ReferenceError: StringDecoder is not defined

1、错误描述
> var sd=require("string_decoder").StringDecoder;
undefined
> var decoder=new StringDecoder();
ReferenceError: StringDecoder is not defined
at repl:1:17
at REPLServer.defaultEval (repl.js:262:27)
at bound (domain.js:287:14)
at REPLServer.runBound [as eval] (domain.js:300:12)
at REPLServer.<anonymous> (repl.js:431:12)
at emitOne (events.js:82:20)
at REPLServer.emit (events.js:169:7)
at REPLServer.Interface._onLine (readline.js:211:10)
at REPLServer.Interface._line (readline.js:550:8)
at REPLServer.Interface._ttyWrite (readline.js:827:14)
>
2、错误原因
     由错误提示可知，StringDecoder没有定义

3、解决办法
> var StringDecoder=require("string_decoder").StringDecoder;
undefined
> var decoder=new StringDecoder();
undefined