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+# Externs
+
+This directory contains externs files, which tell the Closure compiler about symbols and properties that it should not rename.
+
+## oli.js and olx.js
+
+These two files are special externs that belong to ol3, and this document explains their purpose and how they are used.
+
+### Prevent class properties from being renamed
+
+For events, we make properties available to the application. Other than methods, which can be made available by just marking them with the `@api` annotation, properties are exported using `oli.js`:
+```js
+/** @interface */
+oli.MapBrowserEvent;
+
+/** @type {ol.Coordinate} */
+oli.MapBrowserEvent.prototype.coordinate;
+```
+In the source file (`src/ol/MapBrowserEvent.js`), the class needs to implement this interface:
+```js
+/**
+ * ...
+ * @constructor
+ * @implements {oli.MapBrowserEvent}
+ */
+ol.MapBrowserEvent = function(type, map, browserEvent, opt_frameState) {
+
+  // ...
+
+  /**
+   * @type {ol.Coordinate}
+   * @todo stability experimental
+   */
+  this.coordinate = map.getEventCoordinate(this.originalEvent);
+
+  // ...
+
+};
+
+### Override methods in custom classes
+
+For custom subclasses in applications, which can be created using `ol.extends`, the API may want to make certain methods available to override. In addition to marking such methods as `@api`, they need also be added to an interface in `oli.js`:
+```js
+/**
+ * @interface
+ */
+oli.control.Control;
+
+/**
+ * @param {ol.Map} map Map.
+ * @return {undefined} Undefined.
+ */
+oli.control.Control.prototype.setMap = function(map) {};
+
+```
+This interface must be implemented by the class in the source file (`src/ol/control/control.js`):
+```js
+/**
+ * ...
+ * @constructor
+ * @implements {oli.control.Control}
+ */
+ol.control.Control = function(options) {
+  // ...
+};
+
+// ...
+
+/**
+ * Application subclasses may override this.
+ * @param {ol.Map} map Map.
+ * @todo stability experimental
+ */
+ol.control.Control.prototype.setMap = function(map) {
+  // ...
+};
+```
+
+### Export object literals
+
+Object literals cannot be exported like classes. To make sure that their properties do not get renamed, they go in `olx.js`:
+```js
+/**
+ * @typedef {{element: (Element|undefined),
+ *     target: (Element|string|undefined)}}
+ * @todo stability experimental
+ */
+olx.control.ControlOptions;
+
+/**
+ * The element is the control's container element. This only needs to be
+ * specified if you're developing a custom control.
+ * @type {Element|undefined}
+ */
+olx.control.ControlOptions.prototype.element;
+
+/**
+ * Specify a target if you want the control to be rendered outside of the map's
+ * viewport.
+ * @type {Element|string|undefined}
+ */
+olx.control.ControlOptions.prototype.target;
+```
+In the source code, the name used for the typedef is used as type whenever this object literal is expected:
+```js
+ * ...
+ * @param {olx.control.ControlOptions} options Control options.
+ */
+ol.control.Control = function(options) {
+  // ...
+};
+```