Related
I have been wondering for a while if I can use a variable in JS before it is defined,
such as the following:
var country = "USA";
switch (country) {
case "USA":
country = i;
case "blach":
//not finished yet
}
/*
put a whole
bunch more code here
*/
var i = 10;
Is this valid? Is it allowed? And if so, what is the technical term for it?
This is a technique used by the JavaScript engine called hoisting. The parser will read through the entire function before running it, and any variable declarations (i.e. using the var keyword) will be executed as if they were at the top of the containing scope. So your code behaves like:
var country;
var i;
country = "USA";
switch (country) {
case "USA":
country = i;
case "blach":
//not finished yet
}
i = 10;
So, i is declared throughout the entire scope, but its value is undefined until the i = 10 statement runs.
In ECMAScript terms, when a function is invoked, the function's new lexical scope builds its VariableEnvironment before any of the function's code runs. In ECMAScript 10.5, step 8:
8. For each VariableDeclaration... d in code, in source text order do
a. Let dn be the Identifier in d.
...
i. Call env’s CreateMutableBinding concrete method passing dn and configurableBindings as the arguments.
ii. Call env’s SetMutableBinding concrete method passing dn, undefined, and strict as the arguments.
This is quite a mouthful, but basically it says:
Before you run a function, look through the function's source code for declarations like var [identifierName].
For each declaration you find, create a new variable in the function's scope with the name [identifierName] used in the declaration and then set its value to undefined
It's called variable hoisting, and it's a good concept to understand as it can occasionally create bugs that are hard to track down.
For example:
var stuff = 'stuff';
function() {
console.log(stuff); //outputs 'undefined'
var stuff = 'other stuff';
console.log(stuff); //outputs 'other stuff'
}
The first console.log outputs undefined because the var stuff in the function was hoisted to the top of the function.
//theoretical compiled output
var stuff = 'stuff';
function() {
var stuff; //has not been defined
console.log(stuff);
stuff = 'other stuff'; //defined here
console.log(stuff);
}
Without knowledge of variable hoisting, this result may be confusing.
For this reason, if you look at professionally developed JavaScript code, typically you'll see all variables in a function are declared at the top.
Yes. In JavaScript, variables are hoisted
A variable statement declares variables that are created as defined in 10.5. Variables are initialised to undefined when created. A variable with an Initialiser is assigned the value of its AssignmentExpression when the VariableStatement is executed, not when the variable is created.ES5 §12.2
Where 10.5 step 8 is the part of interest
The guys answers are correct. for your example however it is worth noting that country is undefined. as aspillers has mentioned your code behaves as below
var country;
var i;
country = "USA";
switch (country) {
case "USA":
country = i;
case "blach":
//not finished yet
}
i = 10;
alert(country) //undefined;
but when your case statement ran for "USA", i was undefined so this was assigned to country. try it here in this fiddle.
I guess that you just need to be aware that although the variable declarations are hoisted, the value assignments aren't.
Yes, you can use a variable before declaring it. It's called hoisting in JavaScript.
One thing to keep in mind though is that only a variable's declaration is hoisted, not its initialization.
It means that you'll not able to use the value of the variable which you'll assign later in the code. For example,
console.log(myVar); //undefined
myVar = 10;
var myVar;
So you'll not get the error (myVar is not defined) even when you're using myVar before declaring it. But the console logged value of myVar will be undefined. At the starting of code execution, variable was declared, but its assignment was not. So the JS engine would see the above code block as
var myVar;
console.log(myVar); //undefined
myVar = 10;
var myVar;
It will put the declaration for all the variables used in the file at the top of the code.
This question already has answers here:
What is the purpose of the var keyword and when should I use it (or omit it)?
(19 answers)
Closed 7 years ago.
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?
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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
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.
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
I'm currently working on a little project in order to learn the Js and the OOP. But I have a problem, let me explain the differents steps :
1) My constructor :
function Ammo(ammoId,posX,posY){
this.id=ammoId;
this.x=posX;
this.y=posY;
$('body').append('<div class="ammo ammo'+this.id+'"></div>');
}
2) Prototypes :
Ammo.prototype.position = function(){
$('.ammo'+this.id).css({
'top': this.y+"px",
'left': this.x+"px"
});
}
Ammo.prototype.ammomove = function(){
this.y-=1;
this.position();
}
3) The call :
ammo=[];
var a=1;
function createAmmo(){
var ammoX=main.x;
var ammoY=main.y;
ammo[a] = new Ammo(a, ammoX, ammoY);
ammo[a].position();
var animAmmo= setInterval("ammo["+a+"].ammomove()",5);
a++;
}
createAmmo();
My program works perfectly. But when i declare my array with this syntax :
var ammo = [];
it doesn't work and i have the error "ammo is not defined", whereas this works :
ammo = [];
Anybody have an idea ? thx !
EDIT : here is my JsFiddle : https://jsfiddle.net/seabon/c057f1oj/
Press Space Bar for call createAmmo()
var keyword defines local variable. That means variable is visible only in current scope.
When you use ammo = []; it creates global variable, that can be accessed from createAmmo().
But when you use var ammo = [];, it creates local variable. There is method createAmmo() declared in same scope, but you can't access this
scope from body of createAmmo.
Pass it as parameter into createAmmo.
Consider this!
This is exactly the answer to your problem!
When using var keyword, you can't define it for a second time inside the same scope! you can't override this in the same scope.
JavaScript scope is not code inside {...}. It covers all codes inside function xx(){...}. So, when you write ammo=[] and a=1 and ammo[a], JavaScript tries to define another variable with the same name and then successfully override the global ammo array! This succeeds because there is no var keyword. When var keyword is present, ammo[1] is not known, so it is not defined! Remember that array index starts from 0.
with ammo[a] it works as if you try to access/assign a value to a property whose name is 1 of a new object ammo.
Just change a=1 to a=0
If you use the var Keyword to declare a variable this one will be local to the scope where it's declared and it can't be accessed in other scopes, and here if you use:
var ammo = [];
Your ammo variable is local and it can't be accessed inside functions (because every function has its own scope), but when you use:
ammo = [];
Your ammo variable is global and can be accessed in the whole Application.
EDIT:
To correct my answer, in that case you can access both a and ammo inside the createAmmo() function because they are all in the same scope, but the problem here is caused by this instruction:
var animAmmo= setInterval("ammo["+a+"].ammomove()",5);
It's causing an infinite loop and throws ammo is not defined everytime, I tried to debug your code and I think it's caused by the fact that setInterval() is interpreting ammo as a function and also note that it's an Antipattern to pass a String rather than a function to the setInterval() which is stated in JavaScript Anti-Patterns section here, you can avoid this by calling ammo[a].ammomove() without the double quotes like this:
var animAmmo= setInterval(ammo[a].ammomove(),5);
And it will work perfectly, try this DEMO it logs in the console all the changes of ammo.
This question already has answers here:
What is the purpose of the var keyword and when should I use it (or omit it)?
(19 answers)
Closed 7 years ago.
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?
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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
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.
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
This question already has answers here:
What is the purpose of the var keyword and when should I use it (or omit it)?
(19 answers)
Closed 7 years ago.
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?
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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
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.
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