Find in Mountain Array.js

/**
 * // This is the MountainArray's API interface.
 * // You should not implement it, or speculate about its implementation
 * function MountainArray() {
 *     @param {number} index
 *     @return {number}
 *     this.get = function(index) {
 *         ...
 *     };
 *
 *     @return {number}
 *     this.length = function() {
 *         ...
 *     };
 * };
 */

var binarySearch = function(left, right, condition) {
    while (left < right) {
        var mid = left + Math.floor((right - left) / 2);
        if (condition(mid)) {
            right = mid;
        } else {
            left = mid + 1;
        }
    }
    return left;
}

/**
 * @param {number} target
 * @param {MountainArray} mountainArr
 * @return {number}
 */
var findInMountainArray = function(target, mountainArr) {
    var n = mountainArr.length();
    var maxNumIdx = binarySearch(1, n - 2, function(idx) {
        return mountainArr.get(idx) > mountainArr.get(idx + 1);
    });
    var leftIdx = binarySearch(0, maxNumIdx, function(idx) {
        return mountainArr.get(idx) >= target;
    });
    if (mountainArr.get(leftIdx) === target) {
        return leftIdx;
    }
    var rightIdx = binarySearch(maxNumIdx, n - 1, function(idx) {
        return mountainArr.get(idx) <= target;
    });
    if (mountainArr.get(rightIdx) === target) {
        return rightIdx;
    }
    return -1;
};