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why can't I declare the same variable twice in Java?

There are similar questions here but they didn't really answer my questions.
So I am curious why we can't declare the same variable twice in Java?
for example:
int a = 4;
int a = 6;
this won't really work in Java.
However in javascript, this actually works:
var a = 1;
var a = 2;
In javascript, people said that the declaration immediately got moved to the start so that it became like this:
var a;
a = 1;
a = 2;

The simple, obvious answer is because the compiler doesn't let you. But now let's go a step further - why would this be desired?
The reason here is that declaring a variable twice is a sign of a mistake. It usually means one of three things:
Your variable names are not specific enough. Perhaps you used int length twice and it barks at you. You probably should make your name more specific to what it holds the length of, for example int originalLength and int extendedLength when copying an array or something.
Your method is too long. Why is your method so long that you need two of the same variable? Chances are you're duplicating code, so consolidate that into a method.
You haven't really thought out your method. This is sort of an extension of number 2, but the truth is you should decide what a method does before you write it. If you're adding a variable that already exists, it probably means that you haven't decided exactly what this method is doing.
Each of those is a major code smell, and is probably the source of bugs down the road. (And not far down the road!) In each of the cases, allowing you to declare a variable twice is going to cause ambiguity that would have been prevented if it stopped you from compiling.
Now, does this mean there aren't cases where it might be nice? Sure. There might be. Maybe you've covered all your bases and you're absolutely sure it's okay to reuse that variable. In that case, just reassign it instead of redeclaring it. Personally, I'd advise against that, but it's your foot to shoot if you want to. :)

You can use the same variable name if the scopes don't overlap, for instance i could have a variable in a private method called "var1" and then in another method have the same thing, these two would not conflict
However since everytime i use "int var1" in the same scope, java is re-declaring the variable, it wont allow it, as it's a conflicting variable name, whereas in java script the declaration happens once, as it's weakly typed

now it has been rectified or improvised in javascript too with the new let keyword
if you try to intialize the same variable name more than once it will throw an error
let a = 4;
let a = 5;
will throw an error in ES6

Why does not JSLint allow "var" in a for loop?

Something is wrong with my code or with plovr. I went to JSLint to get some help. However, JSLint seems to consider this as a fatal error and refuses to check more of the code:
for (var i = 0; i < data.length; i += 4) {
Why? I like this way of declaring "I".
JSHint does not seem to be an alternative if you are on Windows. I am trying The Online Lint at the moment. I have a feeling that this bug will be a bit difficult to find so I would be glad for suggestions about good code checkers.

I agree with Niet the Dark Absol that the tool is opinion-based. Reflect on all the rules it imposes and whether they actually make sense in your specific project or not. It's also not the only "lint" tool for JavaScript. Maybe ESLint or another such tool is more suited to your needs.
But I also disagree with him: It is not good practice to declare all your variables at the start of your function, especially not if the function is rather long, since this makes comprehension of your program much harder. IMHO this holds regardless of how the scoping of JavaScript works: It's not about program semantics, it's about code readability!
I would argue that a variable should be declared as close to its first use as possible (i.e. in your case: in the for loop). This ensures that someone reading your code (a colleague or yourself three months from now) has too keep as little information in their head as possible. Declaring all your variables at the start forces the reader to keep all those variables in mind throughout the entire function. Questions like "what's the current value of this variable?" or "what is its purpose?" become harder to answer.
Furthermore, you are much more tempted to reuse a variable for more than one purpose. This is not only confusing, but also dangerous! Values might "leak" from the first use to the second. This can lead to subtle, hard-to-debug problems.

My personal opinion is that JSLint is retarded. But opinions may vary on that sensitive topic.
— dystroy, 45 secs ago
Hey, that's actually a valid point. JSLint is entirely constructed on opinion, and it not the word of god.
However, general good practice is to declare all your variables in one place, at the start of the function block they are in:
function doSomething() {
var a, b, c, d;
a = 1;
c = 10;
for( b = a; b < c; b++) {
d = b*2 + a-c;
alert(d);
}
}

Because creating a bar in the for loop creates a global var. it's one of those things that JavaScript does and most people don't realize. And if you don't and you or someone else creates that same var in the scope of that function or global scope then that my friend would be one of the famous JavaScript foot guns.

Consequences of missing a comma when declaring multiple variables in javascript?

I'm new to javascript and have picked up an application developed by another team recently.
In this program in one place where they declare several variables at once there is a missing comma like:
var me = this,
missing = this.missingComma
grid = something.Something;
What if any are the consequences of there not being a comma after the second entry. The relevant bit appears to work when just running it. The code has no tests and since it's javascript I cant compile it, also I dont really know what its supposed to do so unfortunately 'not falling over' is currently my best guess at 'does what its supposed to do'!
Why does it work? Isn't this a syntax error?

