What should every JavaScript programmer know? [closed]

array.length method is not a count of array's items, but the highest index. even when the item was set to undefined

var a = [];
a.length;   // === 0
a[10];      // === undefined
a[10] = undefined;
a.length;   // === 11
a.pop();    // === undefined
a.length;   // === 10

this behavior is hardly distinguishable from a language design bug..

Learning a language really well and understanding its various quirks comes from (years of) experience. If you want to be a better programmer, I would say, understanding design patterns, how and when to use them and/ or even when you are using them without realising it; technical architecture & user experience.

Knowing the (JavaScript) language means you can pick up any framework and use it at will. You'll inevitably need to dive into the source code, and if all you know is the syntax a framework or 2 or 3, then you won't go far. In saying that, getting into a few different frameworks' source code is probably one of the best ways to see how JavaScript can be used. Messing about by stepping through the code in Firebug or Web Inspector, then checking JavaScript Documentation, especially the Mozilla and Webkit docs, to get further understanding of what you're looking at.

Understanding the difference between Object-Oriented and Functional Programming, that JavaScript is a sexy mix of the two and when and how to use both to create a killer codebase and awesome applications will make you a better JavaScript Programmer.

Simply reading some books, especially Crockford's "good parts" which merely presents his opinions on what is good in JavaScript, while skipping most of the AWESOME parts of JavaScript is going to get you off on the wrong foot.

Checking out code written by someone like Thomas Fuchs on the other hand is going to give you much more insight into the power of writing amazing and efficient JavaScript.

Trying to memorise a few gotchas or WTFs is not going to help much either, you'll pick that up if you start coding and stepping through a library/ frameworks' code, especially a helpfully commented one, to see why they've used certain properties/ values and not others why and when it's good to use specific operands and operators, this is all there in the code of the framework's people use. How better than to learn by example? :^)

JavaScript is not support separating return keyword and return statement with newline character like the following code (or try it at my jsFiddle page)

function foo()
{
    return
    {
        bar: 'something'
    };
}

$(function()
{
    document.write(foo());
});

I don't understand why JavaScript does not support this style because it's much easier to read very complex JavaScript source code when compare with default style of JavaScript.

PS. I have written JavaScript almost 6 years. But I just found this bug with myself when I try to execute the following function. It always returns undefined. When I use debugger and step into this function everything is work fine. I think it should be the worst programming bug in my life.

function JqGridInlineEditor_GenerateTool(cellvalue, options, rowObject, disableEdit, disableDelete)
{
    return 
        (!disableEdit ? '<a class="button edit" href="javascript: void(0);" onclick="JqGridInlineEditor_EditRow(this, \'{0}\');return false;" title="{2}"><img src="{1}" alt="{2}" /></a>'.format(options.rowId, getUrl('~/Content/Icons/Edit.gif'), 'Click here to Edit or \nDouble-click row to edit.') : '') +
        (!disableDelete ? '<a class="button delete" href="javascript: void(0);" onclick="JqGridInlineEditor_DeleteRow(this, \'{0}\');return false;" title="{2}"><img src="{1}" alt="{2}" /></a>'.format(options.rowId, getUrl('~/Content/Icons/Delete.png'), 'Click here to Delete or \nSelect row and then press Delete') : '') +
        (!disableEdit ? '<a class="button save" style="display:none" href="javascript: void(0);" onclick="JqGridInlineEditor_SaveRow(this, \'{0}\');return false;" title="{2}"><img src="{1}" alt="{2}" /></a>'.format(options.rowId, getUrl('~/Content/Icons/Save.png'), 'Click here to Save or \nPress Enter') : '') +
        (!disableEdit ? '<a class="button cancel" style="display:none" href="javascript: void(0);" onclick="JqGridInlineEditor_RestoreRow(this, \'{0}\');return false;" title="{2}"><img src="{1}" alt="{2}" /></a>'.format(options.rowId, getUrl('~/Content/Icons/Cancel.png'), 'Click here to Cancel or \nPress Esc') : '');
}

That Javascript is not something which can be learnt in an hour!

In Javascript, Performance matters.

There is not an intelligent compiler to optimize your code so You should be more careful while you are writing javascript code than languages like C#, Java...

Understanding the stuff written in Crockford's Javascript: The Good Parts is a pretty good assumption that a person is a decent JS programmer.

You can pretty much know how to use a good library like JQuery and still not know the hidden parts of Javascript.

Another note is Debugging tools on various browsers. A JS programmer should know how to debug his code in different browsers.

