Backwards Compatibility Vs Forwards Compatibility

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I often witness developers discussing languages like JavaScript being backward compatible, making it tolerant to new updates without breaking old legacy code.

However, I recently discovered the existence of a counter concept known as “Forward Compatibility” in programming. Now, let’s explore the significance of both these terms within the context of HTML, CSS, and JavaScript.

Backwards Compatibility

Backwards compatibility means that once something is accepted as valid Javascript, there will not be a future change to the language that causes that code to become invalid Javascript.

Implying, any code written decades ago however primitive or legacy it may appear, should still work today. This principle serves as a token of trust for developers, assuring the longevity of code, even if archaic or widely considered legacy.

While providing confidence, it also entails a significant responsibility and cost. Liberty becomes restricted as decisions affecting code are effectively permanent and even mistakes once made in JavaScript cannot be rectified without the risk of program breakage.

Forwards Compatibility

Being forwards-compatibility means that including a new addition to the language in a program would not cause that program to break if it were run in an older JS engine. Despite aspirations for such forward compatibility in JavaScript, it turns out that this is not currently the case.

HTML & CSS are forwards-compatible but not backwards-compatible. So, it will come as no surprise if you find certain HTML code from early 2000s not working today – for example <center> tag used to horizontally center align the content is deprecated in HTML5 and doesn’t work anymore.


In summary, backward compatibility allows old JS to function seamlessly with newer JS engines, while forward compatibility enables newer versions of HTML/CSS to work harmoniously with legacy browsers and platforms.

Suggested Reading : You Don’t Know JS Yet by Kyle Simpson

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