SOURCE

var arr = [3,48,66,71,99,101,120,151,188,209];

function searching(arr,target) {
    var start=0, end=arr.length-1, midIndex, midValue;
    while(start <= end) {
        midIndex = Math.floor((start + end) / 2);
        midValue = arr[midIndex];
        if(target === midValue) {
            return midIndex;
        }else if(target < midValue) {
            end = midIndex - 1;
        }else {
            start = midIndex + 1;
        }
    }
    return -1;
}

document.write(searching(arr,120));