Restore mobile PDF preview compatibility for browsers without `Array.prototype.at` by Copilot · Pull Request #729 · kekingcn/kkFileView

Copilot · 2026-04-09T02:39:53Z

Mobile PDF preview could fail during PDF.js initialization with pattern.at is not a function, leaving the viewer blank on older mobile browsers. The failure also broke worker startup, cascading into fake-worker initialization errors.

Problem scope
- PDF.js 5.x assets in kkFileView rely on .at() in both the viewer/runtime path and the worker path.
- Older mobile WebViews/Safari variants do not implement .at(), so PDF rendering fails before the document can load.
Compatibility shim
- Added server/src/main/resources/static/pdfjs/web/compatibility.mjs
- Polyfills .at() for:
  - Array
  - String
  - typed arrays
- Keeps the shim narrowly scoped to the missing API instead of changing viewer behavior more broadly.
Viewer/worker wiring
- Load the compatibility module before pdf.mjs in server/src/main/resources/static/pdfjs/web/viewer.html
- Import the same module from server/src/main/resources/static/pdfjs/build/pdf.worker.mjs
- This covers both:
  - normal viewer boot
  - worker / fake-worker fallback paths
Regression coverage
- Added server/src/test/java/cn/keking/PdfViewerCompatibilityTests.java
- Verifies the compatibility module remains wired into both the viewer entrypoint and the worker entrypoint.

Representative change

<link rel="resource" type="application/l10n" href="locale/locale.json" />
<script src="compatibility.mjs" type="module"></script>
<script src="../build/pdf.mjs" type="module"></script>

Screenshot
- Reporter-provided reproduction screenshot: https://github.com/user-attachments/assets/23f309c7-d1d8-4ed3-bdaa-8ed906622c4f

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Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

repository.aspose.com
- Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --enable-native-access=ALL-UNNAMED -classpath /usr/share/apache-maven-3.9.14/boot/plexus-classworlds-2.9.0.jar -Dclassworlds.conf=/usr/share/apache-maven-3.9.14/bin/m2.conf -Dmaven.home=/usr/share/apache-maven-3.9.14 -Dlibrary.jansi.path=/usr/share/apache-maven-3.9.14/lib/jansi-native -Dmaven.multiModuleProjectDirectory=/home/REDACTED/work/kkFileView/kkFileView org.codehaus.plexus.classworlds.launcher.Launcher -B clean test (dns block)
- Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --enable-native-access=ALL-UNNAMED -classpath /usr/share/apache-maven-3.9.14/boot/plexus-classworlds-2.9.0.jar -Dclassworlds.conf=/usr/share/apache-maven-3.9.14/bin/m2.conf -Dmaven.home=/usr/share/apache-maven-3.9.14 -Dlibrary.jansi.path=/usr/share/apache-maven-3.9.14/lib/jansi-native -Dmaven.multiModuleProjectDirectory=/home/REDACTED/work/kkFileView/kkFileView org.codehaus.plexus.classworlds.launcher.Launcher -B clean package -Dmaven.test.skip=true (dns block)
- Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --enable-native-access=ALL-UNNAMED -classpath /usr/share/apache-maven-3.9.14/boot/plexus-classworlds-2.9.0.jar -Dclassworlds.conf=/usr/share/apache-maven-3.9.14/bin/m2.conf -Dmaven.home=/usr/share/apache-maven-3.9.14 -Dlibrary.jansi.path=/usr/share/apache-maven-3.9.14/lib/jansi-native -Dmaven.multiModuleProjectDirectory=/home/REDACTED/work/kkFileView/kkFileView org.codehaus.plexus.classworlds.launcher.Launcher -f pom.xml -B -V -e -Dfindbugs.skip -Dcheckstyle.skip -Dpmd.skip=true -Dspotbugs.skip -Denforcer.skip -Dmaven.javadoc.skip (dns block)

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Agent-Logs-Url: https://github.com/kekingcn/kkFileView/sessions/287f0212-d368-4b59-ae70-0374fb3fe76f Co-authored-by: klboke <18591662+klboke@users.noreply.github.com>

klboke · 2026-04-09T02:55:30Z

整体看下来没问题，这个修复方式比较克制，风险也相对可控。

我的 review 结论：

修复点收得比较准，只补了 .at() 兼容层，没有去大范围改 pdf.js 主逻辑。
同时覆盖了 viewer 和 worker 两边，这点是对的，能避免只修一半导致 fake worker 路径继续出问题。
新增的测试虽然偏“接线验证”，但至少能防止后续升级 pdf.js 资源时把 compatibility 引用丢掉。

