如何将STDIN传递给node.js子进程

这是我如何使其工作的方法：

var cp = require('child_process');var optipng = require('pandoc-bin').path; //This is a path to a commandvar child = cp.spawn(optipng, ['--from=markdown', '--to=html']); //the array is the argumentschild.stdin.write('# HELLO'); //my command takes a markdown string...child.stdout.on('data', function (data) {    console.log('stdout: ' + data);});child.stdin.end();