Not understanding the concept of local and global variables

Question

Ok so just a quick one... I REAAAAAALLLLY am not understanding scope at all. Do some reason whenever I put a variable inside of a function it's undefined. It seems the only way it works it to declare it OUTSIDE of a function. So what gives? Am I simply misunderstanding the concept of local and global variables? Because it seems global variables are the only ones that work...

Edit:

Simple example...

let a = 'something';
function myFunction() {
    // Code block
};

This works

But the following gives me undefined variables:

function myFunction() {

    let a = 'something';

    // Code block
};

Possible duplicate of [What is the scope of variables in JavaScript?](https://stackoverflow.com/questions/500431/what-is-the-scope-of-variables-in-javascript) — Andrew Marshall, Jun 18 '18 at 22:50
Some examples would help us resolve your specific confusion. — Andrew Marshall, Jun 18 '18 at 22:51
Once you add more examples, I can edit my answer to resolve your specific confusion — Sheshank S., Jun 18 '18 at 22:52
@AndrewMarshall I edited that post and added an example if that helps. Anytime I declare a variable like the second example, I get only undefined variables... And I'm worried doing it the other way could overwrite some other variable from some other script. — Curtis Steele, Jun 19 '18 at 02:23
@CurtisSteele If my answer helped you please upvote it and mark it as the correct answer — Sheshank S., Jun 19 '18 at 13:32
@SheshankS. No answer yet. I edited the post and added an example if you'd like to check that out. — Curtis Steele, Jun 19 '18 at 13:34
@SheshankS. Well the script does nothing on the site so I check the console and each variable returns "undefined". So when I switch it to the FIRST example, it works and the variables come up as they should. I just don't know why when I call it inside the function nothing works — Curtis Steele, Jun 19 '18 at 13:37

Sheshank S. · Accepted Answer · 2018-06-22T15:03:57.090

4

First lets take a look at the differences between local and global variables.

LOCAL VARIABLES - Any variable defined inside of a function or loop. If the variable is defined in a certain function/loop, it can only be accessed in that function/loop.

For example:

function number() {
  let a = 0;
  
  // Can access a here.
}

// Cannot access a here.

GLOBAL VARIABLES - Any variable that is not defined inside of any function or loop. These variables can be accessed anywhere.

For example:

let b = 0;
function number() {
  // b can be accessed here.
}
// b can be accessed here.

The scope of a variable is where it can be accessed in. For ex, a scope of a local variable that is defined inside of function number() is the number() function since it can only be accessed there.

One more quick thing (Credit to Mikael Lennholm) var doesn't scope a variable to a loop (or any non-function statement block), it only scopes it to the current function. To scope a variable to any statement block you have to use let/const

Now let's look at your examples:

Here is the first example:

let a = 'something';
myFunction();
function myFunction() {
    console.log(a);
};

That works fine, and that's what you said.

Here is the second example you said doesn't work:

function myFunction() {

    let a = 'something';

    console.log(a);
};

myFunction();

But, if you click the run button, you'll actually see it does work, it does get logged. If you after this go and check in the console what the variable a is of course it will be undefined, since a can be only accessed in the myFunction() function, and the console is not in that function.

You said that the script doesn't work, it's most probably because of something else (if it was because of this there would be an error in the console). You can try posting the website here and I can help you.

Hope this helped you :)

edited Jun 22 '18 at 15:03

answered Jun 18 '18 at 22:47

Sheshank S.

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`var` doesn't scope a variable to a loop (or any non-function statement block), it **only** scopes it to the current function. To scope a variable to any statement block you have to use `let`/`const` – Lennholm Jun 18 '18 at 23:02
@MikaelLennholm Is that better? – Sheshank S. Jun 18 '18 at 23:04
Not quite. `var` is "Function Local". `let` is "Block Local". Every `{ //stuff }` (when not an object literal) is a block. So `var global; function() { var function_local; if (true) { let block_local; var function_local2; } }` – Cliff Armstrong Jun 18 '18 at 23:23
@SheshankS.The only difference between mine and yours is how i called the function. Could that be the reason? Also thank you... – Curtis Steele Jun 19 '18 at 13:46
Hmm... how did you call the function? – Sheshank S. Jun 19 '18 at 13:58
@SheshankS. Im sure i wrote it wrong and maybe thats why but i was using something i saw on a different site which was window.onload = function { and then the code... – Curtis Steele Jun 19 '18 at 14:00
ohhh you need to do `window.onload = function() { //code}` – Sheshank S. Jun 20 '18 at 22:09
@CurtisSteele ^^ – Sheshank S. Jun 22 '18 at 15:04

score 1 · Answer 2 · answered Jun 18 '18 at 22:48

Global variables are variables that you have access to them anywhere in your codes.

Local variables are limited to their scopes like a function, module, class and etc.

if you define a local variable it can be accessible in the nested level like closures.

if you define a global variable and you define the same name as local variable , the local overwrite the global, then be careful.

Not understanding the concept of local and global variables

2 Answers2