From 525fba6405d7a34f8eb3872ab91a11bcf6870643 Mon Sep 17 00:00:00 2001
From: Tim Schaub <tim.schaub@gmail.com>
Date: Thu, 13 Apr 2017 05:54:51 -0600
Subject: [PATCH 1/3] Assume latest if no ol.VERSION

---
 src/ol/assertionerror.js            | 5 +++--
 test/spec/ol/assertionerror.test.js | 5 ++++-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/ol/assertionerror.js b/src/ol/assertionerror.js
index 341e623a49..2f0af35e02 100644
--- a/src/ol/assertionerror.js
+++ b/src/ol/assertionerror.js
@@ -13,11 +13,12 @@ goog.require('ol');
  */
 ol.AssertionError = function(code) {
 
+  var path = ol.VERSION ? ol.VERSION.split('-')[0] : 'latest';
+
   /**
    * @type {string}
    */
-  this.message = 'Assertion failed. See ' +
-      (ol.VERSION ? 'https://openlayers.org/en/' + ol.VERSION.split('-')[0] : '') +
+  this.message = 'Assertion failed. See https://openlayers.org/en/' + path +
       '/doc/errors/#' + code + ' for details.';
 
   /**
diff --git a/test/spec/ol/assertionerror.test.js b/test/spec/ol/assertionerror.test.js
index 19c18bbcd8..6ff509c588 100644
--- a/test/spec/ol/assertionerror.test.js
+++ b/test/spec/ol/assertionerror.test.js
@@ -6,8 +6,9 @@ goog.require('ol.AssertionError');
 describe('ol.AssertionError', function() {
   it('generates a message', function() {
     var error = new ol.AssertionError(42);
-    expect(error.message).to.be('Assertion failed. See /doc/errors/#42 for details.');
+    expect(error.message).to.be('Assertion failed. See https://openlayers.org/en/latest/doc/errors/#42 for details.');
   });
+
   it('generates a message with a versioned url', function() {
     var origVersion = ol.VERSION;
     ol.VERSION = 'foo';
@@ -15,10 +16,12 @@ describe('ol.AssertionError', function() {
     expect(error.message).to.be('Assertion failed. See https://openlayers.org/en/foo/doc/errors/#42 for details.');
     ol.VERSION = origVersion;
   });
+
   it('has an error code', function() {
     var error = new ol.AssertionError(42);
     expect(error.code).to.be(42);
   });
+
   it('has a name', function() {
     var error = new ol.AssertionError(42);
     expect(error.name).to.be('AssertionError');

From 4a9114cd4e17604a7902c05c7147b600902eb29f Mon Sep 17 00:00:00 2001
From: Tim Schaub <tim.schaub@gmail.com>
Date: Thu, 13 Apr 2017 06:26:02 -0600
Subject: [PATCH 2/3] Set ol.VERSION when creating ol package

---
 transforms/module.js | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/transforms/module.js b/transforms/module.js
index 6597dae7ea..e3c276ad41 100644
--- a/transforms/module.js
+++ b/transforms/module.js
@@ -1,4 +1,5 @@
 const pkg = require('../package.json');
+const version = require('../package/package.json').version;
 
 const defines = {
   'ol.ENABLE_WEBGL': false
@@ -118,6 +119,12 @@ module.exports = function(info, api) {
   // store any initial comments
   const {comments} = root.find(j.Program).get('body', 0).node;
 
+  // replace `ol.VERSION = ''` with correct version
+  root.find(j.ExpressionStatement, getMemberExpressionAssignment('ol.VERSION'))
+    .forEach(path => {
+      path.value.expression.right = j.literal(version);
+    });
+
   const replacements = {};
 
   // replace all uses of defines

From 6604172c1c00be481d072bfb2030494f4d91bf73 Mon Sep 17 00:00:00 2001
From: Tim Schaub <tim.schaub@gmail.com>
Date: Thu, 13 Apr 2017 06:37:42 -0600
Subject: [PATCH 3/3] Throw a byte-saving AssertionError

---
 doc/errors/index.md  | 4 ++++
 src/ol/collection.js | 3 ++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/doc/errors/index.md b/doc/errors/index.md
index 5da35ba9ff..e6b14708a8 100644
--- a/doc/errors/index.md
+++ b/doc/errors/index.md
@@ -220,3 +220,7 @@ The `{-y}` placeholder requires a tile grid with extent.
 ### 57
 
 At least 2 conditions are required.
+
+### 58
+
+Duplicate item added to a unique collection.  For example, it may be that you tried to add the same layer to a map twice.  Check for calls to `map.addLayer()` or other places where the map's layer collection is modified.
diff --git a/src/ol/collection.js b/src/ol/collection.js
index c21bc2f867..d168cb7d16 100644
--- a/src/ol/collection.js
+++ b/src/ol/collection.js
@@ -6,6 +6,7 @@
 goog.provide('ol.Collection');
 
 goog.require('ol');
+goog.require('ol.AssertionError');
 goog.require('ol.CollectionEventType');
 goog.require('ol.Object');
 goog.require('ol.events.Event');
@@ -256,7 +257,7 @@ ol.Collection.prototype.updateLength_ = function() {
 ol.Collection.prototype.assertUnique_ = function(elem, opt_except) {
   for (var i = 0, ii = this.array_.length; i < ii; ++i) {
     if (this.array_[i] === elem && i !== opt_except) {
-      throw new Error('Duplicate item added to a unique collection');
+      throw new ol.AssertionError(58);
     }
   }
 };