diff --git a/src/ol/extent.js b/src/ol/extent.js
index 0edfa2a8ca..5595382e7c 100644
--- a/src/ol/extent.js
+++ b/src/ol/extent.js
@@ -109,9 +109,9 @@ ol.Extent.prototype.getTopRight = function() {
  */
 ol.Extent.prototype.transform = function(transformFn) {
   var input = [this.minX, this.minY, this.maxX, this.maxY];
-  var output = transformFn(input, 2);
-  return new ol.Extent(Math.min(output[0], output[2]),
-      Math.min(output[1], output[3]),
-      Math.max(output[0], output[2]),
-      Math.max(output[1], output[3]));
+  input = transformFn(input, input, 2);
+  return new ol.Extent(Math.min(input[0], input[2]),
+      Math.min(input[1], input[3]),
+      Math.max(input[0], input[2]),
+      Math.max(input[1], input[3]));
 };
diff --git a/src/ol/geolocation.js b/src/ol/geolocation.js
index 69052832b2..bb90723f15 100644
--- a/src/ol/geolocation.js
+++ b/src/ol/geolocation.js
@@ -233,6 +233,7 @@ goog.exportProperty(
 /**
  * @private
  * @param {Array.<number>} input Input coordinate values.
+ * @param {Array.<number>=} opt_output Output array of coordinate values.
  * @param {number=} opt_dimension Dimension (default is 2).
  * @return {Array.<number>} Output coordinate values.
  */
diff --git a/src/ol/projection.js b/src/ol/projection.js
index 80e8c1dbe2..86ac70b6a5 100644
--- a/src/ol/projection.js
+++ b/src/ol/projection.js
@@ -398,20 +398,24 @@ ol.projection.getTransform = function(source, destination) {
     transform =
         /**
          * @param {Array.<number>} input Input coordinate values.
+         * @param {Array.<number>=} opt_output Output array of coordinates.
          * @param {number=} opt_dimension Dimension.
          * @return {Array.<number>} Output coordinate values.
          */
-        function(input, opt_dimension) {
+        function(input, opt_output, opt_dimension) {
       var length = input.length,
-          dimension = goog.isDef(opt_dimension) ? opt_dimension : 2,
-          output, proj4jsPoint;
-      if (dimension > 2) {
-        // preserve values beyond second dimension
-        output = input.slice();
-      } else {
-        output = new Array(length);
+          dimension = opt_dimension > 1 ? opt_dimension : 2,
+          output = opt_output;
+      if (!goog.isDef(output)) {
+        if (dimension > 2) {
+          // preserve values beyond second dimension
+          output = input.slice();
+        } else {
+          output = new Array(length);
+        }
       }
       goog.asserts.assert(output.length % dimension === 0);
+      var proj4jsPoint;
       for (var i = 0; i < length; i += dimension) {
         proj4jsPoint = new Proj4js.Point(input[i], input[i + 1]);
         proj4jsPoint = Proj4js.transform(
@@ -449,21 +453,23 @@ ol.projection.getTransformFromCodes = function(sourceCode, destinationCode) {
 
 /**
  * @param {Array.<number>} input Input coordinate array.
+ * @param {Array.<number>=} opt_output Output array of coordinate values.
  * @param {number=} opt_dimension Dimension.
  * @return {Array.<number>} Input coordinate array (same array as input).
  */
-ol.projection.identityTransform = function(input, opt_dimension) {
+ol.projection.identityTransform = function(input, opt_output, opt_dimension) {
   return input;
 };
 
 
 /**
  * @param {Array.<number>} input Input coordinate array.
+ * @param {Array.<number>=} opt_output Output array of coordinate values.
  * @param {number=} opt_dimension Dimension.
  * @return {Array.<number>} Output coordinate array (new array, same coordinate
  *     values).
  */
-ol.projection.cloneTransform = function(input, opt_dimension) {
+ol.projection.cloneTransform = function(input, opt_output, opt_dimension) {
   return input.slice();
 };
 
