Merge Sort (TypeScript)

mergesort.ts

const sort = (a: number[]): number[] => {
    const split = (a: number[]): [number[], number[]] => {
        const take_left = Math.ceil(a.length / 2);
        const take_right = Math.ceil(a.length / 2);
        const left = a.slice(0, take_left);
        const right = a.slice(take_right);
        return [left, right];

    };
    const merge = (a: number[], b: number[]): number[] => {
        if (a.length === 0) return b;
        if (b.length === 0) return a;
        const [a1, ...a_rest] = a;
        const [b1, ...b_rest] = b;
        return (a1 < b1) ? [a1, ...merge(a_rest, b)] : [b1, ...merge(b_rest, a)];
    };
    if (a.length < 2) return a;
    const [left, right] = split(a);
    return merge(sort(left), sort(right));
};

console.log(sort([1, 9, 2, 8, 3, 7, 4, 6, 5]));