javascript - Fungsi teras underscore.js, mari analisa~

Question

    // Internal function that returns an efficient (for current engines) version
    // of the passed-in callback, to be repeatedly applied in other Underscore
    // functions.
    var optimizeCb = function(func, context, argCount) {
        if (context === void 0) return func;
        switch (argCount) {
            case 1: return function(value) {
                return func.call(context, value);
            };
            // The 2-parameter case has been omitted only because no current consumers
            // made use of it.
            case null:
            case 3: return function(value, index, collection) {
                return func.call(context, value, index, collection);
            };
            case 4: return function(accumulator, value, index, collection) {
                return func.call(context, accumulator, value, index, collection);
            };
        }
        return function() {
            return func.apply(context, arguments);
        };
    };

Answer 1

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参考