schm-extend.js

const schema = require("schm");

const exclaim = prevSchema => prevSchema.merge({
  parsers: {
    exclaim: v => `${v}!!!`,
  },
});

const userSchema = schema(
  {
    name: { type: String, exclaim: true },
    age: Number,
  }, 
  exclaim
);

userSchema.parse({
  name: "Haz",
  age: 27,
});