Same variable name inside of two different functions

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#1

I got this code from someone’s project from github… When I was trying to read the code, I found out that he/she uses same variable name inside of two different functions, is that going to cause a problem/bug? are both variables(err) somehow going to intervene with each other due to using the same name?

const getUser = async () => {
  try{
    const user = await getUserDataFromDB();
  }catch(error){
    // Same variable name here!
    const err = [array of errors];
    throw err;
  }
};

const getPost = () => {
  try{
    const post = await getPostDataFromDB();
  }catch(error){
    // Same variable name here!
    const err = [array of errors];
    throw err;
  }
};
#2

const and let are only available in the current scope. That’s why this is fine.
If var was used, that would be more problematic.

#3

so you mean even though both variables have the same name but they are each in different function, so there is going to be no issues?

so if I have something like this is fine:

const one = () => {
  let/const data = [array of objects];
};

const two= () => {
  let/const data = [array of objects];
};

but if I have something like this will cause some kind of problem/bug?

const one = () => {
  var data = [array of objects];
};

const two= () => {
  var data = [array of objects];
};

how come if I use var, it will cause problems?

sorry js beginner here

#4

In your example you can even use var without running into trouble as they are each wrapped inside a function scope. But once your code becomes more complex, using var can cause trouble. Generally speaking, the newer let (introduced in ES6 2015) should be preferred over the older var.
Here is an example how using var differs from using let:

var var0 = 'var0';
if(true){
  var var0 = 'var1'; // overwrites the global variable
}
console.log(var0); // var1

let let0 = 'let0';
if(true){
  let let0 = 'let1'; // only available in this scope
}
console.log(let0); // let0

As long as you’re not redeclaring a variable inside the same scope, you will not get a syntax error.

Here you find information about the difference of var and let: MDN docs