# Getting Started

## Input and Output

It's convention to write a program to print out "hello world" when first learning a language, so let's do exactly that. In the browser console that you opened up earlier, type in the following:

```js
console.log("hello world");
```

And hit enter. You should see "hello world" printed (without the quotation marks) in the line below, and `undefined` just below it (on Firefox). Here, `undefined` is the return value of the expression that you just evaluated. In general, assignment and output expressions return `undefined`, as do functions that have no return value.

<figure><img src="https://2807223923-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FTUqAJOgHs57S8lmqdxRV%2Fuploads%2Fgit-blob-cd70f7c2f4dee31652f3cfc97e643e6db80b951b%2Fconsole-demo1.png?alt=media" alt=""><figcaption></figcaption></figure>

This is one way to give output. The second way is to use the `alert` function. Type in the following in the console:

```js
alert("hello world");
```

And hit enter again. Now instead of some output in the console, you should see a popup window with the message "hello world". Clicking "ok" will result in the function evaluation completing, which results in the function returning `undefined` as before.

<figure><img src="https://2807223923-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FTUqAJOgHs57S8lmqdxRV%2Fuploads%2Fgit-blob-8c811883e4082a2fa76ee6dea453d08d6f1768b2%2Fconsole-demo2.png?alt=media" alt=""><figcaption></figcaption></figure>

Now let's try to take some user input. JavaScript provides the `prompt` function, which takes in a string to prompt the user with, and returns the user's input. It works like `alert`: a pop-up appears on the page, except this time the user can also type in some text into an input field.

```js
let x = prompt("Enter a number:"); // 'let' declares a variable, see below
console.log(x); // prints out the number the user typed in
```

<figure><img src="https://2807223923-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FTUqAJOgHs57S8lmqdxRV%2Fuploads%2Fgit-blob-a13c59852f99100bc25aee7837d7206c2ac5b587%2Fconsole-demo3.png?alt=media" alt=""><figcaption></figcaption></figure>

## Variable declaration

To declare a variable in JavaScript, you can either use the `let` keyword or the `var` keyword. The key difference between the two is that declaring a variable with the `let` keyword will only allow it to be used within the code block it was declared in, whereas a variable declared with the `var` keyword will allow it to be used within its parent function block and overrides any previous declaration. This is explained in detail [here](https://sentry.io/answers/difference-between-let-and-var-in-javascript/).

To get started, let's declare two variables, `x` and `y` and assign them values of `5` and `10`.

```js
let x = 5;
let y = 10;
```

Now we can check what their values are by printing them out as before, or by just entering the name of the variable:

```js
x; // will return 5
y; // will return 10
```

As you can guess, `//` is used to begin a comment. To write comments spanning multiple lines, start a comment with `/*` and end it with `*/`.

## Next steps

Next, we'll look at some data types and values in JavaScript.