Oh! And knowing JSLint will totally hurt your feelings!!

1. Knowing that there is a life with and without with() and where to draw the line.
2. You can create custom errors with the throw statement to purposely stop the javascript runtime.

You know javascript if you can use Array, Number, String, Date and Object effectively. Plus points for Math and RegExp. You should be able to write functions and use variables (in correct scope, i.e. as 'methods' of an object).

I see some comments about knowing closures, extravagant function syntax, blabla. All that is quite irrelevant for this question. That's like saying you are a runner if you can run the 100m dash under 11 seconds.

I say it takes maybe a couple of weeks to become proficient in javascript. After that it takes years and dozens of books and thousands of lines of programming to become an expert, a ninja, etc.

But that wasn't the question.

Oh, and the DOM is not a part of javascript, and neither is jQuery. So I think both are equally irrelevant to the question too.

What every javascript coder should know?

How about, I can turn off your efforts with 2 clicks. So provide a fallback if possible.

You don't know JavaScript if you don't know:

1. Closures
2. Prototype-based inheritance
3. The module pattern
4. The W3C-DOM
5. How events work

Having read all the above, it's also perfectly fine to learn Javascript by using a framework like jQuery. The truth is it's the first way a lot of folks picked JS up in the first place. No shame in that.

The following things are also important:

1) Variable hoisting. 2) Scope chains and activation objects.

and then things like these: :)

3) wtfjs.com

4)

One should be aware about the following to say "I Know JavaScript":

1. JavaScript is good but DOM is pain point
2. Cross browser issues can make you go crazy
3. Unless code is tested on least 4 different good browsers you can't say its bug free
4. Closure.............. Must know
5. Its prototype based ........... Nice one its fun to learn this
6. debugger keyword ..... Helps in crisis

Variables are global unless declared to be local!!

Bad (DoSomething() is only called 10 times):

function CountToTen()
{
  for(i=0; i< 10; i++)
  {
    DoSomething(i);
  }
}

function countToFive()
{
  for(i=0; i<5; i++)
  {
    CountToTen();
  }
}

CountToFive();

Good (DoSomething() is called 50 times as intended):

function CountToTen()
{
  var i;
  for(i=0; i< 10; i++)
  {
    DoSomething(i);
  }
}

function countToFive()
{
  var i;
  for(i=0; i<5; i++)
  {
    CountToTen();
  }
}

CountToFive();

I strongly recommend to read Javascript: The Good Parts

JSLint http://www.JSLint.com/

If you want to be a true JavaScript ninja, you should know the answers to every question in the Perfection kills JavaScript Quiz.

An example to whet your appetite:

(function f(f){ 
  return typeof f(); 
})(function(){ return 1; });

What does this expression return?

• “number”
• “undefined”
• “function”
• Error

object literals because they are so nice to write.

For knowing that Javascript was originally called LiveScript and the 'Java' prefix was attached for marketing purposes not because Java and Javascript are related (which they are not).

Oh and for owning any version of David Flanagan's 'Javascript: The Definitive Guide' (this information is on page 2).

... and for appreciating those that have gone before in trying to obfuscate Internet Explorer 4's document.all[] and Netscape Navigator 4's document.layers[] before the likes of Jquery took away the pain.

EDIT:

As @Kinopiko points out JavaScript was called project Mocha originally (some sources also reckon it was called project LiveWire) but it is generally accepted that the language (written by Brendan Eich) was slated to be released as LiveScript before the Java prefix was adopted on release in early 1996.

That it can be disabled.

That javascript is the most widely deployed language in the world. (Probably)

Javascript objects and function as first-class citizen, callbacks, not to forget about events and then JQuery.

1. Familiarize yourself with atleast one Javascript library ( Jquery, Prototype, etc ).

2. Learn how to use the debugging tools of the major browsers ( MSIE 7-8, Firefox, Chrome, Safari )

3. Read up on the industry: Douglas Crockford's website is a treasure trove while Ajaxian.com is a good blog to keep up on new, interesting, and or odd ideas for Javascript. There are a number of other resources but those are the ones that helped me the most.

..that javascript is not java :)

Many, many people starting with website development have told me javascript is just simple java!

How to use the good parts, and how to avoid the awful parts.

That JavaScript is much more different than other languages than you might think. Watch this great Google Tech Talk to get an impression: http://www.youtube.com/watch?v=hQVTIJBZook

jQuery would be my best recommendation. Not just for the code itself, it's the idiom, the style, the thinking behind it that's most worthy of emulation.