microsoft/playwright · error · Error

Function "${name}" has been already registered in one of the

Error message

Function "${name}" has been already registered in one of the pages

What it means

Same exposeBinding flow, second guard: even if the context doesn't have the name, if any already-created page has a page-level binding with the same name, Playwright rejects it to prevent shadowing.

Source

Thrown at packages/playwright-core/src/server/browserContext.ts:382

      this.bindingsInitScript = PageBinding.createInitScript(this);
      this.initScripts.push(this.bindingsInitScript);
      await this.doAddInitScript(this.bindingsInitScript);
      await this.safeNonStallingEvaluateInAllFrames(this.bindingsInitScript.source, 'main');
    })();
    return await this._playwrightBindingExposed;
  }

  needsPlaywrightBinding(): boolean {
    return this._playwrightBindingExposed !== undefined;
  }

  async exposeBinding(progress: Progress, name: string, playwrightBinding: frames.FunctionWithSource, forClient?: unknown, noGlobal?: boolean): Promise<PageBinding> {
    if (this._pageBindings.has(name))
      throw new Error(`Function "${name}" has been already registered`);
    for (const page of this.pages()) {
      if (page.getBinding(name))
        throw new Error(`Function "${name}" has been already registered in one of the pages`);
    }
    await progress.race(this.exposePlaywrightBindingIfNeeded());
    const binding = new PageBinding(this, name, playwrightBinding, noGlobal);
    binding.forClient = forClient;
    this._pageBindings.set(name, binding);
    try {
      await progress.race(this.doAddInitScript(binding.initScript));
      await progress.race(this.safeNonStallingEvaluateInAllFrames(binding.initScript.source, 'main'));
      return binding;
    } catch (error) {
      this._pageBindings.delete(name);
      throw error;
    }
  }

  async removeExposedBinding(binding: PageBinding) {
    if (this._pageBindings.get(binding.name) !== binding)
      return;

View on GitHub (pinned to c8fc3bf8d3)

Solutions

Use distinct names for page vs context bindings
Expose at context level before creating pages, or stick to one level
Dispose page-level bindings before promoting to context

Example fix

// before
await page.exposeBinding('cb', h);
await context.exposeBinding('cb', h); // throws

// after
await page.exposeBinding('pageCb', h);
await context.exposeBinding('ctxCb', h);

Defensive patterns

Strategy: validation

Validate before calling

const ctxNames = new Set<string>();
const pageNames = new Set<string>();
async function exposeCtx(ctx: BrowserContext, name: string, fn: Function) {
  if (ctxNames.has(name) || pageNames.has(name))
    throw new Error(`Binding '${name}' already registered (page or context).`);
  ctxNames.add(name);
  await ctx.exposeBinding(name, fn as any);
}

Prevention

Don't mix page-level and context-level bindings with the same name
Expose context bindings before creating pages
Pick one level (page or context) and stay consistent

When it happens

Trigger: `page.exposeBinding('foo', ...)` followed by `context.exposeBinding('foo', ...)`; or any ordering where pages already exist with a colliding binding.

Common situations: Mixing page-level and context-level `exposeBinding`/`exposeFunction` with the same name; adding context bindings after pages were created; global setup registering context bindings after per-page setup ran.

Related errors

AI-assisted analysis of microsoft/playwright@c8fc3bf8d3 (2026-08-12). Data as JSON: /api/errors/465e9ec6d3f237de. Report an issue: GitHub.