var N = 100,
    i;

var FirstWay = function() {
    this.doSomeCalculations = function() {
        // some calculations here
    };

    window.addEventListener( 'scroll', this.doSomeCalculations );
};

for ( i = 0; i < N; i++ ) {
    new FirstWay();
}

var N = 100,
    i,
    instances = [];

var SecondWay = function() {
    this.doSomeCalculations = function() {
        // some calculations here
    };
};

var doCommonCalculations = function() {
    var i;

    for ( i = 0; i < N; i++ ) {
        instances[ i ].doSomeCalculations();
    }
};

for ( i = 0; i < N; i++ ) {
    instances.push( new SecondWay() );
}

window.addEventListener( 'scroll', doCommonCalculations );