The function below will return b is not defined because the compiler will look in the function and then into the global scope in search for a variable b.
However, I assumed that defining b without the word var would automatically create a global variable?
Can anyone explain the rules when omitting the word var?
function foo(a) {
console.log( a + b );
b = a;
}
foo( 2 );
Not using var in a function for a variable declaration does make it global, but in your case, the JavaScript engine is hitting this line:
console.log( a + b );
before it hits this line:
b = a;
And, that's the line that declares it (globally).
And, because you didn't use var, the declaration is not hoisted to the top of the code base (which it would have been with var - you would still not have gotten a value for b because only the declaration would be hoisted, not the initialization, but it would not have thrown an error), so you get your error.
See more about var and hoisting here.
In strict mode:
Using an undeclared variable will throw an exception
In non-strict mode:
Assigning to an undeclared variable will create a global, but this is not hoisted
Declaring a variable with var creates a local variable and is hoisted to the top of the function
Reading an undeclared variable will throw an exception
Since you are not in strict mode and you try to read b before you assign a value to it, you get an exception.
Guidelines to follow:
Always "use strict"
Always declare your variables
Declare them in the global scope if you want them there
… but try to avoid global scope. Consider using a closure instead if you think a global would be useful.
Variables declared this way b = a are not hoisted, like variables declared with the var keyword. That means that, at runtime, the compiler reads b, not as undefined (as would happen with var b = a), but as something that doesn´t exist at all, thus throwing a ReferenceError.
Info on Hoisting: https://developer.mozilla.org/en-US/docs/Glossary/Hoisting
Related
In javascript, compilation phase find and associate all declarations with their appropriate scopes.
CASE1
a = 2;
console.log(a);
If not used in strict mode the snippet is assumed to be actually processed as:
var a;
a = 2;
console.log(a);
Output: 2
CASE2
if the declaration happens below console.log()
console.log(a);
var a = 2;
Here at line 1: compiler will check for the declaration of a in its scope if not found, the global scope will assign one and the same code will be assumed again same as
var a;
console.log(a);
a = 2;
Output : undefined // as no value is assigned while interpreter read console
CASE3
console.log(a);
a = 2;
Output: RefErr
Why this throws reference error and not undefined?
If you carefully look at your first two cases, you are always declaring the variable a, and in JavaScript only the declarations are hoisted.
That being said, in your Case 3, variable a is never declared, hence, hoisting is not possible here. With that, there is no variable by the name a in the scope, and hence, ReferenceError.
If you look at the description of ReferenceError, it reads:
The ReferenceError object represents an error when a non-existent variable is referenced.
Implicit globals do not imply a var statement. There is no hoisting of them.
Looking at MDN's introduction to JavaScript, Grammar and Types section - one reads:
Declaring variables
You can declare a variable in three ways:
With the keyword var. For example, var x = 42. This syntax can be used to declare both local and global variables.
By simply assigning it a value. For example, x = 42. This always declares a global variable. It generates a strict JavaScript
warning. You shouldn't use this variant.
With the keyword let. For example, let y = 13. This syntax can be used to declare a block scope local variable. See Variable scope
below.
The following code snippet would seem to fit the "by simply assigning it a value" scenario, meaning the variable should be treated as global.
(function(){
console.log(myVar);
//the following will throw a ReferenceException error
//myVar = 10;
//the following will not, and I can understand it following the defintion of the behavior of using `var` keyword
//var myVar = 10;
})();
But running the code will generate a ReferenceException when myVar is commented, and undefined when not. I would expect it to generate undefined in both cases, since if myVar is a global variable (per definition), than javascript's variable hoisting would make it known before reaching console.log(myVar);
What is the explanation behind such behavior ? (the behavior I described is what I get when trying it in my firefox's console, but running it in jsfiddle will not throw an error).
Are self-executing functions an exception to hoisting ?
the "by simply assigning it a value" scenario
You are reading the value, not assigning it
if myVar is a global variable (per definition),
It isn't.
myVar is:
a variable scoped to the function if the function contains var myVar (or function myVar () { ... }, or it is listed as a parameter in the function definition).
a variable scoped to the block if the block contains let myVar
a global variable if a value has been assigned to it previously and neither of the above conditions are true.
