Unrolled Argument Applier

customApply.js

function apply(functor, thisArg = null, argsArray = null) {
	if (argsArray !== null && argsArray.length > 8) {
		// More than 8 arguments, use apply
		return functor.apply(thisArg, argsArray);
	}
	if (thisArg === null) {
		// thisArg is not present:
		// 0 arguments, call directly
		if (argsArray === null || argsArray.length === 0) return functor();
		// 8 or less arguments, call with directApplier
		return directAppliers[argsArray.length - 1](functor, argsArray);
	}
	// thisArg is present:
	// 0 arguments, call with call
	if (argsArray === null) return functor.call(thisArg);
	// 8 or less arguments, call with callApplier
	return callAppliers[argsArray.length - 1](functor, thisArg, argsArray);
}
const directAppliers = [
	(functor, args) => functor(args[0]),
	(functor, args) => functor(args[0], args[1]),
	(functor, args) => functor(args[0], args[1], args[2]),
	(functor, args) => functor(args[0], args[1], args[2], args[3]),
	(functor, args) => functor(args[0], args[1], args[2], args[3], args[4]),
	(functor, args) => functor(args[0], args[1], args[2], args[3], args[4], args[5]),
	(functor, args) => functor(args[0], args[1], args[2], args[3], args[4], args[5], args[6]),
	(functor, args) => functor(args[0], args[1], args[2], args[3], args[4], args[5], args[6], args[7])
];
const callAppliers = [
	(functor, thisArg, args) => functor.call(thisArg, args[0]),
	(functor, thisArg, args) => functor.call(thisArg, args[0], args[1]),
	(functor, thisArg, args) => functor.call(thisArg, args[0], args[1], args[2]),
	(functor, thisArg, args) => functor.call(thisArg, args[0], args[1], args[2], args[3]),
	(functor, thisArg, args) => functor.call(thisArg, args[0], args[1], args[2], args[3], args[4]),
	(functor, thisArg, args) => functor.call(thisArg, args[0], args[1], args[2], args[3], args[4], args[5]),
	(functor, thisArg, args) => functor.call(thisArg, args[0], args[1], args[2], args[3], args[4], args[5], args[6]),
	(functor, thisArg, args) => functor.call(thisArg, args[0], args[1], args[2], args[3], args[4], args[5], args[6], args[7])
];

function tester() {
	function test(arg1, arg2) {
		"use strict";
		return arg1 + arg2;
	}
	apply(test, null, null);
	apply(test, null, ["test1", "test2"]);
	apply(test, { test: 123 }, null);
	apply(test, { test: 123 }, ["test1", "test2"]);
	apply(test, null, ["test1", "test2", "test3", "test4", "test5", "test6", "test7", "test8"]);
	apply(test, null, ["test1", "test2", "test3", "test4", "test5", "test6", "test7", "test8", "test9", "test10"]);
}
tester();