Scope

Scope in JavaScript defines where variables and functions are accessible in your code, and it can behave differently depending on the environment (such as a browser or Node.js). There are four levels, from highest to lowest:

Global Scope:
- Variables declared globally have the highest level of scope and can be accessed from anywhere in your code.
- In browsers, global variables are properties of the window object. For example, window.myGlobalVar = 'hello world' defines a global variable.
- In Node.js, global variables are properties of the global object: global.myGlobalVar = 'hello world'.
Module Scope:
- In ES modules (both in Node.js and modern browsers), variables declared at the top level of a module are scoped to that module. They are not added to the global scope.
- In the browser, using <script type="module"> creates a module scope for that script.

We'll cover modules in a later chapter.

Function Scope:
- Variables declared with var (we try to avoid this) are limited to the function scope. They are accessible only within that function and any nested functions.
Block Scope:
- ES6 introduced block scope with the let and const keywords. A block is typically defined by curly braces {}, like in if statements, loops, and other blocks of code.
- Variables declared with let and const are confined to their block, making them more predictable and reducing the chances of accidental variable hoisting.

Assignment

While reviewing the Textio messaging system, you’ve come across a bug involving scope issues!

The function getMessageStatus returns the "status" of a message based on its length. It uses a nested helper function, isValidLength, to check if the message has more than 0 characters.

Fix getMessageStatus so that it uses the result of isValidLength to set the messageStatus.