目前我没有看到 blocker，可以合。

一个后续可选增强建议（不阻塞这次 PR）：

可以考虑再补一个更贴近真实运行时的兼容性 smoke，比如模拟 Array.prototype.at 缺失的环境，验证 viewer 至少能正常启动。

总体来说，这个 PR 我这边是 LGTM 的。

klboke · 2026-04-09T03:05:03Z

补充一下我这边的本地验证结果（不是只看 diff）：

我把这个 PR 拉到本地分支后，额外做了几轮验证：

mvn -q -pl server -Dtest=PdfViewerCompatibilityTests test：通过
mvn -q -pl server -DskipTests package：通过
本地启动服务后，现有 PDF 预览主链路正常：sample.pdf文件转换中 -> sample.pdf图片预览
直接访问 pdf.js viewer 页面并加载同源 PDF，控制台里没有再出现 pattern.at is not a function

另外我还补做了一个更贴近问题本身的本地模块级验证：

先临时删除 Array.prototype.at、String.prototype.at 以及 typed arrays 的 .at()
再加载这次新增的 compatibility.mjs
验证 polyfill 是否真的恢复行为

本地结果是：

Array：OK
String：OK
TypedArray：OK

所以从我这边的本地验证看，这个 PR 不只是“把 compatibility 脚本接上了”，而是确实能补上 .at() 缺失场景，同时也没有把现有 PDF 预览链路搞坏。

klboke

本地已补充验证：兼容层可实际修复 .at() 缺失场景，且未破坏现有 PDF 预览链路。Approve。

Copilot

Pull request overview

Restores PDF.js initialization on older mobile browsers/WebViews that lack Array.prototype.at, preventing blank PDF previews and subsequent worker/fake-worker startup failures.

Changes:

Added a small at() compatibility shim and ensured it loads before pdf.mjs in the viewer.
Imported the same shim from pdf.worker.mjs so both worker and fake-worker paths are covered.
Added regression tests to ensure the shim remains wired into both entrypoints.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
server/src/main/resources/static/pdfjs/web/viewer.html	Loads the compatibility shim before `pdf.mjs`.
server/src/main/resources/static/pdfjs/web/compatibility.mjs	Adds an `at()` polyfill for arrays/strings/typed arrays.
server/src/main/resources/static/pdfjs/build/pdf.worker.mjs	Imports the compatibility shim in the worker entrypoint.
server/src/test/java/cn/keking/PdfViewerCompatibilityTests.java	Verifies shim wiring in viewer and worker assets.

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Copilot · 2026-04-09T04:03:43Z

server/src/main/resources/static/pdfjs/web/compatibility.mjs

+  Float32Array,
+  Float64Array,
+  globalThis.BigInt64Array,
+  globalThis.BigUint64Array,
+];


compatibility.mjs references globalThis directly when building atTargets. In environments without native globalThis (which can overlap with browsers missing Array.prototype.at), this will throw before the polyfill installs and prevent PDF.js from booting. Use a safe global-object resolution (e.g., prefer globalThis when defined, otherwise self/window) before accessing BigInt typed array constructors.

Copilot · 2026-04-09T04:03:44Z

server/src/main/resources/static/pdfjs/web/compatibility.mjs

+      if (Number.isNaN(relativeIndex)) {
+        relativeIndex = 0;
+      }
+      relativeIndex = Math.trunc(relativeIndex);


The polyfill implementation depends on Number.isNaN and Math.trunc, but the compatibility module runs before PDF.js (and its bundled polyfills) load. On older engines these methods may be missing, causing the shim itself to crash. Consider avoiding these dependencies (e.g., NaN check via x !== x and integer conversion via Math.floor/Math.ceil) so the shim only requires the minimum baseline needed to run module scripts.

Suggested change

if (Number.isNaN(relativeIndex)) {

relativeIndex = 0;

}

relativeIndex = Math.trunc(relativeIndex);

if (relativeIndex !== relativeIndex) {

relativeIndex = 0;

}

relativeIndex =

relativeIndex < 0 ? Math.ceil(relativeIndex) : Math.floor(relativeIndex);

Copilot · 2026-04-09T04:03:44Z

server/src/main/resources/static/pdfjs/web/compatibility.mjs

+const atTargets = [
+  Array,
+  String,
+  Int8Array,


The String polyfill uses this[resolvedIndex], which returns UTF-16 code units; native String.prototype.at is code-point aware (handles surrogate pairs). Since String.prototype.at is included in atTargets, either implement the code-point semantics or drop String from the targets to avoid introducing subtly incorrect behavior.

