diff --git a/js/src/api/util/format.js b/js/src/api/util/format.js
index 6a4a81d40..e981fb06d 100644
--- a/js/src/api/util/format.js
+++ b/js/src/api/util/format.js
@@ -75,10 +75,13 @@ export function bytesToAscii (bytes) {
 }
 
 export function asciiToHex (string) {
-  return '0x' + string.split('')
-    .map(s => s.charCodeAt(0))
-    .map(s => s < 0x10 ? '0' + s.toString(16) : s.toString(16))
-    .join('');
+  let result = '0x';
+
+  for (let i = 0; i < string.length; ++i) {
+    result += ('0' + string.charCodeAt(i).toString(16)).substr(-2);
+  }
+
+  return result;
 }
 
 export function padRight (input, length) {
diff --git a/js/src/api/util/format.spec.js b/js/src/api/util/format.spec.js
index ba7a3994e..4058f5364 100644
--- a/js/src/api/util/format.spec.js
+++ b/js/src/api/util/format.spec.js
@@ -69,6 +69,14 @@ describe('api/util/format', () => {
       expect(asciiToHex('abc')).to.equal('0x616263');
       expect(asciiToHex('a\nb')).to.equal('0x610a62');
     });
+
+    it('correctly converts where charCode < 0x10', () => {
+      expect(
+        asciiToHex(
+          [32, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0].map((v) => String.fromCharCode(v)).join('')
+        )
+      ).to.equal('0x20100f0e0d0c0b0a09080706050403020100');
+    });
   });
 
   describe('hexToAscii', () => {