Selectors

Selectors are functions that take in a leaf's value and returns another value.

const user = new Forest({
  $value: {
    username: '',
    password: ''
  },
  selectors: {
    canSubmit(leaf) {
      const {username, password} = leaf.value;
      return username && password && leaf.$.passwordIsValid();
    },
    passwordIsValid(leaf) {
      const {password} = leaf.value;
      return typeof password === 'string' && (/a-z/.test(password) && /A-Z/.test(password) && /\d/.test(password))
    }
  }
});
 
user.do.set_username('Bob');
user.do.set_password('password');
console.log('first try valid: ', user.$.canSubmit());
 
user.do.setPassword('AveryGoodPassword1123');
console.log('second try valid:',  user.$.canSubmit());
/**
* first try valid: false
* second try valid: true
*/
 

Selectors can be async, if desired. They can also preform other utilities like calling AJAX endpoints, etc.

State is frozen during a selector.

Its not just a bad idea to change state in a selector - it is not allowed. The point of a selector is to return a derived value from, the state, as is, with no side effects.

Why use selectors?

Actions also can return values. However, actions are wrapped in a transactional context. as they can change state. Selectors are a utility mechanic, and they are faster than actions because they lack the insulation that Forest uses to protect and provide rollback values in the case of a failed action.