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.
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!
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>
The biggest for me is not understanding the JavaScript programming language itself.
Then there are those for the browser runtime:
And finally the deployment issues.
<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>
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)
A few things right on top of my head. I'll edit this list when I think of more.
any use of 'with'
with (document.forms["mainForm"].elements) {
input1.value = "junk";
input2.value = "junk"; }
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)
You should always put a brace after the statement due to automatic semicolon insertion.
For example this:
function()
{
return
{
price: 10
}
}
differs greatly from this:
function(){
return{
price: 10
}
}
Becuase in the first example, javascript will insert a semicolon for you actually leaving you with this:
function()
{
return; // oh dear!
{
price: 10
}
}
You should use setTimeout rather than setInterval for occasions where you need to do something repeatedly.
If you use setInterval, but the function that is executed in the timer is not finished by the time the timer next ticks, this is bad. Instead use the following pattern using setTimeout
function doThisManyTimes(){
alert("It's happening again!");
}
(function repeat(){
doThisManyTimes();
setTimeout(repeat, 500);
})();
This is explained very well by Paul Irish on his 10 things I learned from the jQuery source video
Not using a community based framework to do repetitive tasks like DOM manipulation, event handling, etc.
Effective caching is seldomly done:
