Glossary (Earth Engine context)

Below are short, Earth Engine–focused definitions of common JavaScript and Earth Engine concepts you'll see in the tutorials.

arrays (list)

In JavaScript an array is a zero-indexed list: ['a', 'b', 'c']. In Earth Engine you will encounter both client-side JavaScript arrays (plain JS arrays) and server-side ee.List objects. Use plain arrays for local operations and ee.List when you need to build or manipulate data on the Earth Engine server.

Example (client-side array):

var colors = ['red', 'green', 'blue'];
print(colors[0]); // 'red' (immediate, client-side)

Example (server-side ee.List):

var eeList = ee.List(['red', 'green', 'blue']);
print(eeList.get(0)); // returns a server-side value that the Code Editor shows

function

A reusable block of code. In Earth Engine JavaScript you'll use functions for local computation and as callbacks passed to server-side methods such as ImageCollection.map() or FeatureCollection.map(). Functions that are used as mapping callbacks are serialized and executed on the Earth Engine server, so they must return ee.* objects when operating on server-side inputs.

Example (mapping an ImageCollection):

var addNDBI = function(image) {
  var ndbi = image.normalizedDifference(['B5', 'B4']).rename('NDBI');
  return image.addBands(ndbi);
};

var withNDBI = collection.map(addNDBI);

javascript

The programming language used in the Earth Engine Code Editor. Most standard JavaScript syntax applies (variables, functions, arrays, objects), but Earth Engine adds the ee namespace (e.g., ee.Image, ee.List) for server-side data structures. The Code Editor environment differs from a browser: there is no DOM, and many operations build a computation graph that runs on Google's servers rather than executing immediately in your local JS runtime.

object (dictionary)

In plain JavaScript an object (often called a dictionary) is a set of key-value pairs enclosed in {} (for example: {a: 1, b: 2}). Earth Engine provides a server-side ee.Dictionary for similar key-value storage on the server. Use ee.Dictionary.get() and related methods to access values server-side.

Example (client-side object):

var props = {name: 'lake', area_km2: 12.5};
print(props.name);

Example (server-side ee.Dictionary):

var d = ee.Dictionary({name: 'lake', area_km2: 12.5});
print(d.get('area_km2'));

variable

A named reference to a value. In Earth Engine scripts variables often hold references to ee objects such as ee.Image, ee.ImageCollection, or ee.FeatureCollection. Remember that many ee objects are immutable: most operations return new objects rather than modifying the original.

Examples:

var img = ee.Image('LANDSAT/LC08/C01/T1_SR/LC08_044034_20140318');
var clipped = img.clip(geometry); // returns a new ee.Image

Note on client vs server values: a variable can reference either a local JavaScript value (number, string, JS array) or a server-side ee.* object. To view server-side contents in the Code Editor use print() or export operations; attempting to treat ee.* objects like plain JS values can lead to errors.

Glossary