reselect.js

(function () {
    function createSelectorCreator (valueEquals) {
        return (selectors, resultFunc) => {
            if (!Array.isArray(selectors)) {
                selectors = [selectors];
            }
            const memoizedResultFunc = memoize(resultFunc, valueEquals);
            return state => {
                const params = selectors.map(selector => selector(state));
                return memoizedResultFunc(params);
            }
        };
    }

    function createSelector () {
        return createSelectorCreator(defaultValueEquals).apply(null, arguments);
    }

    function defaultValueEquals (a, b) {
        return a === b;
    }

    // the memoize function only caches one set of arguments.  This
    // actually good enough, rather surprisingly. This is because during
    // calculation of a selector result the arguments won't
    // change if called multiple times. If a new state comes in, we *want*
    // recalculation if and only if the arguments are different.
    function memoize (func, valueEquals) {
        let lastArgs = null;
        let lastResult = null;
        return (args) => {
            if (lastArgs !== null && argsEquals(args, lastArgs, valueEquals)) {
                return lastResult;
            }
            lastArgs = args;
            lastResult = func(...args);
            return lastResult;
        }
    }

    function argsEquals(a, b, valueEquals) {
        return a.every((value, index) => valueEquals(value, b[index]));
    }
    
    window.reselect = {
        createSelectorCreator: createSelectorCreator,
        createSelector: createSelector
    }
}());