This question already has answers here:
A javascript 'let' global variable is not a property of 'window' unlike a global 'var' [duplicate]
(1 answer)
Do let statements create properties on the global object?
(5 answers)
Closed 5 years ago.
var name = 'John';
console.log(this.name, document.name, window.name, name);
const meme = "Bruce";
console.log(this.meme, document.meme, window.meme, meme);
Output:
John undefined John John
undefined undefined undefined "Bruce"
Is global scope differently defined for var and const? I thought the only difference would be the const is immutable.
Yes, window scope and variables defined using var is different from the scope of variables declared using const and let.
See also Is it possible to delete a variable declared using const?, Why does .then() chained to Promise.resolve() allow const declaration to be reassigned?
Related
This question already has answers here:
Do let statements create properties on the global object?
(5 answers)
Closed 2 years ago.
I experimented a bit with this keyword and I found a strange problem. When a variable is declared as var then this correctly refer to this variable. But if i declared the same variable as let or const then this lost reference and show undefined in console.
var prop = "outer"; // not working if let or const.
let foo = {
prop: "inner",
show() {
console.log(this.prop)
}
}
let a = foo.show;
let b = foo.show.bind(foo);
a()
b()
When you run this code outside of strict mode (as here), then the this keyword defaults to the global object - which in the browser is window. (In strict mode it would be undefined so a() will throw an error.)
The difference in behaviour between var and let/const is a simple consequence of the fact that var-declared global variables are the same thing as properties on the global (window) object, whereas that doesn't happen with variables (even global ones) declared with let or const.
See for example MDN:
Just like const the let does not create properties of the window object when declared globally (in the top-most scope).
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:
What is the difference between "let" and "var"?
(39 answers)
Closed 2 years ago.
ES5 introduced use of let, but I don't understand if I already have var then why i need to use keyword let in javascript.
Variables declared with let and const eliminate this specific issue of hoisting because they’re scoped to the block, not to the function. Previously, when you used var, variables were either scoped globally or locally to an entire function scope.
If a variable is declared using let or const inside a block of code (denoted by curly braces { }), then the variable is stuck in what is known as the temporal dead zone until the variable’s declaration is processed. This behavior prevents variables from being accessed only until after they’ve been declared.
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:
"Variable" variables in JavaScript
(9 answers)
Closed 7 years ago.
I wanted to create variable name based on value sent to function in javascript.
like following, when i call function variable : variable("zebra"); this should return variable name as zebra1
function create variable(i){
return i+"1";
}
var variable("zebra")="abc";//this line should create variable zebra1 and initialise as abc
Try:
window['zebra'] = 'abc';
The window object holds all the global variables, assuming this is a request for global variables.
To specifically answer your question, you could put return window[i + '1'] = 'abc'; in your variable naming function.
Alternatively, you could create a global (or local) object named variables to hold all your variables:
function whoknows() {
var variables = {};
variables['zebra'] = 'abc';
}
Read more on working with objects at mozilla.org
You can create global variable with
window["zebra"] = "abc";
and use later ether with the same indexer syntax or directly - zebra.