Since you haven't assigned a value, it isn't a global. Since none of the above conditions are true, it isn't any kind of variable, so you get a reference error.
Regarding your comment:
I left my var when I meant var in the scenario I am trying to depict. Updated question.
… and the edit to which you refer:
Commented out code is not evaluated. Having a comment that uses the keyword var doesn't do anything.
Regarding your further edits.
You get a reference error if you try to read a variable before it has been declared.
var statements (and function declarations) are hoisted so a variable declared with those methods can be read anywhere in the function.
Assignments are not hoisted. A global variable created implicitly by assignment (which is generally not considered to be best practise and is banned in strict mode) can't be read before the value is assigned.
Since my comment seemed to help explain it to you, I will turn it into an answer:
Implicit global variable creation (when you don't actually declare it, but just assign to it) is NOT hoisted. The variable creation happens inline at the moment the assignment occurs.
Thus, when you try to read the variable, it does not exist yet and that is an error.
var or let declarations are hoisted to the top of their appropriate scope.
All of this should hopefully help explain why you should just run in strict mode where implicit global creation is illegal and not allowed and triggers an immediate error. It's basically evil. A simple typo misspelling a variable may not trigger an error when you really want it to.
Thanks a lot in advance for helping me out!
var f2 = function() {
x = "inside f2";
};
f2();
console.log(x);
// → inside f2
Why do I get the x as a global variable with value "inside f2" when I didn't declare it to be a global variable with "var x;" before defining the function?
var f2 = function() {
var x = "inside f2";
};
f2();
console.log(x);
// → Uncaught ReferenceError: x is not defined
Am I right in assuming that x is not defined in this case because there is no global variable x, only the local variable x within the function f2?
Why do I get the x as a global variable with value "inside f2" when I didn't declare it to be a global variable with "var x;" before defining the function?
Because the specification says so. If you assign to an undeclared variable, a global variable is created. In strict mode this will throw an error (which is more reasonable).
Am I right in assuming that x is not defined in this case because there is no global variable x, only the local variable x within the function f2?
Yes.
8.7.2 PutValue (V, W)
[...]
3. If IsUnresolvableReference(V), then
a. If IsStrictReference(V) is true, then
i. Throw ReferenceError exception.
b. Call the [[Put]] internal method of the global object, passing GetReferencedName(V) for the property name, W for the value, and false for the Throw flag.
Declared variables (using var clause) are constrained in the execution context in which they are declared. Undeclared variables are always global.
ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var
Doing x = "inside f2" will climb the scope chain until it hits an x or until the global space, where it will do property assignment on the global object.
It doesn't matter if you've declared x in the global space or not.
A variable can be declared both with an without the var-keyword. The main differences are that with the var-keyword the variable is contained within the closure in which it is declared - without the variable is considered global.
The reason it becomes global is that the runtime looks up the closure hierarchy to find the given variable. When it doesn't find it, it will declare it in the global scope in order for you to use it.
When you do x="inside f2" you are indeed declaring a variable. Because there was no x in that function’s scope, the compiler looks outside the current scope for this variable. It doesn’t exist, so it is set as a global.
Doing var x declares the variable in the current scope, which is not necessarily the global scope.
You should use strict; to catch errors like this.
Is this a variable definition or declaration? And why?
var x;
..and is the memory reserved for x after this statement?
EDIT:
In C extern int x; is a declaration, int x = 5; is a definition. What's the analog in JS? Wikipedia says a declaration allocates memory and the definition assigns a value to this allocated memory.
SECOND EDIT:
I think the explanation of #Deryck sounds great, but there's some output that disagrees with his explanation:
> var x;
undefined
> x
undefined // now it looks like x is defined to the value undefined
> y
ReferenceError: y is not defined
If the ReferenceError output would say y is not declared it would make sense. But often I read that JS has two non-values: null and undefined. So var x would be a definition with the value undefined.
var x is a declaration because you are not defining what value it holds but you are declaring its existence and the need for memory allocation.
var x = 1 is both declaration and definition but are separated with x being declared in the beginning while its definition comes at the line specified (variable assignments happen inline).
I see that you already understand the concept of hoisting but for those that don't, Javascript takes every variable and function declaration and brings it to the top (of its corresponding scope) then trickles down assigning them in order.
You seem to know most of this already though. Here's a great resource if you want some advanced, in-depth exploration. Yet I have a feeling you've been there before.
