This question already has answers here:
Javascript function scoping and hoisting
(18 answers)
Why is no ReferenceError being thrown if a variable is used before it’s declared?
(3 answers)
Closed 5 years ago.
I wonder why the two code snipped below generate two different results as in reference below the two have to be the same as were are in global scope and placing var would not make a difference.
I was under the impression due to what explained below and the Javascript hoisting the two generate same output, but don't! why?
What is the purpose of the var keyword and when to use it (or omit it)?
console.log(a)
var a = 90;
// undefined
vs
console.log(a)
a = 90; // no var keyword
// Uncaught ReferenceError: a is not defined
When you put var keyword, what it does is it moves the var part as a declaration on that scope and makes it undefined. In the first case, with var:
console.log(a)
var a = 90;
// undefined
Gets rewritten as:
var a = undefined;
console.log(a)
a = 90;
// undefined
So, you get the undefined here, vs. your second case has it the same way, where there's no declaration or key in window object named a. Hope this is understandable.
The point here is, when you use a var keyword, immediately that's defined as undefined at the top of the scope before anything.
Related
This question already has an answer here:
Why is the named IIFE logged instead of the variable with the same name?
(1 answer)
Closed 2 years ago.
I have encountered a problem that if the IFEE function's name is the same with variable's name in it. the output is the function itself. Why?
var b = 10;
(function b() {
b = 20;
console.log(b);
})();
Named function expressions create a read only variable in their own scope which matches their name and references themselves.
This is useful for writing recursive functions.
This question already has answers here:
Javascript function scoping and hoisting
(18 answers)
Closed 4 years ago.
What is the difference between the following two cases:
CASE 1:
console.log(c); //console : undefined
var c;
CASE 2:
console.log(c); //console : Uncaught ReferenceError: c is not defined
Why in Case 1 it console undefined? as it was declare after console.log(). At that time variable c is also not defined than why program console undefined
Please check docs for var statement in MDN
variable declarations (and declarations in general) are processed before any code is executed, declaring a variable anywhere in the code is equivalent to declaring it at the top. This also means that a variable can appear to be used before it's declared. This behavior is called "hoisting", as it appears that the variable declaration is moved to the top of the function or global code.
So, in first example variable has been declared, but was not defined (even with var c = 1; before that row it would be not defined). In the second example, it was not declared at all.
This question already has answers here:
Function and variable with the same name
(2 answers)
Closed 5 years ago.
Might be a kind of easy question, but I have a question on the issue of having the same name for a variable and a function.
If there's a variable,
var add = 1;
and a function,
function add(x,y) {return x+y;}
and there're two console.log,
console.log(add)
console.log(add(1,2))
I've expected those 2 console.log would work properly since add contains the Number and add() is classified as a Function, but the second one prints an error. So they aren't considered the same.
But the result says I'm wrong.
Can anyone explain what's going on in my code?
Variables and function definitions(not expressions) are hoisted to up, it means that wherever in scope you wrote your function or variable they will be moved to the start of the scope. First goes functions definitions then variables. So it means that functions will be overwritten by variables.
var add = 1;
function add(x,y) {return x+y;}
console.log(add);
The order doesn't matter. Later will be the variable and will overwrite
function add(x,y) {return x+y;}
var add = 1;
console.log(add);
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 6 years ago.
I noticed that in JavaScript, Inside a function some times a variable created without mentioning var before it.
For example :
function myfunction() {
x = 10;
var y = 10;
}
what is the difference between these both?
function myact() {
x = 20;
var y = 10;
console.log(x);
console.log(y);
}
myact();
console.log(x);
//y not available here
var is used for declaration. So, assigning without declaration will simply give it global scope meaning: it will first search if it is available in any scope stack above, if not creates the variable implicitly in global scope and assigns.
JS Docs says:
The scope of a variable declared with var is its current execution
context, which is either the enclosing function or, for variables
declared outside any function, global.
Assigning a value to an undeclared variable implicitly creates it as a
global variable (it becomes a property of the global object) when the
assignment is executed.
This question already has answers here:
What is the (function() { } )() construct in JavaScript?
(28 answers)
Closed 8 years ago.
I just encountered this variable declaration syntax for the first time.
var myVar = (function() {
return 1;
})();
My 2 main questions are..
What is it called, and why is it legal?
Forgive me if this question has been asked before, I tried searching around but I have no idea what this notation is called so I wasn't able to find anything.
Also, I should add, what is the function of the 2 sets of parentheses? The first of which encloses the function, the second of which is empty.
Self executing functions are typically used to encapsulate context and avoid name collusions. Any variable that you define inside the (function(){..})() are not global.
The following code:
var same_name = 1;
var myVar = (function() {
var same_name = 2;
console.log(same_name);
})();
console.log(same_name);
produces this output:
1
2
By using this syntax you avoid colliding with global variables declared elsewhere in you javascript code.
I am not sure what this is called, other than defining an anonymous function and immediately invoking it.
It is perfectly legal, because
Defining an anonymous function is legal.
Invoking it and assigning the return value is also legal.
The end result is that myVar = 1.
This is an anonymous function (also called lambda function) that is being executed immediately with its return value (1) being assigned to a variable (myVar). It's legal because the specification says it is. It is a very common feature in many languages.