From b757de13b5b4e044d20991131e3ceaba1bbe51e7 Mon Sep 17 00:00:00 2001
From: Anand Thakker <github@anandthakker.net>
Date: Wed, 18 Apr 2018 12:28:44 -0400
Subject: [PATCH 1/2] Change ["to-string", null] from "null" to ""

Closes #6533
---
 src/style-spec/expression/definitions/index.js              | 4 +++-
 test/integration/expression-tests/to-string/basic/test.json | 2 +-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/style-spec/expression/definitions/index.js b/src/style-spec/expression/definitions/index.js
index 4bb480b278b..9dc48cecb94 100644
--- a/src/style-spec/expression/definitions/index.js
+++ b/src/style-spec/expression/definitions/index.js
@@ -112,7 +112,9 @@ CompoundExpression.register(expressions, {
         (ctx, [v]) => {
             v = v.evaluate(ctx);
             const type = typeof v;
-            if (v === null || type === 'string' || type === 'number' || type === 'boolean') {
+            if (v === null) {
+                return '';
+            } else if (type === 'string' || type === 'number' || type === 'boolean') {
                 return String(v);
             } else if (v instanceof Color) {
                 return v.toString();
diff --git a/test/integration/expression-tests/to-string/basic/test.json b/test/integration/expression-tests/to-string/basic/test.json
index c5252ea7856..7a17dc320b3 100644
--- a/test/integration/expression-tests/to-string/basic/test.json
+++ b/test/integration/expression-tests/to-string/basic/test.json
@@ -15,7 +15,7 @@
       "isZoomConstant": true,
       "type": "string"
     },
-    "outputs": ["1", "false", "null", "string", "[1,2]", "{\"y\":1}"],
+    "outputs": ["1", "false", "", "string", "[1,2]", "{\"y\":1}"],
     "serialized": ["to-string", ["get", "x"]]
   }
 }

From 4ff3629789262850e62b7dd5c797dc69844264ca Mon Sep 17 00:00:00 2001
From: Anand Thakker <github@anandthakker.net>
Date: Wed, 18 Apr 2018 12:45:40 -0400
Subject: [PATCH 2/2] Update changelog and style-spec docs

---
 CHANGELOG.md                     | 1 +
 src/style-spec/reference/v8.json | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a1e5fa9ff2a..32fbf8394d1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 ### ⚠️ Breaking changes
 
 * `Evented` is no longer publicly exported, and `Evented#fire` and `Evented#listens` are now private. If you are writing a class that needs event emitting functionality, consider using [`EventEmitter`](https://nodejs.org/api/events.html#events_class_eventemitter) or similar libraries instead.
+* The `"to-string"` expression operator now converts `null` to an empty string rather than to `"null"`. [#6534](https://github.com/mapbox/mapbox-gl-js/pull/6534)
 
 ## 0.44.2
 
diff --git a/src/style-spec/reference/v8.json b/src/style-spec/reference/v8.json
index 7ca76bdcecc..e42ca203d1f 100644
--- a/src/style-spec/reference/v8.json
+++ b/src/style-spec/reference/v8.json
@@ -2173,7 +2173,7 @@
         }
       },
       "to-string": {
-        "doc": "Converts the input value to a string. If the input is `null`, the result is `\"null\"`. If the input is a boolean, the result is `\"true\"` or `\"false\"`. If the input is a number, it is converted to a string as specified by the [\"NumberToString\" algorithm](https://tc39.github.io/ecma262/#sec-tostring-applied-to-the-number-type) of the ECMAScript Language Specification. If the input is a color, it is converted to a string of the form `\"rgba(r,g,b,a)\"`, where `r`, `g`, and `b` are numerals ranging from 0 to 255, and `a` ranges from 0 to 1. Otherwise, the input is converted to a string in the format specified by the [`JSON.stringify`](https://tc39.github.io/ecma262/#sec-json.stringify) function of the ECMAScript Language Specification.",
+        "doc": "Converts the input value to a string. If the input is `null`, the result is `\"\"`. If the input is a boolean, the result is `\"true\"` or `\"false\"`. If the input is a number, it is converted to a string as specified by the [\"NumberToString\" algorithm](https://tc39.github.io/ecma262/#sec-tostring-applied-to-the-number-type) of the ECMAScript Language Specification. If the input is a color, it is converted to a string of the form `\"rgba(r,g,b,a)\"`, where `r`, `g`, and `b` are numerals ranging from 0 to 255, and `a` ranges from 0 to 1. Otherwise, the input is converted to a string in the format specified by the [`JSON.stringify`](https://tc39.github.io/ecma262/#sec-json.stringify) function of the ECMAScript Language Specification.",
         "group": "Types",
         "sdk-support": {
           "basic functionality": {