Return number of "false" Boolean values from an array - MobX & React

Question

I have a mock data file that has the content arranged like so:

{
  id: 6,
  name: 'example',
  time: new Date(),
  viewed: false,
},

My goal is to be able to count the number of "false" values, and return that count to a Component to be used as content.

I have a file that defines a ModelStore. The beginning of it is like so:

const Model = types
  .model('Model', {
    id: types.identifierNumber,
    name: types.string,
    time: types.Date,
    viewed: types.boolean,
  });

/** @typedef {typeof Model} Model */
/** @typedef {import('mobx-state-tree').Instance<typeof Model>} ModelInstance */

const ModelStoreData = types.model({ Models: types.array(Model) });

You can see the mock data is imported as an array.

I then came up with something like the following:

const ModelStore = types
  .compose(
    ModelStoreData,
  )
  .named('ModelStore')
  .views((self) => {
    /**
     * Count number of unviewed models
     * @returns {typeof self.Models}
     */
    const countUnviewed = (Models) => {
      /**
       * Numerical urgency levels for sorting
       * @enum {number}
       */
      const viewedValues = {
        [true]: 0,
        [false]: 1,
      };

      /** Returns a numerical value */
      const value = Model => viewedValues[Model.viewed];

      return Models
        // do something with value here to add up
    };

    return {
      get unviewedCount() {
        return countUnviewed(self.Models);
      },
    };
  });

/** @typedef {typeof ModelStore} ModelStore */
/** @typedef {import('mobx-state-tree').Instance<typeof ModelStore>} ModelStoreInstance */

export default ModelStore;

but I'm unsure how to proceed from here on how to actually count up the number of "false" values.

I then have this Component that needs that counted value to be inserted:

const myComponent = withRouter(({ history }) => {
  const [isDropdownOpen, setIsDropdownOpen] = React.useState(false);

  const toggle = () => {
    setIsDropdownOpen(prevDropdownOpen => !prevDropdownOpen);
  };

  const handleOnClick = () => {
    history.push(routes.home.path);
  };

  return (
    <Dropdown isOpen={isDropdownOpen} toggle={toggle}>
      <DropdownToggle caret>
        Dropdown
      </DropdownToggle>
      <DropdownMenu right>
        <DropdownItem header>viewed items: 2</DropdownItem>
        <Button outline onClick={handleOnClick}>go home</Button>
      </DropdownMenu>
    </Dropdown>
  );
});

where "viewed items: 2" is located, substituting the "2" for the counted value.

Any guidance as to how to approach the situation would be appreciated.

Answer 1

You could use the array method filter to create a new array with every model that has viewed set to false and return the length of that array as unviewedCount.

const ModelStore = types
  .compose(ModelStoreData)
  .named("ModelStore")
  .views(self => {
    return {
      get unviewedCount() {
        return self.Models.filter(model => !model.viewed).length;
      }
    };
  });

You would then need to create an instance of the ModelStore and use the observer function on your component so that your component will re-render if unviewedCount changes.

const store = ModelStore.create();

const myComponent = withRouter(
  observer(({ history }) => {
    const [isDropdownOpen, setIsDropdownOpen] = React.useState(false);

    const toggle = () => {
      setIsDropdownOpen(prevDropdownOpen => !prevDropdownOpen);
    };

    const handleOnClick = () => {
      history.push(routes.home.path);
    };

    return (
      <Dropdown isOpen={isDropdownOpen} toggle={toggle}>
        <DropdownToggle caret>Dropdown</DropdownToggle>
        <DropdownMenu right>
          <DropdownItem header>
            viewed items: {store.unviewedCount}
          </DropdownItem>
          <Button outline onClick={handleOnClick}>
            go home
          </Button>
        </DropdownMenu>
      </Dropdown>
    );
  })
);