let & const

In ES6 there are two new keywords available that allow us to define variables: let and const.

Before ES6, if you wanted to create a variable you would use the var keyword, and along with this keyword you would also define a name.

var author = "Ryan Christiani";

We use the equals sign to assign the value to the variable. To use the variable and the value stored inside, we simply need to write author. This would refer to the value "Ryan Christiani".

You can create a variable without the use of the var keyword, but this is discouraged as it creates an undeclared global variable. But, what do we mean when we say a global variable?

Scope

In programming languages we have this idea of scope: when you define a variable it is scoped to a specific section of code. Let’s look at some code and break it down:

var rocket = "Falcon9";
function launch() {
    console.log('Launching ' + rocket);
}
launch(); // Launching Falcon9

In the example above, we have created a variable called rocket and it has been created outside of our launch function. Because it was created outside of the function it will be accessible inside of it via a closure. When a function is defined in JavaScript it will capture the variables that were available to it and make them available inside of the function.

So when we run launch(); we see the text Launching Falcon9. If we changed the definition of the rocket variable so that is it is defined inside of a function called rocketName(), our launch() function will not work.

function rocketName() {
    var rocket = "Flacon9";
}
function launch() {
    console.log('Launching ' + rocket);
}
launch(); // Launching undefined

Variables created with the var keyword are function scoped, so unless they are created outside of a function, you will have no access to them. The code above will not work, because rocket exists only in the rocketName function.

let

In ES6 one of the new keywords is let, and its usage is just like using var.

let author = "Ryan Christiani";

When you refer to a variable created with let you do it just the same as you would with var. Referencing author would get you the value of Ryan Christiani.

The one key difference here is that a variable defined with let is a block scoped variable. Remember when we said that a var was a function scoped variable? Let’s look at the difference between block and function scope.

block scope vs function scope

When we looked at var we saw that if we defined a variable inside of a function, it created the variable in there, and it is available inside of that function but not outside of it.

function setName() {
    var name = "Ryan";
}
console.log(name); //Undefined

If we declared a variable using var inside of a block statement, or anything with {} (like a conditional for example), it will make that variable available outside of that block!

if(true) {
    var name = "Ryan";
}
console.log(name); //Ryan

And if you are familiar with other languages, say C for example, you might think this to be odd, since C has the ability to create block scope. Enter let, in ES6 let allows us to create a block scoped variable.

{
    let name = "Ryan";
}
console.log(name); //undefined
if(true) {
    let name = "Ryan";
}
console.log(name); //undefined

Variables defined with let will behave the same inside of a function as a variable defined with var would. However, now we have the benefit of block scope.

One place this works particularly well is when we are creating a variable to use in a for loop. Since var is function scoped it does not create a value that stays inside the loop.

for(var i = 0; i < 10; i++) {
    console.log(i);
}
console.log(i); //10

This can be a little confusing. If we change the variable declaration from var to let, however, it will change the behavior.

for(let i = 0; i < 10; i++) {
    console.log(i);
}
console.log(i); //ReferenceError: i is not defined

Temporal Dead Zone

There is one more gotcha that I want to point out, and this is something called the Temporal Dead Zone. In JavaScript, when the browser interprets your code it will do a pass where it looks for any declarations. However, it will not assign the value just yet. Because of this it is possible to use a variable before it has a value: it will simply print undefined.

console.log(person); //undefined
var person = "Ryan";

The browser knows there is a variable called person, it just doesn’t know what the value is. With the new let keyword, this is no longer the case.

console.log(person); //ReferenceError
let person = 'Ryan';

const

The const keyword behaves like let. It is block scope, but there is one difference that is pretty important. When you declare a variable with const it creates a read-only value, meaning you can use the value stored in it, but you can not reassign the value.

Before const, a common convention was to use uppercase names to hint that the value stored in this variable was a constant value.

var API_KEY = '195fhbgBqg268334mf124';

The obvious problem with this is that it COULD be reassigned whenever you wanted. With const this is not possible.

const api_key = '195fhbgBqg268334mf124';
api_key = 'some other key'; //TypeError: Assignment to constant variable

Trying to reassign the value throws a TypeError, which is great. If you defined a variable to be constant, you want to make sure it is just that.

Not immutable.

There is another small gotcha to watch out for: assigning a value here does not make it immutable. We are able to assign objects to const variables.

const person = {
    name: 'Ryan Christiani'
}

And since a const variable is a read-only value, you might assume that altering the object would throw an error, however this is not the case.

const person = {
    name: 'Ryan Christiani'
}
person.age = 31;
console.log(person);

This is completely valid; since the object’s keys are not protected here you are able to alter and add to them. The value stored on person is still an object. If you are looking for immutability, consider using Object.freeze. Introduced in ES5.1, this method will freeze and prevent new properties from being added.

const person = {
    name: 'Ryan Christiani'
}
Object.freeze(person);
// Try changing a field in frozen object
person.name = 'Instructor Ryan Christiani'
// Try adding something to a frozen object
person.age = 31;
// frozen object doesn't change
console.log(person);