JavaScript  Scopes Explained

JavaScript Scopes Explained

hashnode.com - Sep 24

What is a scope anyway?

Fortunately defining what scope isn't super complicated to explain. A scope just refers to how variables can be seen or accessed from within your code/program. Scopes in JavaScript have three levels:

  • Global
  • Local
  • Block

1. Global Scope

All variables that are declared outside of functions are considered to have a global scope. This means they can be accessed anywhere by all functions within your program.

2. Local Scope

If a variable is declared within a function then that variable is considered to have a local scope. That variable only exists within that function and only sub-functions have access to it.

Let's take a look at some examples:

var myName = "Sheldon";

console.log(myName);

Since myName is a global variable this should work just fine and print "Sheldon" on the console.

Let's look at another one:

function printMiddleName(){

    var middleName = "Lee";

}

console.log(middleName)

Now if you tried running this piece of code you would run into an error: "Uncaught ReferenceError: middleName is not defined". Because middleName is defined inside a function we have to ask the function to print out or return the name for us like this:

function printMiddleName(){

    var middleName = "Lee";

    return middleName;
}

console.log(printMiddleName());

And this piece of code should run just fine.

Just keep in mind if you had nested functions (A function inside a function) and you tried accessing a variable from within a sub-function you would get that same error from before.

var myName = "Sheldon"

function printMiddleName(){

    var middleName = "Lee";

    function printLastName(){

        var lastName = "Cooper";

    }

    console.log(myName,middleName,lastName)
}

printMiddleName()

This code above just wouldn't work because we're trying to access a local variable where it doesn't exist.

var myName = "Sheldon"

function printMiddleName(){

    var middleName = "Lee";

    function printLastName(){

        var lastName = "Cooper";

        return lastName
    }

    console.log(myName,middleName,printLastName())
}

printMiddleName()

This should work fine and print out "Sheldon Lee Cooper".

3. Block Scope

The block is pretty new and wasn't really a thing until ES6 (2015). Variables declared within curly brackets get the block scope. The only keywords that can have a block scope are the let and const.

{
  var name = "Sheldon";
  let middleName = "Lee";
  const lastName = "Cooper";
}

console.log(name); ✔ //this will print out "Sheldon"
console.log(middleName);❌  //will throw an error
console.log(lastName); ❌ //will also throw an error

Variables with a block scope can't be accessed outside their block.

Conclusion

I hope you liked and found this blog useful. If there is any missing or inaccurate information I would really appreciate your feedback. Thanks for reading till the end.

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