chapter-23.md

The old var keyword

Before the release of ES6, JavaScript declares a variable by using the var keyword instead of let and const:

var myVariable = 'hereisastring';

So what does ES6 want to fix by releasing let and const keyword to replace var? It's because the var keyword declares a variable on a global level instead of a block level. Let me explain through an example:

if(true) {
  var name = 'Hedwix';
}
console.log(name);

The code above will run just fine, but it really should not, because the variable name is declared inside the if block. It works because when you declare a variable with var, JavaScript will create the variable on a global scope, while it should be on the block scope.

But why bother with the scope of a variable? That's because when you have hundreds and hundreds code lines, it can become very frustrating to trace where the variable's value is being modified. There's a principle in software development called "principles of least exposure", which means you don't really expose any part of the software that is unnecessary. Block scoping a variable means ensures that the variable is exposed and accessible only from parts of your code lines that needs the variable to run properly.

This is the problem that JavaScript tries to solve with let and const. A variable declared with let keyword is almost identical in every way with var, except for the scope level. A let variable can't be accessed from outside of the block it was declared:

if(true) {
  let name = 'Hedwix';
}
console.log(name); // throws error

The const keyword is short for constant, and variables declared with it can never be reassigned. Although if you assign an object or array with const, you can still assign new values its properties:

const name = 'Hedwix';
name = 'Nathan'; // TypeError: Assignment to constant variable

const myObject = {};
myObject.name = 'Nathan';
console.log(myObject.name);

const myArray = [];
myArray.push('Nathan');
myArray.push('Sebhastian');
console.log(myArray);

Since the release of ES6, JavaScript developers tend to avoid using var keyword from their codebase. The general convention is to declare a variable with const first. When you code your application and realize halfway that you need to reassign your constant value, then change const declaration into let.