Computed Properties

Singular computations

Entity classes are just normal classes, so any common derived data can just be added as getters to the class itself.

import { Entity } from '@data-client/rest';

class User extends Entity {
  id = '';
  firstName = '';
  lastName = '';
  username = '';
  email = '';

  get fullName() {
    return `${this.firstName} ${this.lastName}`;
  }

  static key = 'User';
}

If the computations are expensive feel free to add some memoization.

import { Entity } from '@data-client/rest';
import memoize from 'nano-memoize';

class User extends Entity {
  truelyExpensiveValue = memoize(() => {
    // compute that expensive thing!
  });
}

tip

If you simply want to deserialize a field to a more useful form like Temporal.Instant or BigNumber, you can use the declarative static schema.

import { Entity } from '@data-client/rest';
import BigNumber from 'bignumber.js';

class User extends Entity {
  id = '';
  firstName = '';
  lastName = '';
  createdAt = Temporal.Instant.fromEpochSeconds(0);
  lifetimeBlinkCount = BigNumber(0);

  static key = 'User';

  static schema = {
    createdAt: Temporal.Instant.from,
    lifetimeBlinkCount: BigNumber,
  };
}

Global computations

Query can be used for computations of derived data from more than one entity. We generally call these aggregates.

resources/User

UsersPage

import { schema } from '@data-client/rest';
import { useQuery, useSuspense } from '@data-client/react';
import { UserResource, User } from './resources/User';

const getUserCount = new schema.Query(
  new schema.All(User),
  (entries, { isAdmin } = {}) => {
    if (isAdmin !== undefined)
      return entries.filter(user => user.isAdmin === isAdmin).length;
    return entries.length;
  },
);

function UsersPage() {
  useSuspense(UserResource.getList);
  const userCount = useQuery(getUserCount);
  const adminCount = useQuery(getUserCount, { isAdmin: true });
  // this should never happen since we suspense but typescript does not know that
  if (userCount === undefined) return null;
  return (
    <div>
      <div>Total users: {userCount}</div>
      <div>Total admins: {adminCount}</div>
    </div>
  );
}
render(<UsersPage />);

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