In JavaScript the semi-colons aren't required to indicate the end of a line. A linebreak is sufficient to indicate that the next line is a separate statement rather than a continuation of the previous line of code (as is the case when you use the comma to indicate multiple variables).
Your code is essentially the same as this:
var me = this, missing = this.missingComma;
grid = something.Something;
Since that declares the grid variable without the var keyword, you'd end up with grid being created in the global, rather than the current, scope. That's generally something you want to avoid but it's not going to be the end of the world if it does happen - in this case it may even be intended (though I'd guess not).

Javascript is ubiguitous with a a lot of freedom ;)
Maybe it helps you to understand some peculiarity of JS if you read some additional info about semicolons, commas and newlines in Javascript:
http://www.codecademy.com/blog/78-your-guide-to-semicolons-in-javascript
For the sake of readability, I would suggest you to use the classic approach, anyway.
var me = this;
var you = that;
or at least
var me = this, you = that;
For the rest, I think that Anthony Grist has brought it to the point.

Well even though in javascript the semicolon is not required it is a must now a days, because if you want your JavaScript to get minimized, it must have all semicolons. Minimization puts your complete JavaScript in one line... replacing long variable names with short ones, etc.
On the other hand... back to you question.
If you declare your var inside a JavaScript "namespace" (actually an object) then all the variables are "private" and you could choose to make the ones "public" by using the reveal pattern.
This is a good practice, else all you variables are declared on the windows scope... which actually can then be overwritten by any other part of your page that uses the same variable name, even if you thought it was completely independent.
So you could actually do something like this :
var MyNamespace || {}
// this delcares an object MyNamespace only if it doesn't exists yet
MyNamespace.Logic = function(){
var self = this,
myPrivateVariable = "Hello",
self.myPublicVariable = "World",
self.printHello = function(){
alert(myPrivateVariable +' ' +self.myPublicVariable );
};//this semicolon closes the var statement
};
Now you can use somehwer on you page folowing logic
var newInstanceOnMyLogic = new MyNamespace.Logic()
This is equivalent of writing
var newInstanceOnMyLogic = new window.MyNamespace.Logic();
But your variables myPrivateVariable and myPublicVariable are no longer on the windows context and can't be overwritten
Now if you write something like
alert(newInstanceOnMyLogic.myPublicVariable);
you'll get a "World"
But
alert(newInstanceOnMyLogic.myPrivateVariable );
you'll get an undefined
and
newInstanceOnMyLogic.printHello();
will get an alert of "Hello World"

Does it make any sense to use JSLint and follow it? [closed]

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Lately I've been writing some JS code using jQuery and JavaScript as it is and I thought I will give JSLint a try. Let me say that the code contains various functions and jQuery usages and it works fine (without any errors ) in IE8 and latest Firefox.
The code also validatas as XHTML 1.0 Transitional (and Strict too but I mostly want it to be Transitional valid).
However, with JSLint it's like everything is wrong. While I have read about it being very strict, even if I turn on only "The Good Parts" it's still like 70+ errors in a typical HTML page.
It starts out with this (why in the world would I want to remove the type to make my documents XHTML invalid??)
Problem at line 5 character 67: type is unnecessary.
<script src="/Scripts/jquery-1.4.2.min.js" type="text/javascript"></script>
and goes on with esoteric errors like
Problem at line 41 character 41: Use the array literal notation [].
var rows = new Array();
Problem at line 42 character 30: Too many var statements.
for (var i = 0; i < data.length; i++) {
Problem at line 42 character 55: Unexpected use of '++'.
for (var i = 0; i < data.length; i++) {
Problem at line 64 character 50: ['PrettyId'] is better written in dot notation.
var item = $("#item_" + data["PrettyId"]);
If anyone can provide me answers to these errors and especially, how to make JSLint be aware of jQuery and understand it, I will appreciate it.
If not, please explain whether you use it or not and whether you advice to use it or not.
UPDATE:
I'm going to wait one more day for more answers, if any, then I'll accept the answer with the most upvotes.

