I have a global variable in JavaScript (actually a `window` property, but I don&#39;t think it matters) which was already populated by a previous script, but I don&#39;t want another script that will run later to see its value or that it was even defined.

I&#39;ve put `some_var = undefined` and it works for the purpose of testing `typeof some_var == &quot;undefined&quot;` but I really do not think it&#39;s the right way to go about it.

What do you think?


How can I unset a JavaScript variable?

I&#39;m reading &quot;Think in C++&quot; and it just introduced the `extern` declaration. For example: 

    extern int x;
    extern float y;

I think I understand the meaning (declaration without definition), but I wonder when it proves useful. 

Can someone provide an example?



When to use extern in C++

What is the difference between the following declarations:

    int* arr1[8];
    int (*arr2)[8];
    int *(arr3[8]);

What is the general rule for understanding more complex declarations?

C pointer to array/array of pointers disambiguation

**Question #1:** Is declaring a variable inside a loop a good practice or bad practice?

I&#39;ve read the other threads about whether or not there is a performance issue (most said no), and that you should always declare variables as close to where they are going to be used. What I&#39;m wondering is whether or not this should be avoided or if it&#39;s actually preferred.

Example:

    for(int counter = 0; counter &lt;= 10; counter++)
    {
       string someString = &quot;testing&quot;;

       cout &lt;&lt; someString;
    }

**Question #2:** Do most compilers realize that the variable has already been declared and just skip that portion, or does it actually create a spot for it in memory each time?


Declaring variables inside loops, good practice or bad practice?

Is it possible to declare a variable in Python, like so?:

    var

so that it initialized to None? It seems like Python allows this, but as soon as you access it, it crashes. Is this possible? If not, why?

EDIT: I want to do this for cases like this:

    value
    
    for index in sequence:
    
       if value == None and conditionMet:
           value = index
           break

### Duplicate

 * [Uninitialised value in python][1] (by same author)
 * [Are there any declaration keywords in Python?][2] (by the same author)

### Related

* [Python: variable scope and function calls][3]
* [Other languages have &quot;variables&quot;][4]


  [1]: https://stackoverflow.com/questions/664219/uninitialized-value-in-python
  [2]: https://stackoverflow.com/questions/571514/are-there-any-declaration-keywords-in-python
  [3]: https://stackoverflow.com/questions/575196/python-variable-scope-and-function-calls
  [4]: http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables

Is it possible only to declare a variable without assigning any value in Python?

I&#39;m new to VBA and want to know if I can convert the following declaration and assignment into one line:

    Dim clientToTest As String
    clientToTest = clientsToTest(i)

or

    Dim clientString As Variant
    clientString = Split(clientToTest)

Can I simultaneously declare and assign a variable in VBA?

I am just wondering if there would be any loss of speed or efficiency if you did something like this:

    int i = 0;
    while(i &lt; 100)
    {
        int var = 4;
        i++;
    }

which declares `int var` one hundred times.  It seems to me like there would be, but I&#39;m not sure. would it be more practical/faster to do this instead:

    int i = 0;
    int var;
    while(i &lt; 100)
    {
        var = 4;
        i++;
    }

or are they the same, speedwise and efficiency-wise?

Is there any overhead to declaring a variable within a loop? (C++)

I can&#39;t understand why variables act so strange when declared inside a function.

1. In the `first` function I declare with `let` the variables `b` and `c` with the value _10_:

        b = c = 10;

    In the `second` function I show:

        b + &quot;, &quot; + c

    And this shows:

        10, 10

2. Also in `first` function I declare `a` with value _10_:

        let a = b = c = 10;

    But in the `second` function it shows an error:

    &gt; Can&#39;t find variable: `a`

3. Now in the `first` function I declare `d` with value _20_:

        var d = 20;

    But in the `second` function it shows the same error as before, but with the variable `d`:

    &gt; Can&#39;t find variable: `d`

Example:

&lt;!-- begin snippet: js hide: false console: true babel: false --&gt;

&lt;!-- language: lang-js --&gt;

    function first() {
      let a = b = c = 10;
      var d = 20;
      second();
    }

    function second() {
      console.log(b + &quot;, &quot; + c); //shows &quot;10, 10&quot;

      try{ console.log(a); }  // Rreference error
      catch(e){ console.error(e.message) }

      try{ console.log(d); } // Reference error
      catch(e){ console.error(e.message) }
    }
    first()

&lt;!-- end snippet --&gt;



Why do some variables declared using let inside a function become available in another function, while others result in a reference error?

What&#39;s going on here?

	if(int a = Func1())
	{
		// Works.
	}

	if((int a = Func1()))
	{
		// Fails to compile.
	}

	if((int a = Func1())
		&amp;&amp; (int b = Func2()))
	)
	{
		// Do stuff with a and b.
		// This is what I&#39;d really like to be able to do.
	}
	
Section 6.4.3 in the 2003 standard explains how variables declared in a selection statement condition have scope that extends to the end of the substatements controlled by the condition. But I don&#39;t see where it says anything about not being able to put parenthesis around the declaration, nor does it say anything about only one declaration per condition.

