
    at ReadStream.EventEmitter.emit (events.js:126:20)
    at emitKey (readline.js:1058:12)