Convert array into Zig-Zag fashion

const zigzag = <T>(array: T[]): T[] => {
    const zigzaggedArray: T[] = [array[0]];
    let isGreaterThanExpected = true;

    for (let i = 0; i < array.length - 1; ++i) {
        if ((isGreaterThanExpected && zigzaggedArray[i] < array[i + 1]) ||
            (!isGreaterThanExpected && zigzaggedArray[i] > array[i + 1])) {

            zigzaggedArray[i + 1] = zigzaggedArray[i];
            zigzaggedArray[i] = array[i + 1];
        }
        else
            zigzaggedArray[i + 1] = array[i + 1];

        isGreaterThanExpected = !isGreaterThanExpected;
    }

    return zigzaggedArray;
}

const array = [2, 6, 8, 9, 5, 1];
const zigzaggedArray = zigzag(array);

console.log(JSON.stringify(array));
console.log(JSON.stringify(zigzaggedArray));