This limitation is annoying even in cases where only one declaration in the condition is required. Consider this.

	bool a = false, b = true;

	if(bool x = a || b)
	{

	}
	
If I want to enter the &#39;if&#39;-body scope with x set to false then the declaration needs parenthesis (since the assignment operator has lower precedence than the logical OR), but since parenthesis can&#39;t be used it requires declaration of x outside the body, leaking that declaration to a greater scope than is desired. Obviously this example is trivial but a more realistic case would be one where a and b are functions returning values that need to be tested 

So is what I want to do non-conformant to the standard, or is my compiler just busting my balls (VS2008)?

C++, variable declaration in &#39;if&#39; expression

Learning [Python](https://www.python.org/), and has some basic doubts.

1.I have seen [variable declaration](https://www.learnpython.org/en/Variables_and_Types) (path here) as 

    class writer:
        path = &quot;&quot;

sometimes, no explicit declaration but initialize through `__init__`. 

    def __init__(self, name):
        self.name = name

I understand the purpose of `__init__`, but is it advisable to declare variable in any other functions.


2.How can I create variable to hold a custom type?


    class writer:
        path = &quot;&quot; # string value
        customObj = ??



Python Variable Declaration

Looking at an online source code I came across this at the top of several source files.

    var FOO = FOO || {};
    FOO.Bar = …;

But I have no idea what `|| {}` does.

I know `{}` is equal to `new Object()` and I think the `||` is for something like &quot;if it already exists use its value else use the new object.

Why would I see this at the top of a source file?


What does &quot;var FOO = FOO || {}&quot; (assign a variable or an empty object to that variable) mean in Javascript?

I&#39;m declaring some strings that are empty, so it won&#39;t throw errors later on.

I&#39;ve read that this was the proper way:

    string Camnr = Klantnr = Ordernr = Bonnr = Volgnr = Omschrijving = Startdatum = Bonprioriteit = Matsoort = Dikte = Draaibaarheid = Draaiomschrijving = Orderleverdatum = Regeltaakkode = Gebruiksvoorkeur = Regelcamprog = Regeltijd = Orderrelease = &quot;&quot;;

But that doesn&#39;t work. I get this error: `Klantnr does not exist in the current context`.

What did I do wrong?

Declare and assign multiple string variables at the same time

Today I found one interesting thing. I didn&#39;t know that one can&#39;t declare a variable after a goto label. 

Compiling the following code

    #include &lt;stdio.h&gt;
    int main() {
        int x = 5;
        goto JUMP;
        printf(&quot;x is : %d\n&quot;,x);
    JUMP:
        int a = 0;  &lt;=== giving me all sorts of error..
        printf(&quot;%d&quot;,a);
    }

gives errors like

    temp.c: In function ‘main’:
    temp.c:7: error: expected expression before ‘int’
    temp.c:8: error: ‘a’ undeclared (first use in this function)
    temp.c:8: error: (Each undeclared identifier is reported only once
    temp.c:8: error: for each function it appears in.)

Now what is the logic behind that? I heard that **one cannot create variables inside the case statements of switch**. Since JUMP is inside the same scope (the scope of main function, in my case) of the goto statement, I believe that scope is not an issue here. But then, why am I getting this error?

 


Variable declaration after goto Label

I want to do this but it won&#39;t compile:

    Public MyVariable as Integer = 123

What&#39;s the best way of achieving this?


Is it possible to declare a public variable in vba and assign a default value?

What&#39;s the difference between the types of `bar1` and `bar2`?

    int foo = 10;
    auto bar1 = &amp;foo;
    auto *bar2 = &amp;foo;

If both `bar1` and `bar2` are `int*`, does it makes sense to write the pointer declarator (`*`) in the `bar2` declaration?

C++11 auto declaration with and without pointer declarator

Consider the following program (see live demo [here][1]).

    #include &lt;stdio.h&gt;
    int main(void)
    {
          int ;  // Missing variable name
          puts(&quot;Surprise&quot;);
    }

My compiler, gcc 4.8.1, gives the below warning:

&gt; [Warning] useless type name in empty declaration [enabled by default]

Why does it compile fine? Shouldn&#39;t I get a compiler error? g++ 4.8.1 gives the following error when I compile it as a C++ program:

&gt; [Error] declaration does not declare anything [-fpermissive]

  [1]: http://ideone.com/504P4P


Why does `int ;` compile fine in C, but not in C++?

I see that the &lt;a href=&quot;http://www.gigamonkeys.com/book/&quot;&gt;Practical Common Lisp&lt;/a&gt; uses `(defvar *db* nil)` for setting up a *global variable*. Isn&#39;t it OK to use `setq` for the same purpose?

What are the advantages/disadvantages of using `defvar` vs. `setq`?

setq and defvar in Lisp

Does C++11 allow declaring non-static data members as &#39;auto&#39; if they are initialized in the declaration? For example:

    struct S
    {
        auto x = 5;  // in place of &#39;int x = 5;&#39;, which is definitely allowed
    };

GCC 4.7 rejects the above code, while it accepts `int x = 5;`.

Assuming this is not a compiler bug but rather the standard really doesn&#39;t allow it, why not? It would be just as useful as declaring local variables `auto`.

All Variable Declaration Solutions on Gang of Coders

Total of 18 Variable Declaration Solutions