Today I Learned

How to optimize the performance of the map function in JavaScript

To optimize the performance of a map function in JavaScript, you can do a few things:

  1. Use the forEach method instead of map if you are not creating a new array from the iteration. forEach is faster because it does not create a new array.

  2. Use the map method on a smaller array if possible. For example, if you have a large array and you only need to map over a subset of it, create a new array with just the subset and use map on that.

  3. If you are using map to transform each element of the array, use a specialized method for the transformation instead of using map. For example, if you are transforming each element to a string, use the toString method instead of using map.

  4. If you are using map to filter an array, use the filter method instead. filter is optimized for filtering and will be faster than using map for that purpose.

  5. Use a for loop instead of map if you need to perform complex operations on each element of the array. map is optimized for simple transformations, so a for loop will be faster for more complex operations.

Redirect unauthenticated users with meteor.js and react-router 6

The challenge I encountered was accessing the Meteor.user() in React Router's route loader property.

Alternatively, instead of querying the server, I'm picking up the user id from localStorage. The absence of it indicates the user is logged off or never logged in. Until Meteor kicks off, no sensitive data is displayed anyway.

  path: '/',
  element: <App />,
  errorElement: <ErrorPage />,
  loader: async () => window.localStorage.getItem('Meteor.userId'),

and in app.tsx:

const userId: UserId | null = useLoaderData()
const navigate = useNavigate()

useEffect(() => {
  if (userId === null) navigate('/login')
}, [userId])

How to install npm packages from your local machine

If you need to manually test a package you are developing and don't want to make a release each time you modify something, you can install it directly from your local machine.

You can achieve this by using the following commands, depending on the package manager you use:

yarn add [path-to-your-package]
npm install [path-to-your-package]

This will have the following equivalent in package.json:

"dependencies": {
    "[package]": "file:[relative-path-to-package]",

How to dynamically display different types of cards based on a pattern

Let's say you have an array of cards inside which you want add more items.

Example: [card, card, card, itemOne, card, card, itemTwo, ...]

For this we will create another empty array that we will work with:

let posts = [];

const itemsTemplate = [itemOne, itemTwo];
let items = [itemOne, itemTwo];

for (let i = 0; i < cards.length; i++) {

  if (i !== 0) {

    //next index is multiple of 3
    if ((i + 1) % 5 === 3) {
      if (items.length === 0) {
        items = itemsTemplate;

    //next index is multiple of 5
    if ((i + 1) % 5 === 0) {
      if (items.length === 0) {
        items = itemsTemplate;