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:

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.

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

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.

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.

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

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.

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

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:

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.