how to change global variable in function javascript

How do I change the value of a global variable inside of a function

stackoverflow.com › questions › 10872006 › how-do-i-change-the-value-of-a-global-variable-inside-of-a-function

Just reference the variable inside the function; no magic, just use it's name. If it's been created globally, then you'll be updating the global variable.

You can override this behaviour by declaring it locally using var, but if you don't use var, then a variable name used in a function will be global if that variable has been declared globally.

That's why it's considered best practice to always declare your variables explicitly with var. Because if you forget it, you can start messing with globals by accident. It's an easy mistake to make. But in your case, this turn around and becomes an easy answer to your question.

Answer from Spudley on Stack Overflow

Stack Overflow

stackoverflow.com › questions › 10872006 › how-do-i-change-the-value-of-a-global-variable-inside-of-a-function

javascript - How do I change the value of a global variable inside of a function - Stack Overflow

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194

Just reference the variable inside the function; no magic, just use it's name. If it's been created globally, then you'll be updating the global variable.

You can override this behaviour by declaring it locally using var, but if you don't use var, then a variable name used in a function will be global if that variable has been declared globally.

That's why it's considered best practice to always declare your variables explicitly with var. Because if you forget it, you can start messing with globals by accident. It's an easy mistake to make. But in your case, this turn around and becomes an easy answer to your question.

2 of 6

100

var a = 10;

myFunction();

function myFunction(){
   a = 20;
}

alert("Value of 'a' outside the function " + a); //outputs 20

Codecademy

codecademy.com › forum_questions › 4f85a3e0f37bc30003039312

Can you change a global variable's value from within a function? | Codecademy

var global = "Global Variable"; //Define global variable outside of function function setGlobal(){ global = "Hello World!"; }; setGlobal(); console.log(global); //This will print out "Hello World" ...

Discussions

javascript - jquery change global variable inside function - Stack Overflow

Does in possible to change global variable inside function without return ? I need after call function change have output "after" ... It is possible, but why would you use globals to begin with? Why can't the function take one argument? ... You may want to check this question, because that's seems like a pretty common mistake. ... Short answer is yes. Long answer is you should read this, and this, to get to know all you need about scopes in JavaScript... More on stackoverflow.com

stackoverflow.com

How do I update a global variable from inside of an arrow function?

This is an asynchronous issue. You are passing a callback function to getCurrentPosition because the function is not going to give you a result back until later. Your getLocation function by contrast is synchronous. It returns a result right now. That is why "but location is null here" logs before "loc inside function". Everything you are doing with the variables and returns is fine, but the order is wrong. So how do you fix it? Well, once something is asynchronous, everything downstream must also be asynchronous. This can be a pain, but it makes sense if you think about it. Anything synchronous happens now. There is no way for code that is running now to time travel and get values that do not resolve until later. So your function must become asynchronous. It cannot simply return the value. The simplest way to do this is to just use a callback function just like getCurrentPosition does. function getLocation(onDone) { let loc = null; if (navigator) { navigator.geolocation.getCurrentPosition( ({ coords }) => { loc = { "latitude": coords.latitude, "longitude": coords.longitude }; onDone(loc); } ); } else { onDone(loc); } } We could perhaps simplify ths code further, but this is the minimum change to get your function working. Notice we are no longer returning anything. Instead, you pass in an onDone function, and that function will eventually be called with the location. getLocation((loc) => { console.log(loc); }); Now, most modern JS does not use callbacks for asynchronous code like this, instead wrapping callbacks in Promises . This will require a little more work upfront, but will ultimately allow you to use the .then or async/await syntax, which is a little easier. function getLocation() { return new Promise((resolve) => { if (navigator) { navigator.geolocation.getCurrentPosition( ({ coords }) => { resolve(coords); } ); } else { resolve(null); } }) } I've simplified things a bit here, but the basic structure is the same. We're returning a Promise object, which is basically an IOU. Eventually, it will resolve to some value. Notice that the Promise's resolve function has basically replaced our onDone from before. Under the hood, Promises are built on callbacks. But a Promise let's us do neat tricks like this: async function logLoc() { let loc = await getLocation(); console.log(loc); } logLoc(); This async function will pause its execution until the Promise we returned resolves. The resulting code looks almost synchronous. More on reddit.com

r/learnjavascript

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June 5, 2022

Not Clear How a Function Can Overwrite a Global Variable

Using the keyword var within the function creates a new variable with the same name as the global variable that was defined above the function. If you omit the keyword 'var' in the function definition, you'll change the global variable. Call tripOut() with and without the var keyword to see what ... More on teamtreehouse.com

teamtreehouse.com

3

December 1, 2016

javascript - How can I set a Global Variable from within a function - Stack Overflow

No, really though; I know that ... ES (“JavaScript”) specifications are one of the easiest-to-read and most comprehensible specs I've ever seen. They're truly excellent. Of immediate note, and in no particular order, 10.4, Establishing an Execution Context: Covers how ‘global code’ and ‘eval code’ are handled, specifically. 10.2, Lexical Environments: These describe “where variables are stored.” ... More on stackoverflow.com

stackoverflow.com

November 10, 2011

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