A "variable" is a place where you can store information, such as a string, or a number. A variable has a _name_ (that you choose) and a _value_. New variables in JavaScript are declared using one of three keywords: `let`, `const`, or `var`.

Variables are "declared" using the `var`, `let` or `const` keyword. In the following example three variables are declared with the names `x`, `foo` and `bar`.

Note that the chosen names in this example are meaningless (perhaps with the exception of `x`, for instance as part of a mathematical program). You should make an effort to always choose names that best describe what you intend this variable to hold.

Prior to JavaScript ES6 the `var` keyword was the only way to declare a variable. ES6 introduced two new keywords, `let` and `const` for declaring variables. They improve on how the older `var` declaration works (this involves the concept of "scope" that you will learn about in the third lecture). In HackYourFuture we encourage you to use the more modern `let` and `const` keywords over `var`, but you will often come across `var` in existing books, software libraries and examples on the Internet, so you should understand `var` too.

- [let vs const] (http://wesbos.com/let-vs-const/)

Here, we say: "declare variable x and initialize it with the integer (number) 6".

Values are the "things" that you assign to a variable. All values have a type. In our example above, the variable `x` is assigned a value of type `number`. JavaScript supports the following types:

* `undefined`

If you declare a variable without specifying its value, then, by default its value is `undefined`.

To get the type of a value assigned to a variable, use the following code:

However, in practice we will call these variables arrays.

The values `null` and `undefined` are very similar in JavaScript, but they behave a bit differently. The difference is that `null` always has type `'object'`, and `undefined` always has type `'undefined'`.

Whenever you declare a variable, but you don't set a value, the variable will become `undefined`. JavaScript will never make a variable `null` unless you explicitly assign it the value `null`.

You can use the `typeof` operator to get the type of a certain variable as you have seen in the above section 'Value types'. As you can see in the following examples it returns the type of value that you have assigned to your variable.

In JavaScript you can assign a series of characters to a variable, you then call this a string. You can use all sorts of characters (text/numbers, spaces or phrases) in strings. By using the `''` you define that something is a string. You can also use `""` to create a string. Both are fine as long as you are consistent (just make a choice on which one you prefer and stick to it).

let bar = 'I\'m 99 years old ';

Individual characters in a string can be accessed by their position (index) within the string. The index of a string always starts at 0.

String methods are named operations that you can use on string values to create new values. For example, the `toUpperCase` method creates a new string with all uppercase letters.

let baz = 'Hello World!';

baz.toUpperCase(); // -> 'HELLO WORLD'

Methods differ from properties (such as `.length`) in that you must always use them with open and close parentheses `(` and `)`.

Some methods need additional information, and you must supply it in the form of one or more _parameters_. For example:

let baz = 'Hello World!';

baz.slice(3, 8) // -> 'lo Wo'

baz.startsWith('He') // -> true

baz.indexOf('World') // -> 6

All numbers in JavaScript are considered numbers, either with or without a decimal.

let quux = 42;

Arrays are values that contain a list of things, instead of just one thing. What's inside the array, we typically call "elements". So, the array `[1, 2, 3]` has three elements. The array `[]` has no elements and is therefore empty. The number of elements in an array is called its "length".

let arr = ['john', 'jane', 'jack'];

* Identity / strict equality `===` (preferred)

* Non-identity / strict inequality `!==` (preferred)

We strongly recommend that you always use the strict form when comparing for equality (`===`) or inequality (`!==`). Use the non-strict forms only when there is a compelling reason to do so (you will be hard pressed to find such a reason).

* Division `/`

* Remainder (sometimes called modulo) `%`

8 + 9 // -> 17, adds two numbers together.

10 / 5 // -> 2, divides the left number by the right.

More about [Arithmetic operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators#.25_.28Modulus.29)

* OR `||`

x === 5 || y === 5 // -> false

Logical NOT

true === !false

false === !true

We already mentioned the `typeof` operator:

typeof 5 // -> 'number'

In addition to the simple assignment operator `=` there are also compound assignment operators such as `+=`. The following two assignments are equivalent:

|:------:|:--------:|:-------:|

|`=` | `x = y` | `x = y`|

|`+=`| `x += y` | `x = x + y`|

|`-=`| `x -= y` | `x = x - y`|

|`*=`| `x *= y` | `x = x * y`|

|`/=`| `x /= y` | `x = x / y`|

|`%=`| `x %= y` | `x = x % y`|