Is using var to declare variables optional

Question

Is  var  optional   myObj   1    same as    var myObj   1    I found they both work from my test  I assume var is optional  Is that right

User · Answer

Var doesn t let you  the programmer  declare a variable because Javascript doesn t have variables  Javascript has objects  Var declares a name to an undefined object  explicitly  Assignment assigns a name as a handle to an object that has been given a value   Using var tells the Javacript interpreter two things    not to use delegation reverse traversal look up value for the name  instead use this one not to delete the name   Omission of var tells the Javacript interpreter to use the first-found previous instance of an object with the same name    Var as a keyword arose from a poor decision by the language designer much in the same way that Javascript as a name arose from a poor decision    ps  Study the code examples above

User · Answer

Nope  they are not equivalent   With myObj   1  you are using a global variable   The latter declaration create a variable local to the scope you are using   Try the following code to understand the differences   external   5  function firsttry       var external   6    alert  first Try      external      function secondtry       external   7    alert  second Try      external      alert external      Prints 5 firsttry       Prints 6 alert external      Prints 5 secondtry       Prints 7 alert external      Prints 7   The second function alters the value of the global variable  external   but the first function doesn t

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There s a bit more to it than just local vs global   Global variables created with var are different than those created without   Consider this   var foo   1     declared properly bar   2     implied global window baz   3     global via window object   Based on the answers so far  these global variables  foo  bar  and baz are all equivalent   This is not the case   Global variables made with var are  correctly  assigned the internal   DontDelete   property  such that they cannot be deleted   delete foo     false delete bar     true delete baz     true  foo     1 bar     ReferenceError baz     ReferenceError   This is why you should always use var  even for global variables

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They are not the same  Undeclared variable  without var  are treated as properties of the global object    Usually the window object  unless you re in a with block  Variables declared with var are normal local variables  and are not visible outside the function they re declared in    Note that Javascript does not have block scope  Update  ECMAScript 2015 let was introduced in ECMAScript 2015 to have block scope

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Consider this question asked at StackOverflow today   Simple Javascript question  A good test and a practical example is what happens in the above scenario    The developer used the name of the JavaScript function in one of his variables   What s the problem with the code  The code only works the first time the user clicks the button   What s the solution  Add the var keyword before the variable name

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Everything about scope aside  they can be used differently    console out var myObj 1     SyntaxError  Unexpected token var console out myObj 1     1   Something something statement vs expression

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This is one of the tricky parts of Javascript  but also one of its core features  A variable declared with var  begins its life  right where you declare it  If you leave out the var  it s like you re talking about a variable that you have used before   var foo    first time use   foo    second time use     With regards to scope  it is not true that variables automatically become global  Rather  Javascript will traverse up the scope chain to see if you have used the variable before  If it finds an instance of a variable of the same name used before  it ll use that and whatever scope it was declared in  If it doesn t encounter the variable anywhere it ll eventually hit the global object  window in a browser  and will attach the variable to it   var foo    I m global   var bar    So am I    function          var foo    I m local  the previous  foo  didn t notice a thing       var baz    I m local  too        function              var foo    I m even more local  all three  foos  have different values           baz    I just changed  baz  one scope higher  but it s still not global           bar    I just changed the global  bar  variable           xyz    I just created a new global variable             This behavior is really powerful when used with nested functions and callbacks  Learning about what functions are and how scope works is the most important thing in Javascript

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There s so much confusion around this subject  and none of the existing answers cover everything clearly and directly   Here are some examples with comments inline     this is a declaration var foo     this is an assignment bar   3     this is a declaration and an assignment var dual   5    A declaration sets a DontDelete flag   An assignment does not   A declaration ties that variable to the current scope   A variable assigned but not declared will look for a scope to attach itself to   That means it will traverse up the food-chain of scope until a variable with the same name is found   If none is found  it will be attached to the top-level scope  which is commonly referred to as global    function example        is a member of the scope defined by the function example   var foo       this function is also part of the scope of the function example   var bar   function         foo   12     traverses scope and assigns example foo to 12        function something different         foo   15     traverses scope and assigns global foo to 15     For a very clear description of what is happening  this analysis of the delete function covers variable instantiation and assignment extensively

User · Answer

They mean different things  If you use var the variable is declared within the scope you are in  e g  of the function   If you don t use var  the variable bubbles up through the layers of scope until it encounters a variable by the given name or the global object  window  if you are doing it in the browser   where it then attaches  It is then very similar to a global variable  However  it can still be deleted with delete  most likely by someone else s code who also failed to use var   If you use var in the global scope  the variable is truly global and cannot be deleted   This is  in my opinion  one of the most dangerous issues with javascript  and should be deprecated  or at least raise warnings over warnings  The reason is  it s easy to forget var and have by accident a common variable name bound to the global object  This produces weird and difficult to debug behavior

User · Answer

var is optional  var puts a variable in local scope  If a variable is defined without var  it is in global scope and not deletable   edit  I thought that the non-deletable part was true at some point in time with a certain environment  I must have dreamed it

User · Answer

No  it is not  required   but it might as well be as it can cause major issues down the line if you don t  Not defining a variable with var put that variable inside the scope of the part of the code it s in  If you don t then it isn t contained in that scope and can overwrite previously defined variables with the same name that are outside the scope of the function you are in

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The var keyword in Javascript is there for a purpose   If you declare a variable without the var keyword  like this   myVar   100    It becomes a global variable that can be accessed from any part of your script  If you did not do it intentionally or are not aware of it  it can cause you pain if you re-use the variable name at another place in your javascript   If you declare the variable with the var keyword  like this   var myVar   100    It is local to the scope     - braces  function  file  depending on where you placed it    This a safer way to treat variables  So unless you are doing it on purpose try to declare variable with the var keyword and not without

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I just found the answer from a forum referred by one of my colleague  If you declare a variable outside a function  it s always global  No matter if you use var keyword or not  But  if you declare the variable inside a function  it has a big difference  Inside a function  if you declare the variable using var keyword  it will be local  but if you declare the variable without var keyword  it will be global  It can overwrite your previously declared variables  - See more at  http   forum webdeveloperszone com question what-is-the-difference-between-using-var-keyword-or-not-using-var-during-variable-declaration  sthash xNnLrwc3 dpuf

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Check out this Fiddle  http   jsfiddle net GWr6Z 2   function doMe     a    123      will be global var b    321      local to doMe alert  a   a    --  b   b    b    something else      still local  not global  alert  a   a    --  b   b         doMe   alert  a   a    --  b   b       b  will not be defined  check console log

[javascript] Is using 'var' to declare variables optional?

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