micro-reselect: memoize a function based on the state passed to it (mimics reselect npm module)

micro-reselect.js

/**
 * Implement a simplified createSelector function mimicking the reselect module.
 *
 * Memoizes the result of a function called with a state object as its argument.
 * The given function is only called when state values change.
 */

/**
 * A resolver returns a value from a state object (key/value pairs).
 *
 * @typedef {(state: any, props?: any) => any} Resolver
 */

/**
 * @param {Resolver | Resolver[]} resolvers
 * @param {(state: any) => any} fn
 *
 * @returns {(state: any, props?: any) => any}
 */
const createSelector = (resolvers, fn) => {
  let resolved = []
  let cache = null

  // Convert resolvers to an array.
  const res = Array.isArray(resolvers) ? resolvers : [resolvers]

  /**
   * @type {(state: any, props?: any) => any}
   */
  const selector = (state, props) => {
    // Get the current state's values.
    const values = res.map(r => r(state, props))

    // Determine if any of the values have changed.
    const changed = values.some((value, i) => value !== resolved[i])

    if (changed) {
      // Update resolved state values and the cache.
      resolved = values
      cache = fn.apply(null, values)
    }

    return cache
  }

  return selector
}

export default createSelector