filter 是根据返回的值是否为真来判断是否保留元素，所以可以这样实现

Array.prototype.filter = Array.prototype.filter || function(func) {
    var arr = this;
    var r = [];
    for (var i = 0; i < arr.length; i++) {
        if (func(arr[i])) {
            r.push(arr[i]);
        }
    }
    return r;
}

因为目前大部分浏览器有 filter 实现，所以测试时只能改个名字叫 filterx，下面是测试代码

var data = [1,2,3,4,5,6,7];

Array.prototype.filterx = Array.prototype.filterx || function(func) {
    var arr = this;
    var r = [];
    for (var i = 0; i < arr.length; i++) {
        if (func(arr[i])) {
            r.push(arr[i]);
        }
    }
    return r;
}

console.log(data.filter(function(t) {
    return t % 3 > 0;
}));

console.log(data.filterx(function(t) {
    return t % 3 > 0;
}));

官方Array.filter()的polyfill:

if (!Array.prototype.filter) {
  Array.prototype.filter = function(fun/*, thisArg*/) {
    'use strict';

    if (this === void 0 || this === null) {
      throw new TypeError();
    }

    var t = Object(this);
    var len = t.length >>> 0;
    if (typeof fun !== 'function') {
      throw new TypeError();
    }

    var res = [];
    var thisArg = arguments.length >= 2 ? arguments[1] : void 0;
    for (var i = 0; i < len; i++) {
      if (i in t) {
        var val = t[i];

        // NOTE: Technically this should Object.defineProperty at
        //       the next index, as push can be affected by
        //       properties on Object.prototype and Array.prototype.
        //       But that method's new, and collisions should be
        //       rare, so use the more-compatible alternative.
        if (fun.call(thisArg, val, i, t)) {
          res.push(val);
        }
      }
    }

    return res;
  };
}

var arr = [3,5,6];
var newArr = arr.filter(function(v,i){
    if(v>3){
        return v;
    }
});
//[5,6]