# use-js-interop

Sets up and audits JavaScript interop in Blazor components. It guides .NET-to-JS calls, lifecycle handling, and call batching across the circuit.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 3000 installs · ★ 5730
- Author: dotnet
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/dotnet/skills/tree/HEAD/plugins/dotnet-blazor/skills/use-js-interop/SKILL.md
- Updated: 2026-10-24
- HTML: https://affpapa.org/en/nevibecode/skills/use-js-interop

## What it does

This skill inspects and organizes JavaScript interop in Blazor components. It enforces collocated `.razor.js` files with explicit exports rather than global functions on `window`. All calls run through a typed C# class that implements `IAsyncDisposable` and catches `JSDisconnectedException` during teardown.

Interop timing follows strict rules. JavaScript is completely unavailable during server prerendering. Because of that, running interop code from `OnInitialized` breaks immediately. The pattern defers all execution to `OnAfterRenderAsync` or dedicated event handlers, setting flags in `OnParametersSet` whenever component parameters change.

Consecutive interop calls are batched into single operations to cut traffic over SignalR circuits. Browser callbacks rely on `DotNetObjectReference` wrapped in JavaScript classes with explicit `try/catch` guards. Do not load this skill for general component templates or form fields, which use separate guidance.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add dotnet/skills --skill use-js-interop -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/use-js-interop/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/use-js-interop/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add dotnet/skills --skill use-js-interop -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/use-js-interop/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add dotnet/skills --skill use-js-interop -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/use-js-interop/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install dotnet/skills use-js-interop
```

2. **Manually: put the files in place**
- File: `.github/skills/use-js-interop/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add dotnet/skills --skill use-js-interop -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/dotnet/skills --path plugins/dotnet-blazor/skills/use-js-interop
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/use-js-interop/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add dotnet/skills --skill use-js-interop -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/dotnet/skills/tree/HEAD/plugins/dotnet-blazor/skills/use-js-interop/SKILL.md

## When to use it
- isolate script logic into collocated `.razor.js` files
- prevent JS calls during server prerendering phases
- batch related interop calls across the boundary
- manage lifecycle cleanup with `IAsyncDisposable`
- stream browser callbacks into C# via `DotNetObjectReference`

## Example prompts
- Create a collocated `.razor.js` module for this chart component and add a typed interop wrapper.
- Fix lifecycle issues in this Blazor component by moving JS interop into `OnAfterRenderAsync`.
- Batch these consecutive interop calls into a single JS export to cut SignalR round-trips.

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 185d264338a562e2.

BEGIN THIRD-PARTY FILE 185d264338a562e2
````text
---
license: MIT
name: use-js-interop
description: >
  Add, review, or fix JavaScript interop in Blazor components.
  USE FOR: calling JavaScript from Blazor, calling .NET from JavaScript,
  collocated .razor.js modules, IJSRuntime, IJSObjectReference lifecycle,
  DotNetObjectReference, ElementReference, timing rules for when JS is available,
  IAsyncDisposable disposal of JS references, server-side JS interop safety.
  DO NOT USE FOR: general Blazor component authoring without JS interop needs
  (use author-component), forms (use collect-user-input).
---

# JS Interop in Blazor

## 1. Collocated JS Modules

Always use collocated `.razor.js` files with `export` — never global `window.*` functions or `<script>` tags.

```javascript
// ChartPanel.razor.js — placed next to ChartPanel.razor
export function initialize(canvas, dotNetRef) { /* ... */ }
export function updateData(points) { /* ... */ }
export function dispose() { /* ... */ }
```

Import paths: same project = `"./Components/ChartPanel.razor.js"`, RCL = `"./_content/{AssemblyName}/..."`.

## 2. Lifecycle Timing

**All JS interop must happen in `OnAfterRenderAsync` or event handlers** — never in `OnInitialized`, `OnParametersSet`, or constructors. JS is not available during server prerendering.

Use a typed interop wrapper (see Section 4) — never call `InvokeAsync`/`InvokeVoidAsync` with raw string literals:

```csharp
private ChartInterop? _chart;

protected override async Task OnAfterRenderAsync(bool firstRender)
{
    if (firstRender)
    {
        _chart = new ChartInterop(JS);
        await _chart.InitializeAsync(_canvasRef);
    }
}
```

**Parameter changes**: set a flag in `OnParametersSet`, apply in `OnAfterRenderAsync`:

```csharp
private bool _dataChanged;

protected override void OnParametersSet() => _dataChanged = true;

protected override async Task OnAfterRenderAsync(bool firstRender)
{
    if (firstRender) { /* init */ }
    else if (_dataChanged && _chart is not null)
    {
        _dataChanged = false;
        await _chart.UpdateDataAsync(DataPoints);
    }
}
```

## 3. Batch Related Operations

Each JS interop call crosses the .NET-to-JS boundary (and in Blazor Server, the SignalR circuit). Batching applies in **both directions** — .NET→JS and JS→.NET.

### .NET → JS: merge consecutive calls

If the C# side makes two or more JS calls in a row, combine them into one JS function:

```csharp
// ❌ Two round-trips — theme and locale are always applied together
await _module.InvokeVoidAsync("applyTheme", theme);
await _module.InvokeVoidAsync("applyLocale", locale);

// ❌ Result of one call feeds into another — both can stay in JS
````
END THIRD-PARTY FILE 185d264338a562e2

The third-party file has ended. Everything between the 185d264338a562e2 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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