# ParamIndexMap

**Kind:** Type

**Source:** [`src/router.ts`](https://github.com/honojs/hono/blob/main/src/router.ts#L57)

Type representing a map of parameter indices.

`ParamIndexMap` stores the position associated with each route parameter name. The router uses it to locate parameter values after matching a route path.

## Definition

```ts
Record<string, number>
```

## Diagram

```mermaid
graph LR
  RoutePattern[Route pattern parameters] --> ParamIndexMap[ParamIndexMap]
  MatchedValues[Matched parameter values] --> Lookup[Parameter lookup]
  ParamIndexMap --> Lookup
```

## Usage

```ts
import type { ParamIndexMap } from "./router";

const parameterNames = ["userId", "postSlug"];

const parameterIndices = Object.fromEntries(
  parameterNames.map((name, index) => [name, index]),
) as ParamIndexMap;

function getParameter(
  values: readonly string[],
  name: string,
): string | undefined {
  const index = parameterIndices[name];

  return index === undefined ? undefined : values[index];
}
```

## AI Coding Instructions

- Keep parameter names aligned with the names declared in route patterns.
- Store indices that refer to positions in the matched parameter value list.
- Check for an undefined index before reading from parameter values.
- Update the map when route parsing changes the order of captured parameters.