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Scope - MDN Web Docs Glossary: Definitions of Web-related terms 编辑

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

A function serves as a closure in JavaScript, and thus 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() {
    var 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:

var x = "declared outside function";

exampleFunction();

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

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