Binary search

bsearch.js

Array.prototype.bsearch = function (x) {
    var min = 0, max = this.length;
    
    while (min <= max) {
        var mid = Math.floor((min + max) / 2),
            val = this[mid];

        if (val === x) {
            return mid;
        }
        
        if (x > val) {
            min = mid + 1;
        } else {
            max = mid - 1;
        }
    }
    
    return -1;
};

var Letters = [ 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z' ];

alert(Letters.bsearch('A'));
alert(Letters.bsearch('K'));
alert(Letters.bsearch('Z'));
alert(Letters.bsearch('Ъ'));