var indexOf = function(arr, target) {

  var result = -1;

  var min = 0;

  var max = arr.length - 1;

  while (min <= max) {

    // Divide range in half and check value.

    var index = Math.floor(((max - min) / 2) + min);

    console.log('min=' + min + ', max=' + max + ', index=' + index);

    if (arr[index] === target) {

      // Found our target, we're done!

      result = index;

      break;

    }

    else if (arr[index] < target) {

      // Target is higher.

      min = index + 1;

    }

    else {

      // Target is lower.

      max = index - 1;

    }

  }

  return result;

}

​

var numbers = [];

for (var i=500; i<100000; i++) {

  numbers.push(i);

}

​

console.log(indexOf(numbers, 12345));

​