Iterate over an array until a condition is true

array-until.js

Array.prototype.until = function ( test, fn) {
    var l = this.length,
        i = l,
        _;
    for ( ; i && !test( _ = this[l-i] ) ; --i ) {
        fn( _ );
    }
};

array-until.test.js

var a = [ 1, 2, 3, 4 ],
    _,
    test = function ( y ) { return 3 === y; },
    fn = function ( x ) { if (3 === x) _ = x; };

a.until( test, fn );

assert( 3 === _, 'Array.until test expected 3, got ' + _ );