axis Type helper in TS

typeHelper.ts

export const typeHelper = (() => {
    const types = [
        'Array',
        'Object',
        'String',
        'Date',
        'RegExp',
        'Function',
        'Boolean',
        'Number',
        'Null',
        'Undefined',
    ] as const;

    type MethodKeys = `is${typeof types[number] | 'NaN'}`;

    const functionObject: Record<MethodKeys, (elem: unknown) => boolean> = {} as Record<
        MethodKeys,
        (elem: unknown) => boolean
    >;

    function type(this: unknown) {
        return Object.prototype.toString.call(this).slice(8, -1);
    }

    for (let i = types.length; i--; ) {
        functionObject['is' + types[i]] = (function (self: string) {
            return function (elem: unknown) {
                return type.call(elem) === self;
            };
        })(types[i]);
    }

    // Need a special case for NaN for the extra check as NaN is technically a number
    // But it's the only number for which it won't equal itself
    functionObject['isNaN'] = (function (self: string) {
        return function (elem: unknown) {
            return type.call(elem) === self && elem !== elem;
        };
    })('Number');

    return functionObject;
})();