noParameterAssign

import { Tabs, TabItem } from '@astrojs/starlight/components';

<Tabs> <TabItem label="JavaScript (and super languages)" icon="seti:javascript"> ## Summary - Rule available since: `v1.0.0` - Diagnostic Category: [`lint/style/noParameterAssign`](/reference/diagnostics#diagnostic-category) - This rule isn't recommended, so you need to enable it. - This rule doesn't have a fix. - The default severity of this rule is [**warning**](/reference/diagnostics#warning). - Sources: - Same as [`no-param-reassign`](https://eslint.org/docs/latest/rules/no-param-reassign)

How to configure

json

{
	"linter": {
		"rules": {
			"style": {
				"noParameterAssign": "error"
			}
		}
	}
}

Description

Disallow reassigning function parameters.

Assignment to function parameters can be misleading and confusing, as modifying parameters will also mutate the arguments object. It is often unintended and indicative of a programmer error.

Examples

Invalid

function f(param) {
    param = 13;
}

<pre class="language-text"><code class="language-text">code-block.js:2:5 <a href="https://biomejs.dev/linter/rules/no-parameter-assign">lint/style/noParameterAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Assigning a function parameter is confusing. 1 │ function f(param) { > 2 │ param = 13; │ ^^^^^ 3 │ } 4 │ ℹ The parameter is declared here: > 1 │ function f(param) { │ ^^^^^ 2 │ param = 13; 3 │ } ℹ Developers usually expect function parameters to be readonly. To align with this expectation, use a local variable instead. </code></pre>

function f(param) {
    param++;
}

<pre class="language-text"><code class="language-text">code-block.js:2:5 <a href="https://biomejs.dev/linter/rules/no-parameter-assign">lint/style/noParameterAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Assigning a function parameter is confusing. 1 │ function f(param) { > 2 │ param++; │ ^^^^^ 3 │ } 4 │ ℹ The parameter is declared here: > 1 │ function f(param) { │ ^^^^^ 2 │ param++; 3 │ } ℹ Developers usually expect function parameters to be readonly. To align with this expectation, use a local variable instead. </code></pre>

function f(param) {
    for (param of arr) {}
}

<pre class="language-text"><code class="language-text">code-block.js:2:10 <a href="https://biomejs.dev/linter/rules/no-parameter-assign">lint/style/noParameterAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Assigning a function parameter is confusing. 1 │ function f(param) { > 2 │ for (param of arr) {} │ ^^^^^ 3 │ } 4 │ ℹ The parameter is declared here: > 1 │ function f(param) { │ ^^^^^ 2 │ for (param of arr) {} 3 │ } ℹ Developers usually expect function parameters to be readonly. To align with this expectation, use a local variable instead. </code></pre>

class C {
    constructor(readonly prop: number) {
        prop++;
    }
}

<pre class="language-text"><code class="language-text">code-block.ts:3:9 <a href="https://biomejs.dev/linter/rules/no-parameter-assign">lint/style/noParameterAssign</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ⚠ Assigning a function parameter is confusing. 1 │ class C { 2 │ constructor(readonly prop: number) { > 3 │ prop++; │ ^^^^ 4 │ } 5 │ } ℹ The parameter is declared here: 1 │ class C { > 2 │ constructor(readonly prop: number) { │ ^^^^ 3 │ prop++; 4 │ } ℹ Developers usually expect function parameters to be readonly. To align with this expectation, use a local variable instead. </code></pre>

Valid

function f(param) {
    let local = param;
}

Options

propertyAssignment

The noParameterAssign rule can be configured using the propertyAssignment option, which determines whether property assignments on function parameters are allowed or denied. By default, propertyAssignment is set to allow.

json

{
    "options": {
        "propertyAssignment": "allow"
    }
}

allow: Allows property assignments on function parameters. This is the default behavior.
- Example:

json

{
	"linter": {
		"rules": {
			"style": {
				"noParameterAssign": {
					"options": {
						"propertyAssignment": "allow"
					}
				}
			}
		}
	}
}

function update(obj) {
    obj.key = "value"; // No diagnostic
}

deny: Disallows property assignments on function parameters, enforcing stricter immutability.
- Example:

json

{
	"linter": {
		"rules": {
			"style": {
				"noParameterAssign": {
					"options": {
						"propertyAssignment": "deny"
					}
				}
			}
		}
	}
}

function update(obj) {
    obj.key = "value"; // Diagnostic: Assignment to a property of function parameter is not allowed.
}

</TabItem> </Tabs>

How to configure

Description

Examples

Invalid

Valid

Options

propertyAssignment

Related links