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25 Unnoticeable Features of JavaScript

M Mainul Hasan on March 13, 2024

Often, as developers, we write similar types of code, falling into a pattern that, while comfortable, can sometimes feel mundane. However, the wor...

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Jon Randy 🎖️ • Mar 13 • Edited

Beware...

x | 0 is not the same as Math.floor(x), neither is ~~x - as is sometimes written.

Jon Randy 🎖️ • Mar 14 • Edited

They are the same as Math.trunc(x)

Matt Ellen • Mar 14 • Edited

for numbers up to 32 bits. bigger than 32 bits they are different.

developer.mozilla.org/en-US/docs/W...

Dmitriy A. • May 5

@mmainulhasan we need more Bitwise operators examples

Hauke T. • Mar 13

Nice article. It would bei nice if you included links to the according MDN articles.

Your coclusion bothers me: eval, with, document.write() and __proto__ don't belong in anyones "toolkit". But: I'd really enjoy an an article named "Javascripts forbidden fruits".

If you care about your fellow developers and your future self - reading your Code, you also should avoid Labels, the comma operator, functions in blocks, void and any bitwise operators except for actual Bit Manipulation.

I personally dislike Automatic semicolons, because they might lead to hard-to-find Bugs.

Ellis • Mar 14

Many aspects of js ~~might~~ definitely lead to hard-to-find bugs.

Jan Küster • Mar 13

Aaaah nice, labelled break and continue, the fork and spoon of spaghetti code 🍝😄

Eckehard • Mar 14

break isn´t as bad as goto, there are situations where it can be most useful and not too much confusing. Adding some flags to break out a loop is not much clearer and has the same effect.

But it is worth to mention that in JS break without labels always breaks out to the outermost loop, so using labels can be a real life saver.

Fatemeh Paghar • Mar 13 • Edited

one more feature of JS
Memoization for Performance Optimization
Memoization is a technique used to optimize the performance of functions by caching the results of expensive function calls and returning the cached result when the same inputs occur again. This can significantly reduce redundant calculations, especially in recursive or computationally intensive functions.

function fibonacci(n, memo = {}) {
  if (n in memo) return memo[n];
  if (n <= 1) return 1;
  return memo[n] = fibonacci(n - 1, memo) + fibonacci(n - 2, memo);
}
console.log(fibonacci(10)); // Example usage

By memorizing function results, developers can enhance the efficiency of their code, particularly in scenarios where functions are repeatedly called with the same inputs, as it minimizes unnecessary computation. This aligns with the theme of exploring advanced JavaScript features to improve code performance and productivity.

Jon Randy 🎖️ • Mar 14

This isn't a feature of JS, rather a general programming technique.

Fatemeh Paghar • Mar 14

Maybe ...
But there is memoization in Rect to prevent extra rendering with usecallback and use Memo. Maybe this technique is implemented as a feature.

Jon Randy 🎖️ • Mar 14

React is built from plain JS

eshimischi • Mar 14

Not a feature of JS itself, just a practice developed over the years, do not confuse

Ajeesh Sunil • Mar 14

Explained it gre8 though, in easy mode

Samuel Rouse • Mar 14

Thanks for the article! This is fun trip down memory lane of many old features now opposed strongly for daily use. You will see many of them in compiled production code, still.

18 and on are more like the new features that people should definitely know.

Ellis • Mar 14 • Edited

Fun list, useful too.

I recognise two distinct groups, though:

Actually quite well known features which many developers use on a daily basis now,
Esoteric features which few developers do (or should) use.

Jon Randy 🎖️ • Mar 14 • Edited

JavaScript allows for chained assignments, which can assign a single value to multiple variables in one statement

Assignment in JS is not a statement unless combined with var, let, or const.

We use the assignment operator (=) to perform the assignment, and that assignment has a value, which is the second operand of the assignment operator (the value being assigned to the variable).

It's not that JS 'allows for chained assignments', they are possible as a direct result of the fact that an assignment using the assignment operator has a value, i.e. a = 2 has a value of 2.

So, in your example:

let a, b, c;
a = b = c = 5;

The code sets c to the value of 5, b to the value of c = 5 (which we know from the value of the assignment is also 5), and a to the value of b = c = 5 (which is also 5).

Philip Warkentien II • Mar 14

Good list, there were a few things I wasn't aware of. That said: #9 and #17 are the same. #3 missed the opportunity of showing a tagged template literal being used, since the setup is just another function, might be glanced over.

simon • Mar 14

When would one ever use "2 -- comma operator" ?

Mike Talbot ⭐ • Mar 14

Sometimes in for loops:

for (let i = 0, j = 10; i < j; i++, j--) {
  console.log(i, j);
}

simon • Mar 14

I dont think that is the same. let x = (1, 2); console.log(x); would be using the mentioned feature. But this will just log ‘2’

Mike Talbot ⭐ • Mar 14

It's the same feature, you're just not using the return value.

Jon Randy 🎖️ • Mar 14

Code golf

artydev • Mar 14

Thank you :-)

Soldeplata Saketos • Mar 14

number #25 is simply a lie

Wellington Domiciano • Mar 15 • Edited

Is not a lie, it's a feature in DevTools 111!

To better facilitate debugging, DevTools now supports evaluating expressions with private class members.
Source: developer.chrome.com/blog/new-in-d...

Your code produces an error when used in a <script> tag.

To reproduce the error in DevTools, try this:

(() => {
  class Counter {
    #count = 0;
    increment() {
      this.#count++;
    }
    getCount() {
      return this.#count;
    }
  }

  const a = new Counter();

  console.log(a.#count);
})()

Soldeplata Saketos • Mar 15 • Edited

ah, thanks for pointing that out. In any case I dislike very much classes, but I am glad they added the true privacy for properties.

I am more fan for functional programming, factories, pure functions, etc.

To create a private member which is not even visible:

(()=>{
const superPrivateCounter = 0;
  return {
   getCount: () => superPrivateCounter;
}
})()

Tobi • Mar 14

Holy Shot. I’m using comma in swift expressions since for ever and never new it’s possible in js. I will definitely use it now

Tobi • Mar 14

Unfortunately it’s not like I thought it works in conditions. The last one will be returned. No wonder I did not see it in the wild.

codingjlu • Mar 14

The reason so many are "unnoticeable" is because they're considered bad practice and deprecated from the language.

WangLiwen • Mar 14

Nice.

Soldeplata Saketos • Mar 14

I fail to understand the difference from point 9 and 17:

9 — in Operator for Property Checking
17 — The in Operator for Property Existence

gauharnawab • Mar 14

Thanks for sharing

Pieces 🌟 • Mar 13

Great list!

KevinLeeBuchan • Mar 15 • Edited

Number 3 has no example. I can't tell what you're trying to communicate.

RamR • Mar 15

thanks, good article. #1,#2 and #9 are new to me.

Stephen BJ • Mar 15

awesome dude ,,,
more useful :P

Scott Reno • Mar 13

Nice list! I didn't know about a few of these.

Jakub T. Jankiewicz • Apr 18 • Edited

9 and 17 are the same

9 — in Operator for Property Checking

17 — The in Operator for Property Existence

Gunabalan.S • Mar 15

Excellent compilation of features.

S.T. Karthik • Mar 18

1 — Labels for Loop and Block Statements
Was my first impression of this blog

Aliyu Muhammad • Mar 16

they are the same with math.floor