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    Selectors with parameters and dynamic dependencies.

    Dynamic selectors can access state and call each other dynamically, even conditionally or within loops, without needing to register dependencies up-front. As with Reselect and Re-reselect, functions are only re-run when necessary.

    This may be used with a state library like Redux, or on its own as a general memoization util.

    For more information or related packages, see the Dynamic Selectors workspace.


    import { createDynamicSelector } from '@dynamic-selectors/core';
    /* Simple selectors can access state, like normal */
    const getAuthor = createDynamicSelector((getState, authorId) => {
      return getState(`authors[${authorId}]`);
    /* Selectors can call other selectors inline -- even in loops */
    const getBooksForAuthor = createDynamicSelector((getState, authorId) => {
      const author = getAuthor(authorId);
      if (author) {
        return => getState(`books[${bookId}]`));
      // Else: throw, return default value, etc
    /* Because dependencies are dynamic, selectors are easier to compose together */
    const getBooksForMultipleAuthors = createDynamicSelector((getState, authorIds) => {
    getBooksForMultipleAuthors(state, [1, 2, 3]);
    getBooksForMultipleAuthors(state, [4, 5, 6]);
    // This hits the cache
    getBooksForMultipleAuthors(state, [1, 2, 3, 4, 5, 6]);

    getBooksForAuthor(authorId) will be rerun only when its dependencies change: state.authors[authorId] or one of the state.books[bookId] values it accessed.

    The result is cached per authorId. The same cache is used if you call getBooksForAuthor(state, authorId) directly.

    getBooksForMultipleAuthors(state, authorIds) will be cached the same way: it will rerun only when one of its getBooksForAuthor dependencies has changed. Dependencies can be state values (from getState), or other dynamic selectors.


    Pass arguments to selector functions
    Results are memoized by the params you pass to a selector, so selectBooks({ authorId: 3 }) and selectBooks({ authorId: 4 }) will work properly and be cached independently.
    Call selectors from within selectors
    A selector can call other selectors from `if` blocks, loops, or any other controls -- or even recursively.
    Auto-detected dependencies
    When a selector runs, any secondary selectors it calls get marked as dependencies. It won't re-run unless those dependencies return something new. The dependencies can change from one run to the next.
    Equality comparisons / caching strategy
    Like the useSelector hook, you can specify your own comparison function to 'freeze' updates. This may be customized for each selector.

    Syntax comparison between Reselect and Dyanamic Selectors.


    Your selector function receives two arguments: getState and params (optional).

    • getState(path: string, defaultValue?: any) lets you access any path in state. It works like lodash's get.
    • params (optional) can be anything you want, but it's best as either a single primitive value or a small object containing a few values -- like route params.

    The selector's results will be cached using params. See Options to customize the cache key generation.

    Calling a dynamic selector

    To call a selector, pass it your state and params (optional).

    When calling one selector from within another, pass params only.

    const getRawList = createDynamicSelector((getState, { listId }) => {
      return getState(`lists.${listId}`);
    const getSortedList = createDynamicSelector((getState, { listId, sortField }) => {
      // `state` is not pased when one selector calls another
      const rawList = getRawList({ listId });
      if (rawList && sortField) {
        return sortBy(rawList, sortField);
      return rawList;
    // `state` is passed in when you call the outermost selector
    getSortedList(state, { listId: 123, sortField: 'title' });

    Additional selector properties

    These are attached to the selector function returned by createDynamicSelector.


    If there's a usable cached value for the current state and params, it will be returned. Returns undefined otherwise.

    Call this just like the selector itself (i.e., (state, params) from outside, (params) from inside a selector).


    Returns a boolean to indicate whether or not there's a usable value in cache for the current state and params.

    Call this just like the selector itself (i.e., (state, params) from outside, (params) from inside a selector).


    Re-initializes the cache of previous results.

    selector.getDebugInfo() (development only)

    Returns an object with statistics about the selector's activity (runs, cache-checks, results, etc.)

    selector.isDynamicSelector (always true)

    The canonical way to detect that a function is a dynamic selector.


    When creating a selector, you can pass a second argument with options to customize its behavior.

    const mySelectorFn = createSelectorFn(fn, options);

    compareResult (function(oldValue, newValue), default: shallowEqual)

    After a selector runs, compares its previous cached value to the newly-returned value. Return true to discard the new value and reuse the previous value instead. This is useful for selectors that return arrays or other objects which may be new instances but do not actually contain new values.

    createResultCache (function(), default: plain object)

    Used to customize the cache where results are stored. The cache must implement get(cacheKey: string) and set(cacheKey: string, value: any). To limit the cache size or cache time using Limited-Cache you would customize this like:

    createDynamicSelector(myFn, {
      createResultCache: () => LimitedCache({ maxCacheSize: 100, maxCacheTime: 60 * 1000 }),

    debug (boolean | string, default: false, development only)

    Verbose output: logs all selector activity (runs, cache-checks, results, etc) to the console.

    displayName (string, default: displayName of your function)

    Sets the displayName of the returned selector function, and includes it in verbose debug output (if enabled.)

    getKeyForParams (function(params), default: JSON.stringify)

    Generates a string cache key that represents the params. To get constant hashes even when object properties are in different orders from one call to the next, you would customize this like:

    Example using node-object-hash:

    var hashSortCoerce = hasher({ sort: true, coerce: true });
    createDynamicSelector(myFn, {
      getKeyForParams: (params) => JSON.stringify(hashSortCoerce(params)),

    Example using object-hash:

    createDynamicSelector(myFn, {
      getKeyForParams: hash,

    onError (function(error, selectorArgs, selectorFn), default: null)

    Called if the selector function throws an exception. This may recover from the exception -- and supply a new return value for the selector -- by returning any non-undefined value.

    If onError is not set, or if it does not return a value when it runs, then the error will be re-thrown.

    State-Level Options

    In addition to the per-selector options above, you can set state-level options which control the selectors' interactions with state and with other selectors.

    This gives you a new selector-creating function: use that instead of the default createDynamicSelector. This becomes a "zero dependencies" library if you override the default options to not use shallowEqual and _.get.

    const mySelectorFactory = dynamicSelectorForState(stateOptions);
    const mySelectorFn = mySelectorFactory(fn, options);

    Each distinct stateOptions represents a separate store or type of state. You may mix-and-match selectors for different stateOptions, but this may not work as expected: please contact me if you're doing that to let me know your specific use case.

    compareState (function(oldState, newState), default: ===)

    Compares the current state to the state a selector last ran for. Return true to indicate that state is unchanged. A selector that's called with the same state and params will always use its cache.

    get (function(state, pathString, defaultValue?), default: _.get)

    Accessor to retrieve a value from the state. The path will always be a string. To completely avoid lodash, you can customize this to use something like tiny-get instead.

    defaultSelectorOptions (selector options, default: options)

    The default, base options that will be used for each selector (unless overridden when creating the selector.)


    npm i @dynamic-selectors/core

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