Javascript basics - Scope

Scope

In JavaScript, the term "scope" refers to the context in which variables are declared and can be accessed. There are two main types of scope, Global and Local.

Global Scope

Variables declared outside any function or block have global scope and can be accessed throughout the entire program.

Local Scope

Variables declared within a function or block have local scope and are only accessible within that specific function or block.

var teacher = "Kyle";

function otherClass() {
  teacher = "Suzy";
  topic = "React";
  console.log("Welcome!");
}

otherClass(); // Welcome!

teacher;
topic;

In this code snippet, there is a global variable teacher initialised with the value "Kyle." Additionally, there is a function called otherClass that is invoked. Within the otherClass function, there are two assignments:

teacher = "Suzy";: This modifies the global variable teacher and changes its value to "Suzy."
topic = "React";: This creates a new variable named topic without using the var, let, or const keyword. In JavaScript, if you assign a value to a variable without declaring it using var, let, or const, it becomes a global variable. So, topic would also become a global variable in this case.

The console.log("Welcome!"); statement logs "Welcome!" to the console when the otherClass function is invoked.

After calling otherClass(), the code then has two expressions:

teacher;: This expression is not assigned to anything, so it doesn't produce any visible output in most environments. However, it effectively retrieves the value of the global variable teacher.
topic;: Similar to the first expression, this retrieves the value of the global variable topic, which was created inside the otherClass function.

It's worth noting that relying on implicitly creating global variables by omitting the var, let, or const keyword is generally not considered good practice, as it can lead to unexpected behavior and make the code harder to maintain. It's recommended to always declare variables using var, let, or const to explicitly specify their scope.

Vivienne Reay