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Equivalent of set in ES6 to ES5

I have a set over which I am iterating in ES6. I am trying to convert it to its equivalent in ES5. My build is getting failed because of ES6. That's why I am converting it to ES5.
Here's my code in ES6
service.getDevices = function (date) {
var result = [];
var deviceList = devices[date.getTime()];
for (let item of deviceList) { // browser compatibility: support for ECMA6
result.push({"deviceName": item});
}
return result;
}
I am getting error because of 'let'. I tried using for (var item in deviceList), it does not display the charts.
I also tried this:
for(var i = 0; i < deviceList.length(); i++){
result.push({"deviceName" : deviceList[i]});
}
Even this is not working for set. can someone help and tell me how to iterate over a set in ES5 and if that is not possible, is there any equivalent way of doing it?

This is a basic set es5 class that I have used variations on over the years.
function Set(items) {
this._data = {};
this.addItems(items);
}
Set.prototype.addItem = function(value) {
this._data[value] = true;
return this;
}
Set.prototype.removeItem = function(value) {
delete this._data[value];
return this;
}
Set.prototype.addItems = function(values) {
for (var i = 0; i < values.length; i++) {
this.addItem(values[i]);
}
return this;
}
Set.prototype.removeItems = function(values) {
for (var i = 0; i < values.length; i++) {
this.removeItem(values[i]);
}
return this;
}
Set.prototype.contains = function(value) {
return !!this._data[value];
}
Set.prototype.reset = function() {
this._data = {};
return this;
}
Set.prototype.data = function() {
return Object.keys(this._data);
}
Set.prototype.each = function(callback) {
var data = this.data();
for (var i = 0; i < data.length; i++) {
callback(data[i]);
}
}
var set = new Set(['a', 'b', 'c']);
console.log(set.addItems(['a', 'd', 'e']).removeItems(['b', 'e']).data());
console.log(set.contains('a'));
console.log(set.contains('e'));
set.each(console.log)

Why not just iterate through the data and map the result with Array#map.
result = deviceList.map(function (item) {
return { deviceName: item };
});

I think your problem with your second for example is just that length is a property and not a function so you shouldn't add () to the end of it. A working version of this might look like this:
for(var i = 0; i < deviceList.length; i++){
result.push({"deviceName" : deviceList[i]});
}
This assumes (as #grabantot pointed out) that deviceList is an array, however, if it's a Set then you need to use the deviceList.size property.
However, there is a more compatible version of your first for loop which is the forEach() function (which is available on Array and Set), like this:
deviceList.forEach(function (item) {
result.push({"deviceName": item});
});

Find the length of an specific object within an array

In the following code there is a console log of obj['mn'] which returns the length of that specific object which is 2. The problem with the code is that it doesn't count the multidimentional array, and only it counts the first array. The result should be 4 because there are 4 'mn' in total. What am I doing wrong?
var arr = [['ab','pq','mn','ab','mn','ab'],'mn','mn'];
var obj = { };
for (var i = 0, j = arr.length; i < j; i++) {
if (obj[arr[i]]) {
obj[arr[i]]++;
}
}
console.log(obj['mn']);

This is what you're looking for:
var arr = [['ab','pq','mn','ab','mn','ab'],'mn','mn'];
var obj = { };
function count(arr, obj) {
for (var i = 0, j = arr.length; i < j; i++) {
if (Array.isArray(arr[i])) {
count(arr[i], obj);
}
else if (typeof obj[arr[i]] !== 'undefined') {
obj[arr[i]]++;
}
else {
obj[arr[i]] = 1;
}
}
return obj;
}
console.log(count(arr, obj));
This is a recursive implementation. When it gets to an array, the recursion get one level deeper.

You are calling obj[['ab','pq','mn','ab','mn','ab']], which is obviously not what you wanted.
You need a depth first search.
If arr[i] is an array, then you need to loop through that array.

Create array based on object properties

I'd like to use an object to configure some settings for an app. My idea is to start with this:
var obj = {
property_one: 3;
property_two: 2;
property_three: 1;
}
And I would like to end up with this:
var array = [
'property_one','property_one','property_one',
'property_two','property_two',
'property_three'
]
My current solution is to do this for each property:
function theConstructor(){
for(i=1; i <= obj.property_one; i++){
this.array.push('property_one');
};
for(i=1; i <= obj.property_two; i++){
this.array.push('property_two');
};
for(i=1; i <= obj.property_two; i++){
this.array.push('property_two');
};
}
But this gets tedious, because I might have many properties, and these might change as the app evolves.
I know I can loop through object's properties like this:
for (var key in obj) {
if (obj.hasOwnProperty(key)) {
array.push(key);
}
}
But this will push the value to the array, not the key (as a string). Any ideas about how I can do this more efficiently?

