Use const whenever possible

const sets constant variables in JavaScript. This is useful when you don’t want variables to be able to change!

Below, we use var to set up a name variable. We can change that variable at any time after initialization and assignment:

var name = "Kristian"
name = "Someone"
console.log(name) // "Someone"

Using const, we can set up a variable, and when we try to change it, we’ll get an error back:

const name = "Kristian"
name = "Someone" // Uncaught TypeError: Assignment to constant variable.

Use array concat to add items to an array, immutably

You can combine two or more arrays using the concat method:

const arrayOne = [1]
const arrayTwo = [2, 3]
const newArray = arrayOne.concat(arrayTwo)
console.log(newArray) // [1, 2, 3]

concat is immutable — if you call arrayOne.concat, it doesn’t update arrayOne — it returns a new array that you can capture with a different variable:

const arrayOne = [1]
arrayOne.concat([2, 3]) // [1, 2, 3]
console.log(arrayOne)   // [1]

The concat method takes an arbitrary number of arguments, so you can pass as many arrays as you’d like:

const arrayOne = [1]
const newArray = arrayOne.concat([2, 3], [4, 5], [6, 7])
console.log(newArray) // [1, 2, 3, 4, 5, 6, 7]

JSON.parse can be faster than a JS object

For large objects, it can be faster to deserialize them from a JSON string using JSON.parse than to have them live as raw objects:

const data = { foo: 42, bar: 1337 }; // 🐌
const data = JSON.parse('{"foo":42,"bar":1337}'); // 🚀

This was documented by Addy Osmani’s blog post "The cost of JavaScript in 2019", where performance benchmarks found that V8 and other JavaScript engines can decode an object into an object using JSON.parse almost 50% faster:

Because the JSON grammar is much simpler than JavaScript’s grammar, JSON can be parsed more efficiently than JavaScript. This knowledge can be applied to improve start-up performance for web apps that ship large JSON-like configuration object literals (such as inline Redux stores). Instead of inlining the data as a JavaScript object literal.

As long as the JSON string is only evaluated once, the JSON.parse approach is much faster compared to the JavaScript object literal, especially for cold loads. A good rule of thumb is to apply this technique for objects of 10 kB or larger — but as always with performance advice, measure the actual impact before making any changes.