diff --git a/src/ol/expr/expression.js b/src/ol/expr/expression.js
index 8a60e5b2ad..3739ffc54b 100644
--- a/src/ol/expr/expression.js
+++ b/src/ol/expr/expression.js
@@ -1,4 +1,5 @@
 goog.provide('ol.expr');
+goog.provide('ol.expr.functions');
 
 goog.require('ol.Extent');
 goog.require('ol.Feature');
@@ -83,43 +84,55 @@ ol.expr.isLibCall = function(expr) {
  * Library of well-known functions.  These are available to expressions parsed
  * with `ol.expr.parse`.
  *
- * @type {Object}
+ * @type {Object.<string, function(...)>}
  */
-ol.expr.lib = {
+ol.expr.lib = {};
+
+
+/**
+ * Enumeration of library function names.
+ *
+ * @enum {string}
+ */
+ol.expr.functions = {
+  EXTENT: 'extent',
+  GEOMETRY_TYPE: 'geometryType'
+};
+
+
+/**
+ * Determine if a feature's extent intersects the provided extent.
+ * @param {number} minX Minimum x-coordinate value.
+ * @param {number} maxX Maximum x-coordinate value.
+ * @param {number} minY Minimum y-coordinate value.
+ * @param {number} maxY Maximum y-coordinate value.
+ * @return {boolean} The provided extent intersects the feature's extent.
+ * @this {ol.Feature}
+ */
+ol.expr.lib[ol.expr.functions.EXTENT] = function(minX, maxX, minY, maxY) {
+  var intersects = false;
+  var geometry = this.getGeometry();
+  if (geometry) {
+    intersects = ol.extent.intersects(geometry.getBounds(),
+        [minX, maxX, minY, maxY]);
+  }
+  return intersects;
+};
 
-  /**
-   * Determine if a feature's extent intersects the provided extent.
-   * @param {number} minX Minimum x-coordinate value.
-   * @param {number} maxX Maximum x-coordinate value.
-   * @param {number} minY Minimum y-coordinate value.
-   * @param {number} maxY Maximum y-coordinate value.
-   * @return {boolean} The provided extent intersects the feature's extent.
-   * @this {ol.Feature}
-   */
-  'extent': function(minX, maxX, minY, maxY) {
-    var intersects = false;
-    var geometry = this.getGeometry();
-    if (geometry) {
-      intersects = ol.extent.intersects(geometry.getBounds(),
-          [minX, maxX, minY, maxY]);
     }
-    return intersects;
-  },
-
-
-  /**
-   * Determine if a feature's default geometry is of the given type.
-   * @param {ol.geom.GeometryType} type Geometry type.
-   * @return {boolean} The feature's default geometry is of the given type.
-   * @this {ol.Feature}
-   */
-  'geometryType': function(type) {
-    var same = false;
-    var geometry = this.getGeometry();
-    if (geometry) {
-      same = geometry.getType() === type;
-    }
-    return same;
   }
 
+/**
+ * Determine if a feature's default geometry is of the given type.
+ * @param {ol.geom.GeometryType} type Geometry type.
+ * @return {boolean} The feature's default geometry is of the given type.
+ * @this {ol.Feature}
+ */
+ol.expr.lib[ol.expr.functions.GEOMETRY_TYPE] = function(type) {
+  var same = false;
+  var geometry = this.getGeometry();
+  if (geometry) {
+    same = geometry.getType() === type;
+  }
+  return same;
 };
diff --git a/src/ol/layer/vectorlayer.js b/src/ol/layer/vectorlayer.js
index 3487f187d8..264c4c9fa4 100644
--- a/src/ol/layer/vectorlayer.js
+++ b/src/ol/layer/vectorlayer.js
@@ -9,6 +9,7 @@ goog.require('ol.expr');
 goog.require('ol.expr.Literal');
 goog.require('ol.expr.Logical');
 goog.require('ol.expr.LogicalOp');
+goog.require('ol.expr.functions');
 goog.require('ol.geom.GeometryType');
 goog.require('ol.geom.SharedVertices');
 goog.require('ol.layer.Layer');
@@ -93,14 +94,14 @@ ol.layer.FeatureCache.prototype.getFeaturesObject = function(opt_expr) {
   } else {
     // check for geometryType or extent expression
     var name = ol.expr.isLibCall(opt_expr);
-    if (name === 'geometryType') {
+    if (name === ol.expr.functions.GEOMETRY_TYPE) {
       var args = /** @type {ol.expr.Call} */ (opt_expr).getArgs();
       goog.asserts.assert(args.length === 1);
       goog.asserts.assert(args[0] instanceof ol.expr.Literal);
       var type = /** @type {ol.expr.Literal } */ (args[0]).evaluate();
       goog.asserts.assertString(type);
       features = this.geometryTypeIndex_[type];
-    } else if (name === 'extent') {
+    } else if (name === ol.expr.functions.EXTENT) {
       var args = /** @type {ol.expr.Call} */ (opt_expr).getArgs();
       goog.asserts.assert(args.length === 4);
       var extent = [];
@@ -120,13 +121,13 @@ ol.layer.FeatureCache.prototype.getFeaturesObject = function(opt_expr) {
           for (var i = 0; i <= 1; ++i) {
             expr = expressions[i];
             name = ol.expr.isLibCall(expr);
-            if (name === 'geometryType') {
+            if (name === ol.expr.functions.GEOMETRY_TYPE) {
               args = /** @type {ol.expr.Call} */ (expr).getArgs();
               goog.asserts.assert(args.length === 1);
               goog.asserts.assert(args[0] instanceof ol.expr.Literal);
               type = /** @type {ol.expr.Literal } */ (args[0]).evaluate();
               goog.asserts.assertString(type);
-            } else if (name === 'extent') {
+            } else if (name === ol.expr.functions.EXTENT) {
               args = /** @type {ol.expr.Call} */ (expr).getArgs();
               goog.asserts.assert(args.length === 4);
               extent = [];