Lesson 7: Scope

Scope determines the visibility and accessibility to variables. The concept of scope is used in many different programming languages such as Java, C#, JavaScript and PHP.

• Global scope variables can be accessed anywhere.
• A global scope variable is variable that is defined outside of functions /conditions.

Example:

var my_name = "Kevin" //Global scope Defined 

console.log(my_name); //Output - Kevin 
function getName(){
console.log(my_name); //my_name is still accessible 
}
getName(); //Output – Kevin

• Local variables are accessible only within a function.
• When a variable is defined inside a function it is known as a local variable.
• Once a variable is declared inside a function, it can’t be accessed outside the function.
• Use the var keyword to define a variable for function scope.

Example:

var my_name = "Kevin" //Global scope Defined 

function myAge(){
var age = 21; // Local/Function variable defined
console.log ("My age is : " + age);
}
myAge(); //Output will be 21 as the variable is declared inside the function
console.log(age) //Output will be defined as the variable is visible only to the ‘myAge’ function.
console.log(my_name) //Output will be Kevin as it’s a global variable and can be accessed from anywhere.

• Block scope is the area of code inside curly brackets . Block scopes are commonly found within if statements, switch conditions and loops such as for or while.
• Block scope variables are defined using the const and let keywords.
• The variables declared inside the brackets are only visible inside the brackets.

Example:

function aboutMe (){ 

if(true) {
var myName = "Kevin" // Defined function scope 
let myAge = 21; //Defined block scope 
const favouriteColor = "blue"; //Defined block scope
}
console.log(myName);
console.log(myAge);  // Should be defined in the above brackets
console.log(favouriteColor); // Should be defined in the above brackets
}
aboutMe();
//Result of function
// Kevin
// error -as myAge is undefined
// error -as favouriteColor is undefined

If the console.log () for myAge and favouriteColor were defined inside their own braces, it would work.

• It’s better to use let than var as let reduces the scope, which essentially makes variables less accessible/visible.
• Overall, using let allows us to declare variables in a safer way.