assignment to property of function parameter 'row'.eslint no param reassign

• Assignment to property of function parameter no-param-reassign

Last updated: Mar 7, 2024 Reading time · 3 min

# Table of Contents

• Disabling the no-param-reassign ESLint rule for a single line
• Disabling the no-param-reassign ESLint rule for an entire file
• Disabling the no-param-reassign ESLint rule globally

# Assignment to property of function parameter no-param-reassign

The ESLint error "Assignment to property of function parameter 'X' eslint no-param-reassign" occurs when you try to assign a property to a function parameter.

To solve the error, disable the ESLint rule or create a new object based on the parameter to which you can assign properties.

Here is an example of how the error occurs.

The ESLint rule forbids assignment to function parameters because modifying a function's parameters also mutates the arguments object and can lead to confusing behavior.

One way to resolve the issue is to create a new object to which you can assign properties.

We used the spread syntax (...) to unpack the properties of the function parameter into a new object to which we can assign properties.

If you need to unpack an array, use the following syntax instead.

The same approach can be used if you simply need to assign the function parameter to a variable so you can mutate it.

We declared the bar variable using the let keyword and set it to the value of the foo parameter.

We are then able to reassign the bar variable without any issues.

# Disabling the no-param-reassign ESLint rule for a single line

You can use a comment if you want to disable the no-param-reassign ESLint rule for a single line.

Make sure to add the comment directly above the assignment that causes the error.

# Disabling the no-param-reassign ESLint rule for an entire file

You can also use a comment to disable the no-param-reassign ESLint rule for an entire file.

Make sure to add the comment at the top of the file or at least above the function in which you reassign parameters.

The same approach can be used to disable the rule only for a single function.

The first comment disables the no-param-reassign rule and the second comment enables it.

If you try to reassign a parameter after the second comment, you will get an ESLint error.

# Disabling the no-param-reassign ESLint rule globally

If you need to disable the no-param-reassign rule globally, you have to edit your .eslintrc.js file.

If you only want to be able to assign properties to an object parameter, set props to false instead of disabling the rule completely.

The following code is valid after making the change.

If you use a .eslintrc or .eslintrc.json file, make sure to double-quote the properties and values.

If you want to only allow assignment to object parameters, use the following line instead.

Make sure all properties are double-quoted and there are no trailing commas if your config is written in JSON.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• eslint is not recognized as an internal or external command
• Plugin "react" was conflicted between package.json » eslint-config-react-app
• React: Unexpected use of 'X' no-restricted-globals in ESLint
• TypeScript ESLint: Unsafe assignment of an any value [Fix]
• ESLint error Unary operator '++' used no-plusplus [Solved]
• ESLint Prefer default export import/prefer-default-export
• Arrow function should not return assignment. eslint no-return-assign
• TypeError: Cannot redefine property: X in JavaScript [Fixed]
• ESLint: disable multiple rules or a rule for multiple lines
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• Missing return type on function TypeScript ESLint error

Borislav Hadzhiev

Web Developer

Copyright © 2024 Borislav Hadzhiev

no-param-reassign

Disallow reassigning function parameters

Assignment to variables declared as function parameters can be misleading and lead to confusing behavior, as modifying function parameters will also mutate the arguments object. Often, assignment to function parameters is unintended and indicative of a mistake or programmer error.

This rule can be also configured to fail when function parameters are modified. Side effects on parameters can cause counter-intuitive execution flow and make errors difficult to track down.

Rule Details

This rule aims to prevent unintended behavior caused by modification or reassignment of function parameters.

Examples of incorrect code for this rule:

Examples of correct code for this rule:

This rule takes one option, an object, with a boolean property "props" , and arrays "ignorePropertyModificationsFor" and "ignorePropertyModificationsForRegex" . "props" is false by default. If "props" is set to true , this rule warns against the modification of parameter properties unless they’re included in "ignorePropertyModificationsFor" or "ignorePropertyModificationsForRegex" , which is an empty array by default.