A thing to remember when using JSLint is that is built on the opinion about what one person thinks is the "Good Parts".
I think is a great tool but there are rules that I don't agree with.
About the type attribute, you can find more about the opinion of the author here he says that the attribute is "required and non necessary", but if you validate your documents you clearly need it.
With the use of the Array literal notation [] vs. the Array constructor, I agree, there are differences that can make the two syntaxes behave different, for example:
[5]; // one-element array
["5"]; // one-element array
new Array(5); // empty array but its length is initialized with 5
new Array("5"); // one-element array
So, for consistency an brevity the literal notation is better.
About the "Too many var statements", JavaScript has no block-scope, the scope is at function or global level, and all var statements are evaluated before the code execution -aka hoisting- and they are initialized with undefined, while the assignments are made at runtime.
For example:
var x = 0;
if (true) {
var x = 1; // useless var, x already declared
}
x; // 1
And the "hoisting" of variable declaration can be shown in this example:
var x = 5; // global
(function () {
alert(x); // alerts `undefined`, x declared but unassigned in this scope
alert(y); // ReferenceError, y is undeclared
var x = 10;
})();
As you can see x holds undefined because it is declared before the actual code execution, the var statements are hoisted to the top of their enclosing scope:
var x = 5; // global
(function () {
var x;
alert(x); // alerts `undefined`, x declared but unassigned
alert(y); // ReferenceError, y is undeclared
x = 10; // assignment is made
})();
So that rule wants to actually make the code to resemble what will happen, all the var statements at first place.
About the "Unexpected use of '++'", this is another rule that I don't quite like, the author thinks that "those contribute to bad code by encouraging excessive trickiness".
When I use them in some expression, I try to extract the operator usage to an alone statement, for example:
array[++id] = x;
To:
id+=1;
array[id] = x;
Which is more clearer, but anyway in the case of a for statement IMO it can't cause any confusion at all...
About the last one "['PrettyId'] is better written in dot notation.", JSLint expects the usage of the bracket notation to be "dynamic", it expects to see an expression there, not a String literal that contains a valid identifier name, the bracket notation should be used only when you want to access a property with a name that clashes with a reserved word e.g.:
data.function; // SyntaxError in ECMAScript 3 based implementations
data["function"]; // Ok
Or when a property contains characters that are not a valid identifier, e.g.:
data.foo-bar; // it access the property `foo` minus a `bar` variable
data["foo-bar"]; // Ok
data.foo bar; // SyntaxError, unexpected `bar` identifier
data["foo bar"]; // Ok

I use jslint on .js files and find a combination of options and fixes to make it happy. I find it improves my code quality. I would recommend running it, even if you only use it to isolate omitted 'var' declarations.
I avoid the script tag warning by not running against html files. I use jslint systematically on .js files, but not on html. I find it is just too burdensome to declare all the global identifiers that got defined in included scripts, but aren't visible to jslint.
Crockford's book 'Javascript: The Good Parts' explains many if not all of the jslint warnings, and some of them are based on perceived bug proneness. The 'new Array()' vs ' []' warning is based on Doug's aversion to the 'new' operator. Omitting 'new' for various constructors is commonly valid code, but not correct code, and using the alternative syntax avoids that risk.
The 'too many vars' error means just that, a large number of 'var' declarations in a given function: the Crockford endorsed style is to use one or very few 'var' declarations, all at the top of the function. You can declare multiple variables in one var statement, by separating them with commas.
The '++' caution is another bug proneness based warning; Using '+=1' means the same thing, and Doug believes it to be less error prone.
jslint is thus a mixed bag. Some features (locals vs globals) are invaluable, some (script types) are just annoying, and many are of dubious benefit but harmless.
jQuery itself passes jslint checks, as described here:
http://docs.jquery.com/JQuery_Core_Style_Guidelines#JSLint

There's an old joke about a programmer who smoked. A friend said, "You're a really smart guy, can't you read the warning on the pack that says cigarettes will kill you?" Came the reply: "I'm a programmer. We only pay attention to errors, not warnings."
Sometimes JSLint tells you things that are critical, sometimes it just nags. You can get lost trying to make it happy, but you might just be lost without it. Like any other tool, its input should be taken with a grain of salt. I prefer to think of it like the spell-checker or grammar-checker in MS Word. Sometimes it's right, sometimes it's wrong, and sometimes I just don't care what it tells me.

It hurts when you start using it on your old code.
But then it will save you a lot of time.JSLint can detect many problems that you would only see when refreshing your page or worse when your users do.Now it hurts me when I have to use an editor without JSLint
Another advantage is when you start compressing your javascript. If you pass the JSLint checks, you are almost certain to compress without problems.
That being said, jquery has 23 warning, most of them about regex, but is widely used without any problems.

JSLint is a Code Quality tool. It's not like W3C validator. Consider it something like a weekly code review at a company. Coding conventions vary and JSLint is just one of them. There may be points you don't agree with, but if you work in a team and you need to manage a larger codebase JSLint can be a life saver. Why? Because it warns you about things that are confusing or error prone.
The best way to use JSLint is this: read everything it says and if you're unsure about something correct it as suggested (as it seems to be the case here).
The reason is that JSLint is meant to avoid confusion and if you're using something that can lead to errors and you don't know why, then you better off with a clear alternative.
And if you want to know the reasoning behind the suggestions watch Douglas Crockford's videos on Javascript.
Finally, you can turn off filters that you find useless.

you should use it. The fact that it works in one browser doesn't mean it will work in all browsers; some are more lenient than others. for example, having a trailing comma in an array will break in IE but work in FF.
Be aware that sometimes you get a ton of errors, but you remove the first and a dozen go away, like compiling code.
you can do
var rows = [];
for the first error.

