4-programmatic-keyselector-composition.md

Programmatic keySelector composition

This example shows how to decouple keySelector from provided inputSelectors and make it flexible enough to automatically handle changes in inputSelector declarations.

The final keySelector will be the composition of provided inputSelectors' keySelectors.

Since we don't want to know the number of provided inputSelectors upfront, we'll make use of the keySelectorCreator option: a function to generate the actual keySelector at runtime based on the actually provided inputSelectors.

keySelectorCreator might be implemented like this:

• get inputSelectors array
• filter out inputSelectors without a keySelector property
• return the chained result of inputSelectors's keySelector property (res1:res2:res3)

In the future, a similar utility might be shipped with re-reselect itself.

keySelectorCombiner.js

function keySelectorCombiner({inputSelectors = []} = {}) {
  const keySelectors = inputSelectors
    .map(entry => entry.keySelector)
    .filter(keySelector => typeof keySelector === 'function');

  // The actual keySelector
  return (...args) => {
    return keySelectors
      .map(keySelector => keySelector(...args))
      .filter(value => {
        const valueType = typeof value;
        return valueType === 'string' || valueType === 'number';
      })
      .join(':');
  };
}

composedSelector.js

import {createCachedSelector} from 're-reselect';
import keySelectorCombiner from './keySelectorCombiner';

const composedSelector = createCachedSelector(
  inputSelector1,
  inputSelector2,
  inputSelector3,
  (first, second, third) => ({
    first,
    second,
  })
)({
  keySelectorCreator: keySelectorCombiner,
});