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Ambrose Liew
Ambrose Liew

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The Full Practical Guide To Type Coercion In JavaScript: Beware Of These 3 Common Mistakes

Ever used ‘==’ in JavaScript?
Or ‘if (x)’in JavaScript?
Or used String Concatenation?

Hopefully, after reading this article, you can obtain some insights about the common mistakes to avoid in JavaScript!

The Mythical Land of JavaScript

The 7 Primitive Types

Before I begin teaching Type Coercion in JavaScript, I would like to introduce the 7 primitive types in JavaScript. And they are:

  • undefined

  • null

  • boolean

  • string

  • symbol

  • number

  • object (for those wondering what is the type of array, arrays are a special type of object)

But for the focus of this article, we would be focusing only on the most commonly used types which are: boolean, string, and number.

What is Type Coercion?

Type Coercion is the conversion of one type to another. There are two forms of Type Coercion: Explicit and Implicit.

Explicit Coercion is the conversion of a value from one type to another using explicit methods such as Number(), parseInt(), and String().

Implicit Coercion is the automatic conversion of one type to another.
This occurs because the operation performed on the 2 types is incompatible, and JavaScript has to step in to make it compatible.

We would be focusing on Implicit Coercion instead as that is where most of the bugs occur.

String Concatenation

One common place where Implicit Coercion tends to occur is through String Concatenation.

Behavior when a string is concatenated with different types

JavaScript would convert the non-string type to a string, and concatenate the string to deal with these incompatible types.

Evaluation of Truthy Values

Another common use case is through the short-hand usage of Evaluation of Truthy Values.

Behavior when a Non-Boolean type is being treated as a Boolean

These occur typically in if statements (if (x)), ternary operators (x ? 1 : 0), and basically any place in code where a non-Boolean variable is being treated as a Boolean value.

That is why it is dangerous to use these short-hand syntaxes if you do not know all the edge cases when converting a non-Boolean type to a Boolean type.


Another common area where Implicit Coercion occurs is through the usage of ‘==’.

Behavior when using ‘==’ to compare incompatible types

  • For number and string comparisons, JavaScript will convert the string to a number using Number() function.

  • For boolean and number comparisons, JavaScript will convert the boolean to a number using the Number() function. true = 1, false = 0.

  • For boolean and string comparisons, JavaScript will convert the boolean to a number, then the string is converted to a number. That is why (false == ‘0’) is evaluated as true.


If you are curious, you can check out the algorithm for Implicit Coercion, here.

The algorithm for IsLooselyEqual()

You can play with the examples above in this JSFiddle.


I hope that after reading this article you would think twice about using these unsafe methods. You can see how by not carefully considering all the edge cases of Type Coercion, bugs can easily occur.

Consider these safer alternatives:

  • Utilising ‘===’ instead of ‘==’ always

  • Using ESLint, to throw warnings and errors when you write these code

  • Using TypeScript, to help detect potential run-time errors beforehand

However, of course, there is one situation you might want to consider using ‘==’ in JavaScript.


Did you know? Someone even took this Type Coercion concept to the extreme and made a programming language out of it.


It uses only six different characters to write and execute code in JavaScript. Here is a video explanation of the makings of the programming language.

Even though this programming language looks ridiculous, there are two practical uses for it. And that is obfuscation and the crafting of a malicious payload for XSS.

If you felt that you have learned something new, do feel free to follow 🥰, drop a react 💖 or write a comment ✍️!
It helps makes me create and share more valuable content for you to become a better Web Developer! 🤗

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