Related
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've attached an animated gif to illustrate this weird behavior. Essentially, my question is does Chrome console treat var and let differently when used in the same scope? You'll notice that after declaring / assigning a variable, if you try to type that variable's name into the console, Chrome will autocomplete it for you, showing a dropdown list containing what your typing. When using lets, this is not the case. Is this a bug, feature, or is there something I'm missing about var and let in JavaScript?
Note: I'm well aware that let lives & dies within the immediate scope.
When you use var in the console, it executes in the global scope and adds the variable to the window object.
When you use let in the console, it executes in the global scope, which doesn't add the variable to the window object.
When you start typing, autocomplete checks the parent object for properties to complete along with other language constructs, such as function, for, and while.
When there is no content in the console, the parent object is window, which won't have the property you're looking for because let doesn't add the property to window.
As soon as you have a new object for autocomplete to complete, behavior returns to what you'd expect.
> let foo = {bar: 'baz'};
> foo.b //autocompletes bar
Now, with all of that said, there's no reason that autocomplete has to behave that way. In many regards the lack of autocomplete for variables defined in global scope via let could be considered a "bug" worth "fixing". In my opinion it is moderately surprising behavior.
var defines a variable on the global scope, while let defines it only in the local scope. Most likely, the autocomplete is only looking on the global scope for targets.
I need to emulate in firefox the behavior of activeXobject in last versions of ie. I write addon with contentScript and want to implement to pages such variable. It is exactly what I want - It is a challenge in itself.
Example of IE 11 working code:
if (window.ActiveXObject == undefined) {
var but = new window.ActiveXObject; // [ActiveXObject object]
}
I need exactly same behavior in firefox with my variable
For example what i want
<script>
console.log(window.variable) // undefined
console.log(new window.variable); // [object Variable]
console.log(new variable) // [object Variable] if not possible previous string
</script>
for solving, you can change the addon sdk or a browser source, change realization of all getters or something else
In browser Javascript, there is no such thing as a global variable that is not a property of the window object. So, if that's truly what you are trying to do, then it cannot be done. Yes, you might be able to create a getter on the window object with Object.defineProperty(), but that's still a property on the window object so I'm not sure how that helps you.
Likewise, there is no structure in Javascript such that:
window.ActiveXObject === undefined
and this works:
var x = new window.ActiveXObject;
Javascript just simply doesn't work that way. The property either exists or it doesn't. It can't be undefined for one way of accessing it and defined for some other way of accessing it.
You might also be able to create the property on the window object so it is not enumerable if you want it to be less visible for some reason.
If you explained what you're really trying to accomplish, there may be some work-arounds by enclosing the relevant code in a closure and defining the variable in the closure, but without knowing more details about what the actual problem to be solved is, we can't help more specifically than that.
If you just want the variable defined for YOUR code only, then you can just put it in a closure:
(function() {
var myVar = "foo";
// other parts of your code
// elsewhere in your code
console.log(myVar);
})();
Your code can treat it like a global, but it's really not a global, it's just defined within the scope that your code lives and not available to any other code outside this scope.
Theoretically, this is possible with lexically declared variables in ES6:
let variable = 5;
console.log(variable); // 5
console.log(window.variable) // undefined
However, the current let support in FF is not as advanced, and still treats this a property of the global object.
If the variable only needs to be accessible by your project code, wrap all of your project code in a self executing anonymous function.
(function(){
var internalGlobalVar = "some value";
//All your code gets wrapped in here...
})();
Then if you tried to alert(window.internalGlobalVar) you would get undefined.
We use google closure compiler with plovr and wrap all of our code in a self executing anonymous function so basically all of our global variables are project scope, not on the window "output-wrapper": "(function($){%output%})(jQuery);"
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