What anti-patterns exist for JavaScript?

Allain_Lalonde_2443 0
Viewed 11711

I find that what not to do is a harder lesson to learn than what should be done.

From my experience, what separates an expert from an intermediate is the ability to select from among various seemingly equivalent ways of doing the same thing.

So, when it comes to JavaScript what kinds of things should you not do and why?

I'm able to find lots of these for Java, but since JavaScript's typical context (in a browser) is very different from Java's I'm curious to see what comes out.

anti-patterns javascript
10 Answers

Language:

  • Namespace polluting by creating a large footprint of variables in the global context.

  • Binding event handlers in the form 'foo.onclick = myFunc' (inextensible, should be using attachEvent/addEventListener).

  • Using eval in almost any non-JSON context

  • Almost every use of document.write (use the DOM methods like document.createElement)

  • Prototyping against the Object object (BOOM!)

  • A small one this, but doing large numbers of string concats with '+' (creating an array and joining it is much more efficient)

  • Referring to the non-existent undefined constant

Design/Deployment:

  • (Generally) not providing noscript support.

  • Not packaging your code into a single resource

  • Putting inline (i.e. body) scripts near the top of the body (they block loading)

Ajax specific:

  • not indicating the start, end, or error of a request to the user

  • polling

  • passing and parsing XML instead of JSON or HTML (where appropriate)

Many of these were sourced from the book Learning JavaScript Design by Addy Osmati: https://www.oreilly.com/library/view/learning-javascript-design/9781449334840/ch06.html

edit: I keep thinking of more!

annakata_130180

Besides those already mentioned...

  • Using the for..in construct to iterate over arrays
    (iterates over array methods AND indices)

  • Using Javascript inline like <body onload="doThis();">
    (inflexible and prevents multiple event listeners)

  • Using the 'Function()' constructor
    (bad for the same reasons eval() is bad)

  • Passing strings instead of functions to setTimeout or setInterval
    (also uses eval() internally)

  • Relying on implicit statements by not using semicolons
    (bad habit to pick up, and can lead to unexpected behavior)

  • Using /* .. */ to block out lines of code
    (can interfere with regex literals, e.g.: /* /.*/ */)

    <evangelism> And of course, not using Prototype ;) </evangelism>

user_430890

The biggest for me is not understanding the JavaScript programming language itself.

  • Overusing object hierarchies and building very deep inheritance chains. Shallow hierarchies work fine in most cases in JS.
  • Not understanding prototype based object orientation, and instead building huge amounts of scaffolding to make JS behave like traditional OO languages.
  • Unnecessarily using OO paradigms when procedural / functional programming could be more concise and efficient.

Then there are those for the browser runtime:

  • Not using good established event patterns like event delegation or the observer pattern (pub/sub) to optimize event handling.
  • Making frequent DOM updates (like .appendChild in a loop), when the DOM nodes can be in memory and appended in one go. (HUGE performance benefit).
  • Overusing libraries for selecting nodes with complex selectors when native methods can be used (getElementById, getElementByTagName, etc.). This is becoming lesser of an issue these days, but it's worth mentioning.
  • Extending DOM objects when you expect third-party scripts to be on the same page as yours (you will end up clobbering each other's code).

And finally the deployment issues.

  • Not minifying your files.
  • Web-server configs - not gzipping your files, not caching them sensibly.

<plug> I've got some client-side optimization tips which cover some of the things I've mentioned above, and more, on my blog.</plug>

user_200890
  • browser detection (instead of testing whether the specific methods/fields you want to use exist)
  • using alert() in most cases

see also Crockford's "Javascript: The Good Parts" for various other things to avoid. (edit: warning, he's a bit strict in some of his suggestions like the use of "===" over "==" so take them with whatever grain of salt works for you)

user_443300

A few things right on top of my head. I'll edit this list when I think of more.

  • Don't pollute global namespace. Organize things in objects instead;
  • Don't omit 'var' for variables. That pollutes global namespace and might get you in trouble with other such scripts.
user_413600

any use of 'with'

with (document.forms["mainForm"].elements) {
input1.value = "junk";
input2.value = "junk"; }

user_12005580

any reference to

document.all

in your code, unless it is within special code, just for IE to overcome an IE bug. (cough document.getElementById() cough)

user_61440

Not using a community based framework to do repetitive tasks like DOM manipulation, event handling, etc.

Allain_Lalonde_24430
Related