---
url: /docs/guide/usage/linter/rules/typescript/prefer-readonly-parameter-types.md
---

### What it does

Require function and method parameters to use readonly-compatible types.

### Why is this bad?

Mutable parameter types make accidental mutation easier and weaken function contracts.
Readonly parameter types communicate intent and improve API safety.

### Examples

Examples of **incorrect** code for this rule:

```ts
function update(items: string[]) {
  items.push("x");
}

function consume(obj: { value: string }) {
  obj.value = obj.value.trim();
}
```

Examples of **correct** code for this rule:

```ts
function update(items: readonly string[]) {
  return items.length;
}

function consume(obj: Readonly<{ value: string }>) {
  return obj.value;
}
```

## Configuration

This rule accepts a configuration object with the following properties:

### allow

type: `array`

default: `[]`

Type/value specifiers that should be exempt from this rule.

#### allow\[n]

type: `object | string`

Type or value specifier for matching specific declarations

Supports four types of specifiers:

1. **String specifier** (deprecated): Universal match by name

```json
"Promise"
```

2. **File specifier**: Match types/values declared in local files

```json
{ "from": "file", "name": "MyType" }
{ "from": "file", "name": ["Type1", "Type2"] }
{ "from": "file", "name": "MyType", "path": "./types.ts" }
```

3. **Lib specifier**: Match TypeScript built-in lib types

```json
{ "from": "lib", "name": "Promise" }
{ "from": "lib", "name": ["Promise", "PromiseLike"] }
```

4. **Package specifier**: Match types/values from npm packages

```json
{ "from": "package", "name": "Observable", "package": "rxjs" }
{ "from": "package", "name": ["Observable", "Subject"], "package": "rxjs" }
```

##### allow\[n].from

type: `"file"`

Must be "file"

##### allow\[n].name

type: `array | string`

The name(s) of the type or value to match

Name specifier that can be a single string or array of strings

###### allow\[n].name\[n]

type: `string`

##### allow\[n].path

type: `string`

Optional file path to specify where the types or values must be declared.
If omitted, all files will be matched.

### checkParameterProperties

type: `boolean`

default: `true`

Whether to check constructor parameter properties.

### ignoreInferredTypes

type: `boolean`

default: `false`

Whether to ignore parameters without explicit type annotations.

### treatMethodsAsReadonly

type: `boolean`

default: `false`

Whether mutable methods should be treated as readonly members.

## How to use

## Version

This rule was added in v1.49.0.

## References