Count the number of zig zags in an array (switches from increasing to decreasing or from decreasing to increasing)

ZigZag.js

// Counts how often a sequence zig zags.
// ie. switches from increasing to decreasing or from decreasing to increasing
// countZigZags([])              --> 0
// countZigZags([5,5,5,5])       --> 0
// countZigZags([1,2,3])         --> 0
// countZigZags([1,2,3,2,1])     --> 1
// countZigZags([5,6,5,6,5])     --> 3
// countZigZags([2,2,2,6,6,5,5]) --> 1
function countZigZags(numbers) {
    // sweep through the numbers finding points where it switches from increasing to decreasing or from decreasing to increasing

    let i;
    let isIncreasing;
    let count = 0;

    // figure out the initial slope
    for (i=1; i<numbers.length; ++i) {
        if (numbers[i-1] == numbers[i]) continue;

        isIncreasing = numbers[i-1] < numbers[i];
        break;
    }
    ++i;
        
    // count how oftern isIncreasing changes.
    for (; i<numbers.length; ++i) {
        if (numbers[i-1] == numbers[i]) continue;

        const newIsIncreaing = numbers[i-1] < numbers[i];
        if (newIsIncreaing == isIncreasing) continue;

        isIncreasing = newIsIncreaing;
        ++count;
    }
    return count;
}