Scope - MDN Web Docs Glossary&colon; Definitions of Web-related terms

The scope is the current context of execution in which values and expressions are “visible” or can be referenced. If a variable or expression is not in the current scope, it will not be available for use. Scopes can also be layered in a hierarchy, so that child scopes have access to parent scopes, but not vice versa.

JavaScript has the following kinds of scopes:

Global scope: The default scope for all code running in script mode.
Module scope: The scope for code running in module mode.
Function scope: The scope created with a function.

In addition, variables declared with let or const can belong to an additional scope:

Block scope: The scope created with a pair of curly braces (a block).

A function creates a scope, so that (for example) a variable defined exclusively within the function cannot be accessed from outside the function or within other functions. For instance, the following is invalid:

function
 exampleFunction
(
)
 {
  const
 x =
 "declared inside function"
;  
  console.
log
(
"Inside function"
)
;
  console.
log
(
x)
;
}

console.
log
(
x)
;

However, the following code is valid due to the variable being declared outside the function, making it global:

const
 x =
 "declared outside function"
;

exampleFunction
(
)
;

function
 exampleFunction
(
)
 {
  console.
log
(
"Inside function"
)
;
  console.
log
(
x)
;
}

console.
log
(
"Outside function"
)
;
console.
log
(
x)
;

Blocks only scope let and const declarations, but not var declarations.

{
  var
 x =
 1
;
}
console.
log
(
x)
;

{
  const
 x =
 1
;
}
console.
log
(
x)
;

Scope – MDN Web Docs Glossary&colon; Definitions of Web-related terms | MDN

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