This tutorial covers just enough
[JavaScript](https://developer.mozilla.org/en-US/docs/Web/JavaScript/About_JavaScript)
to get you started writing Earth Engine scripts. For more thorough JavaScript tutorials,
see [these Mozilla developer
resources](https://developer.mozilla.org/en-US/docs/Web/JavaScript). For an introduction to programming, with examples in JavaScript, see
[Eloquent JavaScript](http://eloquentjavascript.net/). For suggestions
on JavaScript coding style, see the
[Google JavaScript Style
Guide](http://google.github.io/styleguide/javascriptguide.xml). In this tutorial, you're going to write JavaScript in the Earth Engine
[Code Editor](https://code.earthengine.google.com/). Before getting started,
use [the Code Editor guide](https://developers.google.com/earth-engine/guides/playground) to get familiar
with the Code Editor environment.

## Hello World!

Time to write your first JavaScript for Earth Engine! In your Chrome browser, go to
[code.earthengine.google.com](https://code.earthengine.google.com/) and copy
the following into the [Code Editor](https://developers.google.com/earth-engine/guides/playground):

### Code Editor (JavaScript)

```javascript
print('Hello World!');
```

Click **Run** and observe that 'Hello world!' is printed to the
[Console tab](https://developers.google.com/earth-engine/guides/playground#console-tab). The line above is a JavaScript
statement. In JavaScript, statements end in a semicolon. Earth Engine programs are made
up of a set of statements like this one. You can prevent code from running without
deleting it by commenting it. One of the ways to comment out code is by putting two
forward slashes `//` before the code that you don't want to run. For example:

### Code Editor (JavaScript)

```javascript
// print('Hello World!');
```

It's good practice to put lots of comments in your code, to describe what you're trying
to do. It's also good to delete commented code that doesn't do anything anymore.
Both these practices will improve code readability.

## Basic JavaScript data types

### Strings

Using variables to store objects and primitives helps code readability. For example,
a variable that stores a string object is defined by single `'` or double
`"` quotes (but don't mix them), with
[single quotes
preferred](https://google.github.io/styleguide/javascriptguide.xml#Strings). Make a new string and store it in a variable called
`greetString`:

### Code Editor (JavaScript)

```javascript
// Use single (or double) quotes to make a string.
var greetString = 'Ahoy there!';
// Use parentheses to pass arguments to functions.
print(greetString);
```

### Numbers

Note that variables are defined with the keyword `var`. Variables can also
store numbers:

### Code Editor (JavaScript)

```javascript
// Store a number in a variable.
var number = 42;
print('The answer is:', number);
```

In this example, observe that when `print()` is given two arguments separated
by commas, each argument is printed on a different line.

### Lists

Define lists with square brackets `[]`. A list of numbers, for example:

### Code Editor (JavaScript)

```javascript
// Use square brackets [] to make a list.
var listOfNumbers = [0, 1, 1, 2, 3, 5];
print('List of numbers:', listOfNumbers);
```

Lists can also store strings or other objects. For example:

### Code Editor (JavaScript)

```javascript
// Make a list of strings.
var listOfStrings = ['a', 'b', 'c', 'd'];
print('List of strings:', listOfStrings);
```

### Objects

Objects in JavaScript are dictionaries of `key: value` pairs. Make an object
(or dictionary) using curly brackets `{}`, for example:

### Code Editor (JavaScript)

```javascript
// Use curly brackets {} to make a dictionary of key:value pairs.
var object = {
  foo: 'bar',
  baz: 13,
  stuff: ['this', 'that', 'the other thing']
};
print('Dictionary:', object);
// Access dictionary items using square brackets.
print('Print foo:', object['foo']);
// Access dictionary items using dot notation.
print('Print stuff:', object.stuff);
```

Note that you can get a value from a dictionary by supplying the key. This example
shows you how to do that for JavaScript objects. Later you'll learn how to do it for
dictionaries that are on the Earth Engine server.

## Functions

Functions are another way to improve code readability and reusability by grouping sets
of operations. Define a function with the `function` keyword. Function names
start with a letter and have a pair of parentheses at the end. Functions often take
*parameters* which tell the function what to do. These parameters go inside the
parentheses `()`. The set of statements making up the function go inside curly
brackets. The `return` keyword indicates what the function output is. There
are several ways to declare a function, but here we'll use something like this:

### Code Editor (JavaScript)

```javascript
var myFunction = function(parameter1, parameter2, parameter3) {
  statement;
  statement;
  statement;
  return statement;
};
```

Let's consider the lines one by one. The first line creates a new function and assigns
it to the variable `myFunction`. This variable could have been named
anything. It defines how to call the function later. The terms in the parentheses after
the function name (i.e., parameter1, parameter2, parameter3) are the parameter names and
could have been named anything as well, though it's good practice to give them unique names
that are different from the code outside the function. Whatever you name them, these are
the names that function will use to refer to the values that are passed into the function
when it is called. The value of a parameter once it's been passed into a function is
called an *argument* . Although functions can use variables declared outside
the function (*global* variables), function arguments are not visible outside the
function. Functions can take as many parameters as you need, even zero. Here's a simple
example of a function that just returns its argument:

### Code Editor (JavaScript)

```javascript
// The reflect function takes a single parameter: element.
var reflect = function(element) {
  // Return the argument.
  return element;
};
print('A good day to you!', reflect('Back at you!'));
```

This is an example of a user-defined function. There are also lots of built-in Earth
Engine functions. Explore the Code Editor [Docs
tab](https://developers.google.com/earth-engine/guides/playground#api-reference-docs-tab) to learn about these built-in functions. Here's a very simple example of an
Earth Engine function:

### Code Editor (JavaScript)

```javascript
var aString = ee.Algorithms.String(42);
```

In the next section, learn more about [Earth Engine Objects
and Methods](https://developers.google.com/earth-engine/tutorials/tutorial_js_02).