Scope

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, identifiers declared with certain syntaxes, including let, const, class, or (in strict mode) function, 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"; // x can only be used in exampleFunction
  console.log("Inside function");
  console.log(x);
}

console.log(x); // Causes error

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); // 1

{
  const x = 1;
}
console.log(x); // ReferenceError: x is not defined