function mergeSort(arr) {
    if (arr.length < 2) {
        // Arrays of length 0 or 1 are sorted by definition.
        return arr;
    }

    const left = arr.slice(0, Math.floor(arr.length / 2));
    const right = arr.slice(Math.floor(arr.length / 2), Math.floor(arr.length));

    return merge(mergeSort(left), mergeSort(right));
}

function merge(arr1, arr2) {
    const merged = [];
    let i = 0, j = 0;

    while (i < arr1.length && j < arr2.length) {
        if (arr1[i] <= arr2[j]) {
            merged.push(arr1[i]);
            i++;
        } else if (arr2[j] < arr1[i]) {
            merged.push(arr2[j]);
            j++;
        }
    }

    merged.push(...arr1.slice(i), ...arr2.slice(j));
    return merged;
}

const deepEqual = require('./deepEqual');

console.log(deepEqual(
    mergeSort([]),
    [],
));
console.log(deepEqual(
    mergeSort([1]),
    [1],
));
console.log(deepEqual(
    mergeSort([2, 1]),
    [1, 2],
));
console.log(deepEqual(
    mergeSort([7, 2, 4, 3, 1, 2]),
    [1, 2, 2, 3, 4, 7],
));
console.log(deepEqual(
    mergeSort([7, 2, 4, 3, 1, 2]),
    [1, 2, 2, 3, 4, 7],
));
console.log(deepEqual(
    mergeSort([10, 9, 8, 7, 6, 5, 4, 3, 2, 1]),
    [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
));
console.log(deepEqual(
    mergeSort([98322, 3242, 876, -234, 34, 12331]),
    [-234, 34, 876, 3242, 12331, 98322],
));
console.log(deepEqual(
    mergeSort([1, 2, 3, 4, 5, 0]),
    [0, 1, 2, 3, 4, 5],
));

function mergeSort(arr) {
    if (arr.length < 2) {
        // Arrays of length 0 or 1 are sorted by definition.
        return arr;
    }

    const left = arr.slice(0, Math.floor(arr.length / 2));
    const right = arr.slice(Math.floor(arr.length / 2), Math.floor(arr.length));

    return merge(mergeSort(left), mergeSort(right));
}

function merge(arr1, arr2) {
    const merged = [];
    let i = 0, j = 0;

    while (i < arr1.length && j < arr2.length) {
        if (arr1[i] <= arr2[j]) {
            merged.push(arr1[i]);
            i++;
        } else if (arr2[j] < arr1[i]) {
            merged.push(arr2[j]);
            j++;
        }
    }

    merged.push(...arr1.slice(i), ...arr2.slice(j));
    return merged;
}

const deepEqual = require('./deepEqual');

console.log(deepEqual(
    mergeSort([]),
    [],
));
console.log(deepEqual(
    mergeSort([1]),
    [1],
));
console.log(deepEqual(
    mergeSort([2, 1]),
    [1, 2],
));
console.log(deepEqual(
    mergeSort([7, 2, 4, 3, 1, 2]),
    [1, 2, 2, 3, 4, 7],
));
console.log(deepEqual(
    mergeSort([1, 2, 3, 4, 5, 0]),
    [0, 1, 2, 3, 4, 5],
));
console.log(deepEqual(
    mergeSort([10, 9, 8, 7, 6, 5, 4, 3, 2, 1]),
    [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
));
console.log(deepEqual(
    mergeSort([98322, 3242, 876, -234, 34, 12331]),
    [-234, 34, 876, 3242, 12331, 98322],
));

Fix mergeSort tests