diff --git a/lib/transmuxer/h264.js b/lib/transmuxer/h264.js
index ee772dd681..88f3e919e4 100644
--- a/lib/transmuxer/h264.js
+++ b/lib/transmuxer/h264.js
@@ -175,7 +175,7 @@ shaka.transmuxer.H264 = class {
   static parseFrame(nalus) {
     const H264 = shaka.transmuxer.H264;
     let isKeyframe = false;
-    let data = new Uint8Array([]);
+    const nalusData = [];
     const spsNalu = nalus.find((nalu) => {
       return nalu.type == H264.NALU_TYPE_SPS_;
     });
@@ -240,13 +240,14 @@ shaka.transmuxer.H264 = class {
         naluLength[1] = (size >> 16) & 0xff;
         naluLength[2] = (size >> 8) & 0xff;
         naluLength[3] = size & 0xff;
-        data = shaka.util.Uint8ArrayUtils.concat(
-            data, naluLength, nalu.fullData);
+        nalusData.push(shaka.util.Uint8ArrayUtils.concat(
+            naluLength, nalu.fullData));
       }
     }
-    if (!data.byteLength) {
+    if (!nalusData.length) {
       return null;
     }
+    const data = shaka.util.Uint8ArrayUtils.concat(...nalusData);
     return {
       data,
       isKeyframe,
diff --git a/lib/transmuxer/h265.js b/lib/transmuxer/h265.js
index 07335e1547..601a9cc907 100644
--- a/lib/transmuxer/h265.js
+++ b/lib/transmuxer/h265.js
@@ -590,7 +590,7 @@ shaka.transmuxer.H265 = class {
   static parseFrame(nalus) {
     const H265 = shaka.transmuxer.H265;
     let isKeyframe = false;
-    let data = new Uint8Array([]);
+    const nalusData = [];
     let hvcSample = false;
     for (const nalu of nalus) {
       let push = false;
@@ -636,13 +636,14 @@ shaka.transmuxer.H265 = class {
         naluLength[1] = (size >> 16) & 0xff;
         naluLength[2] = (size >> 8) & 0xff;
         naluLength[3] = size & 0xff;
-        data = shaka.util.Uint8ArrayUtils.concat(
-            data, naluLength, nalu.fullData);
+        nalusData.push(shaka.util.Uint8ArrayUtils.concat(
+            naluLength, nalu.fullData));
       }
     }
-    if (!data.byteLength) {
+    if (!nalusData.length) {
       return null;
     }
+    const data = shaka.util.Uint8ArrayUtils.concat(...nalusData);
     return {
       data,
       isKeyframe,
diff --git a/lib/util/mp4_generator.js b/lib/util/mp4_generator.js
index cb44a7db26..af904c1178 100644
--- a/lib/util/mp4_generator.js
+++ b/lib/util/mp4_generator.js
@@ -1037,10 +1037,8 @@ shaka.util.Mp4Generator = class {
   mdat_(streamInfo) {
     const Mp4Generator = shaka.util.Mp4Generator;
     const samples = streamInfo.data ? streamInfo.data.samples : [];
-    let bytes = new Uint8Array([]);
-    for (const sample of samples) {
-      bytes = shaka.util.Uint8ArrayUtils.concat(bytes, sample.data);
-    }
+    const allData = samples.map((sample) => sample.data);
+    const bytes = shaka.util.Uint8ArrayUtils.concat(...allData);
     return Mp4Generator.box('mdat', bytes);
   }