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Ankur Tyagi
Ankur Tyagi

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Top Mistakes I made as a JavaScript Developer.

Top Mistakes I made as a JavaScript Developer...

Top Mistakes I made as a JavaScript Developer...

A Thread...

— Ankur💻🎧💪 (@TheAnkurTyagi) June 2, 2021

Today, JavaScript is at the core of virtually all modern web applications.

Let me show you some typical mistakes that almost every JS programmer has made during their career.

I've been a JS dev since 2014 & I still make some of these mistakes, especially when coding in a hurry.

▫ Do you remember the difference between «=», «==» and «===»?

You might encounter a problem with code Down pointing backhand index

You'll get “Hello”! Why?

Because you don’t understand the difference between the 3 operators mentioned above.


*Let’s get this thing over with and go further: *

“=” is the equal operator, so it’s used for assignment.

In our example, we assign seven to “x” in the condition and get words of welcome “Hello”.

The correct code looks like this:

We get “Nope”.


“==” is the loose equality comparison operator.

Why loose? Because it allows converting values from one type to another to compare them

“===” is the strict equality comparison operator.

If this operator returns “true” it means that our values are identical both in value & type.

▫ Inefficient DOM manipulation

JavaScript makes it relatively easy to manipulate the DOM (i.e., add, modify, and remove elements).

A common example is a code that adds a series of DOM Elements one at a time.

Adding a DOM element is an expensive operation.

One effective alternative when multiple DOM elements need to be added is to use document fragments instead, thereby improving both efficiency and performance.


▫ Used '&' when I mean to use '&&'

The 'bitwise' & is used to compare Integers, and if the values being compared are not Integers, they are coerced into Integers.

The standard && is used to compare the truthiness of the operands.

So 123 & false === 0 (because the false becomes a 0)

but 123 && false === false 9 & 5 === 1 but 9 && 5 === 5.

But why does 9 && 5 === 5 and not equal true?

Can you tell me why? Try yourself...

▫ Misunderstand the difference between “let”, “const” and “var”

Let’s first look at the code below:

The code is logical as the output, no questions.


Another example:

The reason is that var is function scoped and let is block scoped.

When you declare a variable with a let keyword, they are moved to the beginning of the block.

This may lead to a reference error when you try to access the variable before the initialization.


- Incorrect use of function definitions inside for loops

- Fail to notice that 'this' is not always 'this'

- Incorrect references to instance methods

- Thinking that variables can be scoped to their blocks

▫ Fail to use “strict mode

“strict mode” (i.e., including 'use strict'; at the beginning of your JavaScript source files) is a way to voluntarily enforce stricter parsing and error handling on your JavaScript code at runtime, as well as making it more secure.

Some key benefits of strict mode:

  • Makes debugging easier.
  • Prevents accidental globals.
  • Eliminates this coercion.
  • Makes eval() safer.
  • Throws error on invalid usage of delete.

In the end,

I would say the better you understand why and how JavaScript works and doesn’t work.

The more solid your code will be...
The more you’ll be able to effectively harness the true power of the language...

Happy coding...

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Thanks for reading. If you like this might be you are interested in my eBook as well.

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Top comments (3)

realattila profile image

Tanks to you! I really did some of this mistake in past 😂

qucchia profile image

I believe the && operator returns the last input when the first one is truthy, that is why 9 && 5 === 5.

tyaga001 profile image
Ankur Tyagi

Because the && is a chaining operator, meaning it passes its value along until the final value is needed.

In this example the 9 is truthy, and so this is like saying true && 5 which evaluates to 5.