Using setter in Mongoose

According to mongoose.js doc, it is able to set setters to a field on schema.
However, the doc is not quite detailed and obselete.
Some example:

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var schema=new Schema({
  password:{
    type:String,
    required:true,
    set:hash
  }
})

function hash(plainPwd){
  return require("crypto").createHash("sha1").update(plainPwd).update(secret).digest("hex");
}
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node.js mongoose.js