Examples of correct code for the default { "props": false } option:

Examples of incorrect code for the { "props": true } option:

Examples of correct code for the { "props": true } option with "ignorePropertyModificationsFor" set:

Examples of correct code for the { "props": true } option with "ignorePropertyModificationsForRegex" set:

When Not To Use It

If you want to allow assignment to function parameters, then you can safely disable this rule.

This rule was introduced in ESLint v0.18.0.

Further Reading

JavaScript: Don’t Reassign Your Function Arguments

• Rule source
• Tests source

© OpenJS Foundation and other contributors Licensed under the MIT License. https://eslint.org/docs/latest/rules/no-param-reassign

Disallow Reassignment of Function Parameters (no-param-reassign)

Assignment to variables declared as function parameters can be misleading and lead to confusing behavior, as modifying function parameters will also mutate the arguments object. Often, assignment to function parameters is unintended and indicative of a mistake or programmer error.

This rule can be also configured to fail when function parameters are modified. Side effects on parameters can cause counter-intuitive execution flow and make errors difficult to track down.

Rule Details

This rule aims to prevent unintended behavior caused by modification or reassignment of function parameters.

Examples of incorrect code for this rule:

Examples of correct code for this rule:

This rule takes one option, an object, with a boolean property "props" , and arrays "ignorePropertyModificationsFor" and "ignorePropertyModificationsForRegex" . "props" is false by default. If "props" is set to true , this rule warns against the modification of parameter properties unless they're included in "ignorePropertyModificationsFor" or "ignorePropertyModificationsForRegex" , which is an empty array by default.

Examples of correct code for the default { "props": false } option:

Examples of incorrect code for the { "props": true } option:

Examples of correct code for the { "props": true } option with "ignorePropertyModificationsFor" set:

Examples of correct code for the { "props": true } option with "ignorePropertyModificationsForRegex" set:

When Not To Use It

If you want to allow assignment to function parameters, then you can safely disable this rule.

Further Reading

• JavaScript: Don’t Reassign Your Function Arguments

This rule was introduced in ESLint 0.18.0.

• Rule source
• Documentation source

© OpenJS Foundation and other contributors Licensed under the MIT License. https://eslint.org/docs/rules/no-param-reassign

Disallow Reassignment of Function Parameters (no-param-reassign)

Assignment to variables declared as function parameters can be misleading and lead to confusing behavior, as modifying function parameters will also mutate the arguments object. Often, assignment to function parameters is unintended and indicative of a mistake or programmer error.

This rule can be also configured to fail when function parameters are modified. Side effects on parameters can cause counter-intuitive execution flow and make errors difficult to track down.

Rule Details

This rule aims to prevent unintended behavior caused by modification or reassignment of function parameters.

Examples of incorrect code for this rule:

Examples of correct code for this rule:

This rule takes one option, an object, with a boolean property "props" and an array "ignorePropertyModificationsFor" . "props" is false by default. If "props" is set to true , this rule warns against the modification of parameter properties unless they’re included in "ignorePropertyModificationsFor" , which is an empty array by default.

Examples of correct code for the default { "props": false } option:

Examples of incorrect code for the { "props": true } option:

Examples of correct code for the { "props": true } option with "ignorePropertyModificationsFor" set:

When Not To Use It

If you want to allow assignment to function parameters, then you can safely disable this rule.

Further Reading

• JavaScript: Don’t Reassign Your Function Arguments

This rule was introduced in ESLint 0.18.0.

• Rule source
• Documentation source

Disallow Reassignment of Function Parameters (no-param-reassign)

Assignment to variables declared as function parameters can be misleading and lead to confusing behavior, as modifying function parameters will also mutate the arguments object. Often, assignment to function parameters is unintended and indicative of a mistake or programmer error.

This rule can be also configured to fail when function parameters are modified. Side effects on parameters can cause counter-intuitive execution flow and make errors difficult to track down.