Javascript Garden
PS - your analogy between C variable dec/def and JS was spot on. What you read on Wikipedia was correct.
Declaring a variable is like telling the (javascript) compiler that this token x is something I want to use later. It does point to a location in memory, but it does not yet contain a value. ie. it is undefined
var x;
defining it means to give it a value which you can either do it like:
x = 10; // defining a variable that was declared previously
or like this:
var y = 20; // declaring and defining a variable altogether.
http://msdn.microsoft.com/en-us/library/67defydd(v=vs.94).aspx
http://www.w3schools.com/js/js_variables.asp
I will give you a long answer for better explanation.
When the javascript engine is not able to find a particular variable in memory, it will throw an error. To be more specific, when the javascript engine (Execution Context) is not able to "reference" a variable in memory, it will throw a ReferenceError. This is not exactly the same as a Declaration Error, at least in javascript.
There is a deference between a not defined error and the value undefined.
So doing
var a = undefined;
and
var a;
will both log the same result i.e. undefined. This is because, when you simply do a var a; the javascript engine allocates memory for the variable and automatically sets it's value to undefined, which is different from saying that a doesn't exist at all - in which case it will throw a ReferenceError.
Hoisting
console.log(a); // undefined
var a = 'something';
will log undefined because, the javascript engine knows there's a variable declared somewhere in the code - which means to say that the javascript engine actually does something before it executes the code - one of the thing it does is hoists variables. To put it simply, the above code is the same as
var a; // hoisted (declared and defined the value `undefined`)
console.log(a); // undefined
a = 'something' // update the defined value to `something`
So, yes, declaration and definition happen together in javascript (automatically - if you don't do it yourself) and the default defined value is undefined.
ES6
Just an additional note
const a;
will throw a SyntaxError where a initializer (definition) is necessary. const is the only time when you need to declare and define manually.
> var x;
undefined
> x
undefined // now it looks like x is defined to the value undefined
> y
ReferenceError: y is not defined
Although it is usually said that Javascript is an interpreted language, but there is also a compilation step that happens very fast just before the interpreter runs. The job of this compilation step is to create scope chains, where variables are declared(no read/write operation here, just simple name-keeping) in their respective scopes. These variables will point to some memory location but value in it will be undefined until some execution is carried out by the interpreter.
> Compiler run:
When compiler sees var x;, it will simply book-keep this variable in its respective scope.
The next x; and y; are simply ignored in the compilation step as they are execution statements.
> Interpreter run:
When interpreter sees var x;, it will skip this as there is no read/write operation here.
Now when interpreter sees x;(execution statement), "x" will already be declared in the scope, and it will hold value "undefined", which is what you get on the console.
But when interpreter sees y; similarly, there has been no previous declaration or name-keeping for it in the compilation step, and thus we get the ReferenceError as expected.
Hope someone finds this comment useful.
var x, y, z;
var x;
var h = 4;
i = 4;
all the above are global variables if placed at the top, (outside any functions)
Lets say that the javascript has a function start
function start() {
x = 5*5;
}
the global variable x is now equal to 25
Where as if the var x; was not placed outside of any functions, that variable x would just be local to that function.
You declare JavaScript variables with the var keyword:
var carname;
After the declaration, the variable is empty (it has no value).
To assign a value to the variable, use the equal sign
var carname="Volvo";
In computer programs, variables are often declared without a value. The value can be something that has to be calculated, or something that will be provided later, like user input. Variable declared without a value will have the value undefined.
The variable carname will have the value undefined after the execution of the following statement:
var carname;
var hoisting
In JavaScript, a variable can be declared after being used.
bla = 2
var bla;
// ...
// is implicitly understood as:
var bla;
bla = 2;
For that reason, it is recommended to always declare variable at the top of functions. Otherwise, it may lead to confusing cases
When declaring a variable without assigning a value to it, there still needs to be some memory available for it, otherwise you cannot make a reference to the variable later in the program. I don't think it's a noticeable amount of memory being used and won't make a difference.
var x;
This is a variable declaration. In Js if you don't assign any value to variable in declaration. It will get undefined by default.
var x; // declaring x
console.log(x); // output: undefined
But if you have not even declared the variable in you try to access it. It says that the variable is not defined.
console.log(y); // Output: ReferenceError: y is not defined
If you need access to objects between JS files, it's good practice to expose one object to the global namespace and declare fields and methods on that object.