Try this
function theConstructor(){
for (var key in obj) {
if (obj.hasOwnProperty(key)) {
for(var i=1; i <= obj[key]; i++){
this.array.push(key);
};
}
}
}

Using Array.prototype.reduce():
var obj = {
property_one: 3,
property_two: 2,
property_three: 1
};
var resultArray = Object.keys(obj).reduce(function(result, curItem) {
for (var index = 0; index < obj[curItem]; index++) {
result.push(curItem);
}
return result;
}, []);
document.write(JSON.stringify(resultArray));

Javascript filter function polyfill

I am unable to understand why check corresponding to line if (i in t) - Line no.18 is placed in filter function 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;
};
}

It is to avoid the elements which are not defined yet, in the sparse arrays. See the following example,
var array = [];
array[3] = 10;
console.log(array.length);
// 4
So, the length of the array is 4, but only the element at index 3 is defined, all others are not defined yet. So, if you do
for (var i = 0; i < array.length; i += 1) {
console.log(i, array[i]);
}
you will get
0 undefined
1 undefined
2 undefined
3 10
Arrays are special JavaScript objects. The indices are just properties in the array object. Whenever you extend the array object with an index which is not in the array, the length property will be adjusted internally. In this case, there is only one property in the array object, with the name 3 defined. But we are trying to access elements from 0 to 3. So it returns undefined for all the indices which are not present in the array object yet.
To avoid that, we are checking if the current index is really defined in the array object, with that if statement.
for (var i = 0; i < array.length; i += 1) {
if (i in array) {
console.log(i, array[i]);
}
}
would print
3 10

This is because it's possible for JavaScript arrays to have gaps.
For example, consider the following:
var a = ["hello", "howdy", "welcome"];
delete greetings[1];
"0" in a; // true
"1" in a; // false
"2" in a; // true

Array.prototype.myFilter = function(callBack) {
let newArray = [];
for (let i = 0; i < this.length; i++) {
let result = callBack(this[i], i, this);
if (result) {
newArray.push(this[i]);
}
}
return newArray;

IN is a reserved keyword which can be used in for and if statement.
18th line they are doing exist check
Refer this dot vs in.

Create own filter() method
Array.prototype.newFilter = function(func){
let filtered = [], n = this.length;
for(let i = 0; i<n; i++) {
if(func(this[i],i,this))
filtered.push(this[i]);
}
return filtered;
}
let result = [1,2,3,4,5,6,7].newFilter(item => item > 4);
console.log(result);

Array.prototype.myFilter = function(callBackFn) {
let res = [];
if (!Array.isArray(this)) {
throw new Error(`${this} is not a function`);
}
for (let i = 0; i<this.length; i++) {
if (callBackFn(this[i], i, this)) {
res.push(this[i])
}
}
return res
}

replace items which appear in both arrays in javascript

I have two arrays and want to remove from one all elements which exist in the other as well.
Can this be done with native JS?
Is there a jQuery function to do it?
What are best practices to do so (the
faster the better)
p.s.: just post code in other languages too, maybe I can port it to Javascript
Update, after accepting answer to help the JS dudes ;-)
// Array Remove - By John Resig (MIT Licensed)
Array.prototype.remove = function(from, to) {
var rest = this.slice((to || from) + 1 || this.length);
this.length = from < 0 ? this.length + from : from;
return this.push.apply(this, rest);
};
// Array Contains - By Helmuth Lammer (TU Vienna)
Array.prototype.contains = function(key){
for(var i = 0; i<this.length; i++){
if(this[i] == key) return i;
}
return false;
}
(There is a native JS method named contains too, but it should work)

Given two sets a and b, to remove all elements present in a from b. Translation to array formatting and javascript left as an exercise to the reader. Optimizations exist for sorted arrays.
for(element e : b) {
if(a.contains(e)) {
b.remove(e);
}
}

var foo = [1, 2, 3, 4, 5];
function removeAll(a, b) {
var i = 0,
j;
while (i < b.length) {
for (j = 0; j < a.length; j++) {
if (a[j] === b[i]) {
b.splice(i, 1);
i--;
break;
}
}
i++;
}
}
removeAll([2, 4], foo);
// foo === [1, 3, 5]

[See it in action]
// two arrays
var arr1 = [1,2,3,4,5];
var arr2 = [4,5,6,7,8];
// returns the element's index in an array
// or -1 if there isn't such an element
function indexOf( arr, el ) {
for ( var i = arr.length; i--; ) {
if ( arr[i] === el ) return i;
}
return -1;
}
// go through array1 and remove all
// elements which is also in array2
​for ( var i = arr1.length; i--; )​ {
if ( indexOf( arr2, arr1[i] ) !== -1 ) {
arr1.splice(i, 1);
}
}
// result should be: arr1 = [1,2,3]
alert( arr1 );
​

I had to write a code once where an Ajax call would return an array containing a list of element which must be deleted from a local (client-side) array.
I ended implementing it this way:
// The filter method creates a new array with all elements that pass the
// test implemented by the provided function.
local_array = local_array.filter( function (element) {
// Now I check if this element is present in the "delete" array. In order to do
// that I use the "some" method which runs a callback function for each of the
// array elements and returns true or false.
return !items_to_delete.some( function(item) {
return item.pk === element.pk;
});
});
Edit:
In order to make it cross-browser (I am talking about IE here). You will have to define the some and filter functions. Just put this somewhere in your code:
if (!Array.prototype.some)
{
Array.prototype.some = function(fun /*, thisp*/)
{
var i = 0,
len = this.length >>> 0;
if (typeof fun != "function")
throw new TypeError();
var thisp = arguments[1];
for (; i < len; i++)
{
if (i in this &&
fun.call(thisp, this[i], i, this))
return true;
}
return false;
};
}
if (!Array.prototype.filter)
{
Array.prototype.filter = function(fun /*, thisp*/)
{
var len = this.length >>> 0;
if (typeof fun != "function")
throw new TypeError();
var res = [];
var thisp = arguments[1];
for (var i = 0; i < len; i++)
{
if (i in this)
{
var val = this[i]; // in case fun mutates this
if (fun.call(thisp, val, i, this))
res.push(val);
}
}
return res;
};
}