# MESSAGE_MATCHER_IS_ALREADY_BUILT

**Kind:** Constant

**Source:** [`src/router.ts`](https://github.com/honojs/hono/blob/main/src/router.ts#L21)

Error message indicating that a route cannot be added because the matcher is already built.

`MESSAGE_MATCHER_IS_ALREADY_BUILT` stores the error message used when code attempts to add a route after its matcher has been built. It keeps this validation message consistent within `src/router.ts` when route configuration occurs in the wrong order.

## Definition

```ts
'Can not add a route since the matcher is already built.'
```

## Value

```ts
'Can not add a route since the matcher is already built.'
```

## Diagram

```mermaid
graph LR
  A[Add route] --> B{Matcher built?}
  B -- No --> C[Register route]
  B -- Yes --> D[MESSAGE_MATCHER_IS_ALREADY_BUILT]
  D --> E[Throw error]
```

## Usage

```ts
import { MESSAGE_MATCHER_IS_ALREADY_BUILT } from "./router";

function addRoute(matcherBuilt: boolean) {
  if (matcherBuilt) {
    throw new Error(MESSAGE_MATCHER_IS_ALREADY_BUILT);
  }

  // Register the route before building the matcher.
}
```

## AI Coding Instructions

- Use `MESSAGE_MATCHER_IS_ALREADY_BUILT` instead of duplicating the matcher-built error text.
- Check matcher build state before registering a new route.
- Add routes before the router builds its matcher.
- Keep errors related to route registration in `src/router.ts` aligned with this constant.

## Used by

2 references from 2 files. Each is a place in this repository where the symbol is actually used — go read one rather than trusting an example.

### Imported by (2)

- `RegExpRouter` — `src/router/reg-exp-router/router.ts`:47
- `SmartRouter` — `src/router/smart-router/router.ts`:4