The Difference Between Function and Block Scope in JavaScript

Back to the basics with the var, let and const variables

Photo by Tony Webster on Unsplash

The difference between var, let and const variables

var width = 100;
let height = 200;
const key = 'abc123';

var

// Define the variable:
var width = 100;
// Call the variable:
width;
// It returns:
100
// Reassign the variable and call it again:
width = 200;

width;
// Returns:
200
function setWidth(){
var width = 100;
console.log(width);
}
width;
// Returns:
Uncaught ReferenceError: width is not defined

Understanding scope

Example: var age = 100;
if (age > 12){
var dogYears = age * 7;
console.log(`You are ${dogYears} dog years old!`);
}
100
You are 700 dog years old!
dogYears;
// returns:
700
var age = 100;
if (age > 12){
let dogYears = age * 7;
console.log(`You are ${dogYears} dog years old!`);
}

Recap

var is function scope.
let and const are block scope.
Function scope is within the function.
Block scope is within curly brackets.

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Technical Support Engineer at Linode | Linux & Kubernetes

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