From 7b5a1552e20adc0ba9eb749cbf7cb5c0ab69fa54 Mon Sep 17 00:00:00 2001
From: Anand Thakker <anandthakker@users.noreply.github.com>
Date: Fri, 2 Mar 2018 13:26:24 -0500
Subject: [PATCH] Fix off-by-one error in array expression error message

h/t @drewbo
---
 src/style-spec/expression/definitions/at.js          | 8 ++++++--
 test/integration/expression-tests/at/basic/test.json | 4 ++--
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/src/style-spec/expression/definitions/at.js b/src/style-spec/expression/definitions/at.js
index 306eaee83bf..c47d8102c48 100644
--- a/src/style-spec/expression/definitions/at.js
+++ b/src/style-spec/expression/definitions/at.js
@@ -42,8 +42,12 @@ class At implements Expression {
         const index = ((this.index.evaluate(ctx): any): number);
         const array = ((this.input.evaluate(ctx): any): Array<Value>);
 
-        if (index < 0 || index >= array.length) {
-            throw new RuntimeError(`Array index out of bounds: ${index} > ${array.length}.`);
+        if (index < 0) {
+            throw new RuntimeError(`Array index out of bounds: ${index} < 0.`);
+        }
+
+        if (index >= array.length) {
+            throw new RuntimeError(`Array index out of bounds: ${index} > ${array.length - 1}.`);
         }
 
         if (index !== Math.floor(index)) {
diff --git a/test/integration/expression-tests/at/basic/test.json b/test/integration/expression-tests/at/basic/test.json
index 800c8a6ca19..47ba77bccfc 100644
--- a/test/integration/expression-tests/at/basic/test.json
+++ b/test/integration/expression-tests/at/basic/test.json
@@ -18,8 +18,8 @@
     },
     "outputs": [
       8,
-      {"error": "Array index out of bounds: -1 > 3."},
-      {"error": "Array index out of bounds: 4 > 3."},
+      {"error": "Array index out of bounds: -1 < 0."},
+      {"error": "Array index out of bounds: 4 > 2."},
       {"error": "Array index must be an integer, but found 1.5 instead."}
     ],
     "serialized": [