File 1:
var myObject;
myObject.myField = "Field!";
File 2:
myObject.prototype.myFunction = function () {
return this.myField;
};
I have taken from a really good discussion on : Equivalent of C extern declaration in JavaScript
https://github.com/ganqqwerty/123-Essential-JavaScript-Interview-Questions
Various trivia regarding the difference between undefined and null completely aside, the short answer is: there is no equivalence in Javascript. There are no bare "forward declarations". Javascript variable declarations are definitions. Variables that have been defined but not explicitly initialized will contain the value 'undefined'. There is no "external linkage".
If you refer to an identifier that is not in any accessible scope (perhaps because it doesn't exist the first place), you will get your "ReferenceError: y is not defined". This has nothing to do with variable value or storage.
In simple terms,
undefined means value to the variable is not defined.
not defined means the variable itself is not defined.
var x; //value is not defined. So,
x //undefined
//y variable is not declared or defined. So,
y // y is not defined
I get a strange behavior when declaring an object with the logical OR.
my_var = my_var || {}; // throws TypeError
If I add the var keyword
var my_var = my_var || {}; // returns empty object
Why is this? I can't seem to find an explanation. my_var is global scope, so why is var changing the behavior?
The first example tries to assign to a property on the global object named my_var by reading the value from an identifier called my_var (OR an empty object). However, the identifier my_var is not defined at that point, so it fails.
In the second example, due to how javascript variable hoisting works, the my_var variable is already declared, when you read from it by assign to it.
Also have a look at this example:
a = a; // fails, undeclared identifier
a = 0;
With var keyword it will work!
b = b; // succeeds allthough identifier undeclared?!
var b = 0;
This is because variable hoisting will turn it into this:
var b; // declaration of b hoisted to the top of scope
b = b;
b = 0;
when defining a variable without var, you're directly accessing the global object. That means, your Javascript engine trys to lookup my_var on the global object (window if you're in a browser). Since that property does not exist yet, your JS engine will throw.
That happens on the right side of your statement, when your engine trys to read a variable with the name my_var. assigning like
my_var = {};
would work tho. But accessing an identifier without var will cause the browser to lookup the scopechain. Since the variable object for the global object is the global object itself, the lookup procedure will end up nowhere ( = exception ).
By putting the var keyword infront, your js engine knows at parsetime that it has to declare a new variable with that identifier name. It actually does declare that variable with an undefined value (that is called "hoisting"). That means, using var will prematurely create a property in the current context with that name. So if that code is not located in any function or eval context, it'll create the property my_var on the window object.
In your second example, the var defines the scope of my_var. This allows the value to be set properly. Here's a quick and dirty look at what's happening in both examples:
With example 1, the JS engine has to do the following:
Check the current scope to see if my_var exists.
Check the scope above (and the scope above that) until it finds my_var.
Create my_var in the global scope when it's not found.*
*Step 3 has not yet happened when you try to assign a value to my_var.
Example 2:
Is there a variable named my_var in the current scope? Yes, you just created it using the var keyword.
Assign its value.
In the 2nd situation, my var is undefined so the variable is assigned an empty object which is {}
Although I cannot quote the specs, I assume that the initialization in a var statement is just syntactic sugar:
var my_var = my_var || {};
is equivalent to:
var my_var;
my_var = my_var || {};
In the first line, my_var is declared and implicitely set to the special value undefined. Then, in the second line, my_var exists, undefined evaluates to false and no error is raised.
In your first example, however, my_var is unknown when the right hand side is evaluated, hence the error.
In the first case you try to assign a variable that was not defined. Its the same as you would write :
my_var = a || {};
in the second case my_var is only undefined as you use the var keyword an create my_var in the actual scope. Undefined is converted to false and it will return the empty object.
Interestingly, this will work:
a = my_var || {}; // 'a' is now an empty object
When you use the 'var' keyword you're literally giving the command "create a new variable and assign to it this value." If you don't use 'var' then you're implying that the variable already exists within the global scope somewhere.
In your example the JavaScript engine attempts to look up the current value of 'my_var', finds that it doesn't exist, cannot create it because you're currently attempting to assign a value to it, and fails.