Copilot · 2026-04-09T04:03:44Z

server/src/test/java/cn/keking/PdfViewerCompatibilityTests.java

+    void shouldLoadCompatibilityModuleBeforePdfJs() throws IOException {
+        String viewerHtml = readResource("/static/pdfjs/web/viewer.html");
+
+        assertTrue(viewerHtml.contains("<script src=\"compatibility.mjs\" type=\"module\"></script>"));
+        assertTrue(viewerHtml.indexOf("compatibility.mjs") < viewerHtml.indexOf("../build/pdf.mjs"));
+    }


These assertions are very brittle because they depend on exact HTML/script formatting (attribute order, whitespace, self-closing style). A small upstream PDF.js update could break the test without changing behavior. Consider making the check more resilient (e.g., regex for a module script whose src ends with compatibility.mjs, and order based on match positions) while still ensuring the shim loads before pdf.mjs.

Copilot · 2026-04-09T04:03:44Z

server/src/test/java/cn/keking/PdfViewerCompatibilityTests.java

+    void shouldLoadCompatibilityModuleInPdfWorker() throws IOException {
+        String workerScript = readResource("/static/pdfjs/build/pdf.worker.mjs");
+
+        assertTrue(workerScript.contains("import \"../web/compatibility.mjs\";"));


This assertion requires the worker import line to match exactly (including quote style and semicolon). To reduce false negatives on future PDF.js asset updates, consider loosening it to a substring/regex that tolerates whitespace/quotes while still verifying the compatibility module is imported from the worker entrypoint.

Suggested change

assertTrue(workerScript.contains("import \"../web/compatibility.mjs\";"));

assertTrue(workerScript.matches("(?s).*import\\s+[\"']\\.\\./web/compatibility\\.mjs[\"']\\s*;?.*"));

Initial plan

405a12e

Copilot AI assigned Copilot and klboke Apr 9, 2026

Copilot started work on behalf of klboke April 9, 2026 02:39 View session

Copilot AI linked an issue Apr 9, 2026 that may be closed by this pull request

移动端预览PDF报错 pattern.at is not a function #728

Closed

3 tasks

Copilot AI and others added 3 commits April 9, 2026 02:46

fix: add pdfjs at compatibility polyfill

5499150

Agent-Logs-Url: https://github.com/kekingcn/kkFileView/sessions/287f0212-d368-4b59-ae70-0374fb3fe76f Co-authored-by: klboke <18591662+klboke@users.noreply.github.com>

test: use junit assertions for pdf viewer compatibility

530304b

Agent-Logs-Url: https://github.com/kekingcn/kkFileView/sessions/287f0212-d368-4b59-ae70-0374fb3fe76f Co-authored-by: klboke <18591662+klboke@users.noreply.github.com>

fix: refine pdfjs at compatibility polyfill

3fbf015

Agent-Logs-Url: https://github.com/kekingcn/kkFileView/sessions/287f0212-d368-4b59-ae70-0374fb3fe76f Co-authored-by: klboke <18591662+klboke@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Fix mobile PDF preview error when using pattern.at~~ Restore mobile PDF preview compatibility for browsers without Array.prototype.at Apr 9, 2026

Copilot AI requested a review from klboke April 9, 2026 02:52

Copilot finished work on behalf of klboke April 9, 2026 02:52

klboke approved these changes Apr 9, 2026

View reviewed changes

klboke marked this pull request as ready for review April 9, 2026 04:00

Copilot AI review requested due to automatic review settings April 9, 2026 04:00

Copilot started reviewing on behalf of klboke April 9, 2026 04:01 View session

Copilot AI reviewed Apr 9, 2026

View reviewed changes

klboke merged commit c41c14b into master Apr 9, 2026
6 checks passed

klboke deleted the copilot/fix-pdf-preview-error branch April 9, 2026 04:04

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Restore mobile PDF preview compatibility for browsers without `Array.prototype.at`#729

Restore mobile PDF preview compatibility for browsers without `Array.prototype.at`#729
klboke merged 4 commits intomasterfrom
copilot/fix-pdf-preview-error

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	assertTrue(workerScript.contains("import \"../web/compatibility.mjs\";"));
	assertTrue(workerScript.matches("(?s).import\\s+[\"']\\.\\./web/compatibility\\.mjs[\"']\\s;?.*"));

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