What leads to a poor JavaScript application?

badjs.md

What makes a bad JavaScript application?

We've all seen some horrible JavaScript. Name the top five things you see that lead to horrible JavaScript applications. Feel free to elaborate on each of your points.

I'll start, in no particular order:

1. Poor organization in the file system
2. Poor abstraction
3. Seemingly endless indentation in nested callbacks / if / else if / etc.
4. Poor understanding of "the bad parts" (forgot to var)
5. Poor understanding of what's fast and what's slow (querying the DOM instead of caching elements, delegating mouseover, etc)

• a bad developer.
• poor knowledge of basic programming logic (e.g. expecting that some async code runs synchronously).
• anything that would also be considered a bad practice in other languages (code duplication, tight coupling, etc..)

1. Poor understanding of JavaScript asynchronous nature.
2. Building your application's entire architecture on the DOM.