noParameterAssign

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

Description

Disallow reassigning function parameters.

Assignment to a 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

1
function f(param) {
2
    param = 13;
3
}

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

  ⚠ Reassigning 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 Use a local variable instead.

1
function f(param) {
2
    param++;
3
}

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

  ⚠ Reassigning 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 Use a local variable instead.

1
function f(param) {
2
    for (param of arr) {}
3
}

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

  ⚠ Reassigning 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 Use a local variable instead.

1
class C {
2
    constructor(readonly prop: number) {
3
        prop++
4
    }
5
}

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

  ⚠ Reassigning 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 Use a local variable instead.

Valid

1
function f(param) {
2
    let local = param;
3
}

How to configure

1
{
2
  "linter": {
3
    "rules": {
4
      "style": {
5
        "noParameterAssign": "error"
6
      }
7
    }
8
  }
9
}