Switch

If is great for conditions, but sometimes you have a value with several possible states. Switch mounts the first Case whose value equals the subject, falling back to Default. Where If evaluates conditions, Switch matches values.

import { Switch } from "@implementjs/core";

type Status = "todo" | "in-progress" | "done";
const status = signal<Status>("todo");

Switch(status)
	.Case("todo", Icon("circle"))
	.Case("in-progress", Icon("half-circle"))
	.Case("done", Icon("check"))
	.Default(Icon("question"));

Subjects

Pass a single readable, or several plus a getter that computes the subject from their values:

Switch([user, page], (u, p) => (u ? p : "login"))
	.Case("login", LoginForm())
	.Case("settings", Settings())
	.Default(Home());

Matching

Cases compare with deep equality, so object values work too:

Switch(position).Case({ x: 0, y: 0 }, P("At origin")).Default(P("Somewhere else"));

The first matching case wins. When nothing matches, the Default children mount (or nothing, if there is no Default).

Exhaustiveness

The type of Case narrows as you chain, so each case removes its value from the remaining union. .Exhaustive() only typechecks once every member has a case. That gives you a compile-time guarantee that adding a variant to the union breaks the build until it is handled:

Switch(status)
	.Case("todo", Todo())
	.Case("in-progress", InProgress())
	.Case("done", Done())
	.Exhaustive(); // ✅ compiles — all of Status is covered

Switch(status).Case("todo", Todo()).Exhaustive(); // ❌ type error — "in-progress" and "done" unhandled

The check is purely type-level and changes nothing at runtime. Duplicate cases are also caught, because a value already handled is no longer in the remaining union.

Mounting semantics

Same as If. The children of the matched case are created when it shows and discarded when another case takes over. Switch only chooses which branch is mounted, children close over reactive values themselves for updates within a branch.

Branches cover one element at a time. For rendering a whole list of them, it's time to meet ForEach.