It returns the date of easter - see unit test: http://jsfiddle.net/6ZtTP/

getEasterSunday.js

// Unit testing: http://jsfiddle.net/6ZtTP/

function getEaster(year) {
    "use strict";
    var a, k, m, d, s, r, og, sz, oe, os, date;
    date = new Date(year,2,1,0,0);
    
    // Moon parameter
    a = year % 19;        
    // Secular number
    k = Math.floor(year / 100); 
    
    // Secular Moon shift
    m = 15 + Math.floor((3 * k + 3) / 4) - Math.floor((8 * k + 13) / 25);
     // Seed for 1st full Moon in spring
    d = (19 * a + m) % 30;   
    // Secular sun shift
    s = 2 - Math.floor((3 * k + 3) / 4);  
    // Calendarian correction quantity
    r =  Math.floor((d + Math.floor(a / 11)) / 29); 
    // Easter limit
    og = 21 + d - r;
    
  // 1st sunday in March
    sz = 7 - (year + Math.floor(year / 4) + s) % 7;
    // Distance Easter sunday from Easter limit in days
    oe = 7 - (og - sz) % 7;// Distance Easter sunday from Easter limit in days
    // Easter sunday as number of days in March
    os = (og + oe)  ; 
    
    // Convert to dare
    date.setDate(date.getDate() + os - 1);
 
    return date;
}