We need to create validators for each status. The validator will check if transition from current state is allowed or not.

Script

var todo = {
    status: 'open',
    validators: {
      'open': ['close'],
      'close': ['open'],
      'assigned': ['open', 'close']
    },
    init: function(status) {
      this.status = status;
      return this;
    },
    change: function(newStatus) {
      if (this.validators[this.status].indexOf(newStatus) > -1) {
        this.status = newStatus;
        return this;
      } else {
        return 'Error';
      }
    }
}