Here are the best answers I can give. Perhaps someone can fill in some blanks. It doesn't appear as though any of these relate to jQuery, though.
Also, see this document for some explanations.
Problem at line 5 character 67: type is unnecessary.
<script src="/Scripts/jquery-1.4.2.min.js" type="text/javascript"></script>
and goes on with esoteric errors like
I believe that for modern browsers, there's no need for text/javascript when linking to external js files.
Problem at line 41 character 41: Use the array literal notation [].
var rows = new Array();
It is recommended to not call the constructor when creating an Object or Array. I'm honestly not sure why. Anyone?
Problem at line 42 character 30: Too many var statements.
for (var i = 0; i < data.length; i++) {
I suppose you have the i variable declared elsewhere in the same scope. A javascript block does not create scope, so if you have another for loop (for example) that uses i, it is using the same variable. No need to declare it again.
Problem at line 42 character 55: Unexpected use of '++'.
for (var i = 0; i < data.length; i++) {
Not sure about this one, except that if I recall correctly, Crockford doesn't like the ++ and --.
Problem at line 64 character 50: ['PrettyId'] is better written in dot notation.
var item = $("#item_" + data["PrettyId"]);
I believe . notation is preferred because it is shorter and faster (in some browsers anyway). So data.PrettyID instead.
As far as I can tell, nothing here appears to be explicitly wrong. Mostly just best practice advice from JSLint creator's perspective.

What does "with" do in JavaScript?

I saw JavaScript code which begins with with. That's a bit confusing. What does it do and how can it be used correctly?
with (sObj) return options[selectedIndex].value;

It adds to the scope of the statements contained in the block:
return sObj.options[selectedIndex].value;
can become:
with (sObj)
return options[selectedIndex].value;
In your case, it doens't do a whole lot...but consider the following:
var a, x, y;
var r = 10;
a = Math.PI * r * r;
x = r * Math.cos(PI);
y = r * Math.sin(PI /2);
Becomes:
var a, x, y;
var r = 10;
with (Math) {
a = PI * r * r;
x = r * cos(PI);
y = r * sin(PI / 2);
}
...saves a couple of keystrokes. The Mozilla documentation actually does a pretty good job of explaining things in a little more detail (along with pros and cons of using it):
with - Mozilla Developer Center

the with statement is pure syntactical sugar, but it also can cause some nasty bugs.
See with Statement Considered Harmful for clarification:
If you can't read a program and be confident that you know what it is going to do, you can’t have confidence that it is going to work correctly. For this reason, the with statement should be avoided.

In that with block you dont have to type:
sObj.options[selectedIndex].value
but you can just use:
options[selectedIndex].value

Its the equivalent of
return sObj.options[selectedIndex].value;
With lets you issue a block of statements in the context of a particular object. Therefore all of the statements in the with block are taken to be members of the object in parenthesis.
This can make code more readable at times, but it also can lead to ambiguity, since the variable references can either be with sObj or global.
legitimate uses for javascript's "with" statement :D

I would recommend NOT using this because of performance issues, but what the above means is:
for the object sObj (here presumably a select element), all children and properties referenced on this one (or between following curly braces) treat that as their parent scope.

Your example could be rewritten as...
return sObj.options[selectedIndex].value;
...as the 'with' statement places all related statements in the scope of the supplied object. In this case, it's pretty pointless but, if you were doing lots of operations on 'sObj', then it saves a lot of typing.
Totally ficticious example..
with (sObj)
{
if(options[selectedIndex].value < 10){
options[selectedIndex].value++;
total+ = options[selectedIndex].value;
}
}
But, having said that, it's often the case that saving typing can be achieved in better ways.

It isn't a function (as was indicated in the question title before it was edited) but a statement. It may make more sense if the code sample is formatted like so:
with (sObj){
return options[selectedIndex].value;
}
Regarding what it does (Source)
The with statement establishes the default object for a set of statements. JavaScript looks up any unqualified names within the set of statements to determine if the names are properties of the default object. If an unqualified name matches a property, then the property is used in the statement; otherwise, a local or global variable is used.
Which means that in the code sample, it is first checked if options is a property of sObj. If it is then options refers to sObj.options, otherwise it checks other scopes for a variable defined by the name options
The downside of using a with statement is that it is impossible to know from just glancing at the code what gets accessed. There are other better alternatives as shown in this article

It brings pain and suffering among you and others