getClosestValues.js

var getClosestValues = function(a, x) {
    var lo = -1, hi = a.length;
    while (hi - lo > 1) {
        var mid = Math.round((lo + hi)/2);
        if (a[mid] <= x) {
            lo = mid;
        } else {
            hi = mid;
        }
    }
    if (a[lo] == x) hi = lo;
    return [a[lo], a[hi]];
}

getLyricByTimestamp.js

var getLyricByTimestamp = function(a, x) {
    var lo = -1, hi = a.length;

    if(x >= a[hi-1]["timestamp"]) {
        return [hi, hi]
    }

    if(x <= a[0]["timestamp"]) {
        return [0, 0];
    }

    while (hi - lo > 1) {
        var mid = Math.round((lo + hi)/2);
        if (a[mid]["timestamp"] <= x) {
            lo = mid;
        } else {
            hi = mid;
        }
    }
    if (a[lo]["timestamp"] == x) hi = lo;
    return hi;
}

main.js

var getClosestIndex = function(a, x) {
    var lo = -1, hi = a.length;

    if(x >= a[hi-1]) {
        return [hi, hi]
    }

    if(x <= a[0]) {
        return [0, 0];
    }

    while (hi - lo > 1) {
        var mid = Math.round((lo + hi)/2);
        if (a[mid] <= x) {
            lo = mid;
        } else {
            hi = mid;
        }
    }
    if (a[lo] == x) hi = lo;
    return [lo, hi];
}


var arr = [1.2, 2.4, 3.6, 4.8];
console.log(getClosestIndex(arr, -2));