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Is JavaScript backward- and/or forward compatible?

10 Apr 2024

Do you know if JavaScript is backwards compatible? Or if it is forwards compatible? What do these terms even mean? This blog post will answer those questions.

Backward compatibility

According to WikiPedia: Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, software, real-world product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially in telecommunications and computing.

In JavaScript this would mean that once something is accepted as valid JavaScript there won't be a future change in the language that causes that to be invalid. This would mean that you can be sure that code you have written, even the code from 10 years ago, will work today and in the future.

Forward compatibility

Again, according to WikiPedia: Forward compatibility or upward compatibility is a design characteristic that allows a system to accept input intended for a later version of itself.

In JavaScript that would mean that code that the JavaScript engine does not understand, because it is part of a future spec that the engine is not aware of yet, will be skipped and ignored. It will not throw an error!

HTML and CSS on the other hand are forward compatible. Since these have a declarative nature, being forward compatible works for them. A CSS rule that is unsupported by a browser is skipped over and ignored.

What applies to JavaScript?

You might have already concluded that JavaScript is backwards compatible, but is not forwards compatible.

It being backwards compatible makes JavaScript an incredibly safe and durable language to choose for a program. You can count the fact that a program you write right now will still work in 10 years without any maintenance!

Now, imagine JavaScript being forwards compatible. You don't want a JavaScript engine skipping code that your program relies on in later stages. That would be disastrous.


Now you know that JavaScript is a declarative language. It's good to know the ins and outs of the language you're working with. You could use this knowledge when configuring transpilers or bundlers. Or maybe even apply this in decision making for your own applications.

If you liked this article and want to read more make sure to check the my other articles. Feel free to contact me on Twitter with tips, feedback or questions!