{
  "schema": {
    "title": "Custom validation",
    "description" :
      "This form defines custom validation rules checking that the two passwords match.",
    "type": "object",
    "properties": {
      "pass1": {
        "title": "Password",
        "type": "string",
        "minLength": 3
      },
      "pass2": {
        "title": "Repeat password",
        "type": "string",
        "minLength": 3
      },
      "age": {
        "title": "Age",
        "type": "number",
        "minimum": 18
      }
    }
  },
  "uiSchema": {
    "pass1": { "ui:widget": "password" },
    "pass2": { "ui:widget": "password" }
  },
  "formData": {},
  "validate": function ({ pass1, pass2 }, errors) {
    if (pass1 !== pass2) {
      errors.pass2.addError("Passwords don't match.");
    }
    return errors;
  },
  "transformErrors": function(errors) {
    return errors.map(error => {
      if (error.name === "minimum" && error.property === "instance.age") {
        return Object.assign({}, error, {
          message: "You need to be 18 because of some legal thing",
        });
      }
      return error;
    });
  }
}