/use-js-interop

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.

// 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:

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`:

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:

// ❌ 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
var token = await _module.InvokeAsync<string>("createAccessToken");
await _module.InvokeVoidAsync("storeToken", token);

// ✅ One call applies both — no data dependency, no reason for two trips
export function applyPreferences(theme, locale) {
    document.documentElement.dataset.theme = theme;
    document.documentElement.lang = locale;
}

// ✅ Chain stays in JS — the token never needs to cross the boundary
export function createAndStoreToken() {
    const token = crypto.randomUUID();
    sessionStorage.setItem('access-token', token);
    return token;
}

JS → .NET: batch callbacks

When JS needs to send multiple pieces of data back to .NET, send them in a single `invokeMethodAsync` call rather than making separate callbacks:

// ❌ Two .NET round-trips from JS
await dotNetRef.invokeMethodAsync(ON_VOLUME_CHANGED, volume);
await dotNetRef.invokeMethodAsync(ON_PLAYBACK_CHANGED, isPlaying);

// ✅ One callback with all data
await dotNetRef.invokeMethodAsync(ON_PLAYER_STATE_CHANGED, { volume, isPlaying });

**Rule**: if two interop calls always happen together from either side, merge them into one function.

4. Typed Interop Wrapper

Encapsulate interop for a feature in a plain class that owns the module lifecycle:

public sealed class ChartInterop : IAsyncDisposable
{
    internal const string ModulePath = "./Components/ChartPanel.razor.js";
    internal const string InitMethod = "initialize";
    internal const string UpdateMethod = "updateData";
    internal const string DisposeMethod = "dispose";

    private readonly IJSRuntime _js;
    private IJSObjectReference? _module;

    public ChartInterop(IJSRuntime js) => _js = js;

    private async ValueTask<IJSObjectReference> GetModuleAsync()
        => _module ??= await _js.InvokeAsync<IJSObjectReference>("import", ModulePath);

    public async ValueTask InitializeAsync(ElementReference canvas)
    {
        var module = await GetModuleAsync();
        await module.InvokeVoidAsync(InitMethod, canvas);
    }

    public async ValueTask UpdateDataAsync(IReadOnlyList<DataPoint> points)
    {
        var module = await GetModuleAsync();
        await module.InvokeVoidAsync(UpdateMethod, points);
    }

    public async ValueTask DisposeAsync()
    {
        try
        {
            if (_module is not null)
            {
                await _module.InvokeVoidAsync(DisposeMethod);
                await _module.DisposeAsync();
            }
        }
        catch (JSDisconnectedException) { }
    }
}

The component creates and uses the wrapper with no magic strings:

@inject IJSRuntime JS
@implements IAsyncDisposable

<canvas @ref="_canvasRef" width="600" height="400"></canvas>

@code {
    private ElementReference _canvasRef;
    private ChartInterop? _chart;

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

    async ValueTask IAsyncDisposable.DisposeAsync()
    {
        if (_chart is not null)
            await _chart.DisposeAsync();
    }
}

Prefer a concrete class over interface + implementation for interop wrappers. For unit testing, substitute `IJSRuntime` directly (it is already an interface).