noParameterAssign

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JavaScript (and super languages)

Summary

Rule available since: v1.0.0
Diagnostic Category: lint/style/noParameterAssign
This rule doesn’t have a fix.
The default severity of this rule is warning.
Sources:
- Same as no-param-reassign

How to configure

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

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.

In contrast to the ESLint rule, this rule cannot be configured to report assignments to a property of a parameter.

Examples

Invalid

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function f(param) {
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    param = 13;
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}

code-block.js:2:5 lint/style/noParameterAssign ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ 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.

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function f(param) {
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    param++;
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}

code-block.js:2:5 lint/style/noParameterAssign ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ 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.

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

code-block.js:2:10 lint/style/noParameterAssign ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ 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.

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

code-block.ts:3:9 lint/style/noParameterAssign ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ 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.

Valid

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

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.

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

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

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

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

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

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

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

code-block.js:2:5 lint/style/noParameterAssign ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Assigning to a property of a function parameter is confusing.
  
    1 │ function update(obj) {
  > 2 │     obj.key = “value”; // Diagnostic: Assignment to a property of function parameter is not allowed.
      │     ^^^
    3 │ }
    4 │ 
  
  ℹ Function callers usually don’t expect the parameters they pass in to be modified. To avoid mutation, create a new instance and return it to the caller.