Context

Every component body has an ambient ctx context.Context. The context passed to Render(ctx, w) is available throughout the component tree without adding it as a parameter.

Read from context

Pass ctx to an ordinary typed helper that owns the key and fallback behavior.

package views

import "context"

type ctxKey struct{}

// userName reads an authenticated username from the context, or falls
// back to "guest" when the value is absent.
func userName(ctx context.Context) string {
	if v, ok := ctx.Value(ctxKey{}).(string); ok {
		return v
	}
	return "guest"
}

component Greeting() {
	<p>Hello, { userName(ctx) }</p>
}

Renders:

<p>Hello, guest</p>

▶ Open in Playground

An unexported key type avoids collisions with keys from other packages. Context works well for request-scoped concerns such as authentication, locale, request IDs, tracing, and feature flags.

Derive a context

Rebind ctx in a block with =, or with := alongside at least one new name:

component Traced() {
	{{ ctx, span := tracer.Start(ctx, "traced"); defer span.End() }}
	{{ ctx, cancel := context.WithCancel(ctx); defer cancel() }}
	<Report/>
}

The derived ctx applies to everything rendered after it in the component, including child components, up to the end of the enclosing if/for/switch body or component children; plain elements don't scope it. A defer runs when the component finishes rendering. Declaring a new ctx (var ctx, or ctx := x with no other new name) is rejected.

Prefer parameters for application data

Use explicit, typed component parameters for data that directly determines what a component renders; the declaration and call site then show the dependency. Reserve context for values that are genuinely ambient across a request.