Rule Details

This rule aims to prevent unintended behavior caused by modification or reassignment of function parameters.

Examples of incorrect code for this rule:

Examples of correct code for this rule:

This rule takes one option, an object, with a boolean property "props" and an array "ignorePropertyModificationsFor" . "props" is false by default. If "props" is set to true , this rule warns against the modification of parameter properties unless they're included in "ignorePropertyModificationsFor" , which is an empty array by default.

Examples of correct code for the default { "props": false } option:

Examples of incorrect code for the { "props": true } option:

Examples of correct code for the { "props": true } option with "ignorePropertyModificationsFor" set:

When Not To Use It

If you want to allow assignment to function parameters, then you can safely disable this rule.

Further Reading

• JavaScript: Don’t Reassign Your Function Arguments

This rule was introduced in ESLint 0.18.0.

• Rule source
• Documentation source

Disallow Reassignment of Function Parameters (no-param-reassign)

Assignment to variables declared as function parameters can be misleading and lead to confusing behavior, as modifying function parameters will also mutate the arguments object. Often, assignment to function parameters is unintended and indicative of a mistake or programmer error.

This rule can be also configured to fail when function parameters are modified. Side effects on parameters can cause counter-intuitive execution flow and make errors difficult to track down.

Rule Details

This rule aims to prevent unintended behavior caused by modification or reassignment of function parameters.

Examples of incorrect code for this rule:

Examples of correct code for this rule:

This rule takes one option, an object, with a boolean property "props" and an array "ignorePropertyModificationsFor" . "props" is false by default. If "props" is set to true , this rule warns against the modification of parameter properties unless they’re included in "ignorePropertyModificationsFor" , which is an empty array by default.

Examples of correct code for the default { "props" : false } option:

Examples of incorrect code for the { "props" : true } option:

Examples of correct code for the { "props" : true } option with "ignorePropertyModificationsFor" set:

When Not To Use It

If you want to allow assignment to function parameters, then you can safely disable this rule.

Further Reading

• JavaScript: Don’t Reassign Your Function Arguments

This rule was introduced in ESLint 0.18.0.

• Rule source
• Documentation source

优雅解决： assignment to property of function parameter ‘state‘

在airbnb的eslint规则中，有这样一条规则 no-param-reassign

目的是提醒你不要直接修改函数的入参。因为假如入参是一个对象，修改入参可能会导致对象的属性被覆盖。

但有一些情况下，我们必须要这样做，比如在 vuex 中

其实，不仅仅是vuex，再比如express的 req res ，前端事件处理的 e.returnvalue 都需要直接给入参赋值。这时候我们显然不希望直接disable掉这条规则，或者在发生冲突的代码处单独disable。

这时候可以使用 ignorePropertyModificationsFor 这个属性，他可以为这个规则添加一个白名单，即指定的入参名称不予限制。看代码就明白了：

如上代码配置即可避免vuex与eslint的冲突。

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TypeScript Reactjs: Assignment to property of function parameter 'state' #13253

sptGabriel commented May 3, 2020

anikethsaha commented May 3, 2020

Sorry, something went wrong.

sptGabriel commented May 3, 2020 • edited Loading

eslint-deprecated bot commented Jun 4, 2020

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Assignment to function parameter 'value' no-param-reassign

I am trying to get rid off the no-param-reassign error from the following code.

Tried with adding the followings:

Nothing has worked. Also tried creating a new variable and assign to it. Still didn't work. I can't commit code due to that error.

I need to update this ( properties.color ) array element with function parameter value.

• typescript-eslint

• See if it helps, stackoverflow.com/a/35637900/13262332 –  DJ Hemath Commented Mar 25, 2022 at 4:20
• Unfortunately I tried most of the suggestion from the post you suggested. But nothing worked. –  PineCone Commented Apr 4, 2022 at 7:55

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