Вивчайте Scopes

Definition

A Scope is simply an area in the code where a variable can be accessed or used.

There are two types of scopes:

Global Scope;
Local Scope.

If a variable is defined inside a block of code (between curly brackets {}), it is said to have a local scope. This means that it can only be accessed from inside that function or code block, or any nested blocks:


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function exampleFunc() {
  let exampleVariable = 10;

  console.log(exampleVariable); // Valid

  if(10 + 10 == 20) {
    console.log(exampleVariable); // Valid
  }
}

exampleFunc();
console.log(exampleVariable); // Shows error

A variable that is defined outside of any code block is said to have a Global Scope, and it can be accessed from anywhere:


              123456789101112
            
let exampleVariable = 10;

function exampleFunc() {
  console.log(exampleVariable); // Valid

  if(10 + 10 == 20) {
    console.log(exampleVariable); // Valid
  }
}

exampleFunc();
console.log(exampleVariable); // Valid

A variable defined in a lower (nested) scope cannot be accessed from a higher (parent) scope:


              1234567891011
            
function exampleFunc() {
  if(10 + 10 == 20) {
    let exampleVariable = 10;
    console.log(exampleVariable); // Valid
  }

  console.log(exampleVariable); // Shows error
}

exampleFunc();
console.log(exampleVariable); // Shows error

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