diff --git a/src/ol/projection/epsg3857.js b/src/ol/projection/epsg3857.js
index 2050aca7d2..0d183c3e66 100644
--- a/src/ol/projection/epsg3857.js
+++ b/src/ol/projection/epsg3857.js
@@ -73,18 +73,22 @@ ol.projection.EPSG3857.PROJECTIONS = goog.array.map(
  * Transformation from EPSG:4326 to EPSG:3857.
  *
  * @param {Array.<number>} input Input array of coordinate values.
+ * @param {Array.<number>=} opt_output Output array of coordinate values.
  * @param {number=} opt_dimension Dimension (default is 2).
  * @return {Array.<number>} Output array of coordinate values.
  */
-ol.projection.EPSG3857.fromEPSG4326 = function(input, opt_dimension) {
+ol.projection.EPSG3857.fromEPSG4326 = function(
+    input, opt_output, opt_dimension) {
   var length = input.length,
-      dimension = goog.isDef(opt_dimension) ? opt_dimension : 2,
-      output;
-  if (dimension > 2) {
-    // preserve values beyond second dimension
-    output = input.slice();
-  } else {
-    output = new Array(length);
+      dimension = opt_dimension > 1 ? opt_dimension : 2,
+      output = opt_output;
+  if (!goog.isDef(output)) {
+    if (dimension > 2) {
+      // preserve values beyond second dimension
+      output = input.slice();
+    } else {
+      output = new Array(length);
+    }
   }
   goog.asserts.assert(output.length % dimension === 0);
   for (var i = 0; i < length; i += dimension) {
@@ -100,18 +104,21 @@ ol.projection.EPSG3857.fromEPSG4326 = function(input, opt_dimension) {
  * Transformation from EPSG:3857 to EPSG:4326.
  *
  * @param {Array.<number>} input Input array of coordinate values.
+ * @param {Array.<number>=} opt_output Output array of coordinate values.
  * @param {number=} opt_dimension Dimension (default is 2).
  * @return {Array.<number>} Output array of coordinate values.
  */
-ol.projection.EPSG3857.toEPSG4326 = function(input, opt_dimension) {
+ol.projection.EPSG3857.toEPSG4326 = function(input, opt_output, opt_dimension) {
   var length = input.length,
-      dimension = goog.isDef(opt_dimension) ? opt_dimension : 2,
-      output;
-  if (dimension > 2) {
-    // preserve values beyond second dimension
-    output = input.slice();
-  } else {
-    output = new Array(length);
+      dimension = opt_dimension > 1 ? opt_dimension : 2,
+      output = opt_output;
+  if (!goog.isDef(output)) {
+    if (dimension > 2) {
+      // preserve values beyond second dimension
+      output = input.slice();
+    } else {
+      output = new Array(length);
+    }
   }
   goog.asserts.assert(output.length % dimension === 0);
   for (var i = 0; i < length; i += dimension) {
diff --git a/src/ol/transformfunction.js b/src/ol/transformfunction.js
index 65b5a6c585..b5fb315d69 100644
--- a/src/ol/transformfunction.js
+++ b/src/ol/transformfunction.js
@@ -2,10 +2,11 @@ goog.provide('ol.TransformFunction');
 
 
 /**
- * A transform function accepts an array of input coordinate values and an
- * optional dimension (default should be 2).  The function transforms the
- * coordinate values and returns an array of the same length as the input.
+ * A transform function accepts an array of input coordinate values, an optional
+ * output array, and an optional dimension (default should be 2).  The function
+ * transforms the input coordinate values, populates the output array, and
+ * returns the output array.
  *
- * @typedef {function(Array.<number>, number=): Array.<number>}
+ * @typedef {function(Array.<number>, Array.<number>=, number=): Array.<number>}
  */
 ol.TransformFunction;
diff --git a/test/spec/ol/projection.test.js b/test/spec/ol/projection.test.js
index a840a6f0f4..5a217e0f40 100644
--- a/test/spec/ol/projection.test.js
+++ b/test/spec/ol/projection.test.js
@@ -215,6 +215,21 @@ describe('ol.projection', function() {
       expect(output[5]).toRoughlyEqual(52.4827802220782, 1e-9);
     });
 
+    it('accepts an optional destination array', function() {
+      var transform = ol.projection.getTransformFromCodes(
+          'EPSG:3857', 'EPSG:4326');
+      var input = [-12000000, 5000000];
+      var output = [];
+
+      var got = transform(input, output);
+      expect(got).toBe(output);
+
+      expect(output[0]).toRoughlyEqual(-107.79783409434258, 1e-9);
+      expect(output[1]).toRoughlyEqual(40.91627447067577, 1e-9);
+
+      expect(input).toEqual([-12000000, 5000000]);
+    });
+
     it('accepts a dimension', function() {
       var transform = ol.projection.getTransformFromCodes(
           'GOOGLE', 'EPSG:4326');
@@ -225,7 +240,7 @@ describe('ol.projection', function() {
         -626172.13571216376, 6887893.4928337997, 100,
         -12000000, 5000000, 200,
         -626172.13571216376, 6887893.4928337997, 300
-      ], dimension);
+      ], undefined, dimension);
 
       expect(output[0]).toRoughlyEqual(-5.625, 1e-9);
       expect(output[1]).toRoughlyEqual(52.4827802220782, 1e-9);
@@ -247,7 +262,7 @@ describe('ol.projection', function() {
     it('removes functions cached by addTransform', function() {
       var foo = new ol.Projection('foo', units, extent);
       var bar = new ol.Projection('bar', units, extent);
-      var transform = function(intput, dimension) {return input};
+      var transform = function(input, output, dimension) {return input};
       ol.projection.addTransform(foo, bar, transform);
       expect(ol.projection.transforms_).not.toBeUndefined();
       expect(ol.projection.transforms_.foo).not.toBeUndefined();