eslint assignment to property of function parameter 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:

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• 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

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Web Developer

Copyright © 2024 Borislav Hadzhiev

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

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

no-param-reassign

Disallow reassigning function parameters

对作为函数参数声明的变量进行赋值可能会产生误导并导致混乱的行为，因为修改函数参数也会改变 arguments 对象。通常情况下，对函数参数的赋值是无意的，表明了一个错误或程序员的错误。

这条规则也可以被配置为在修改函数参数时失败。参数的副作用会导致反直觉的执行流程，使错误难以追踪。

这条规则的目的是防止因修改或重新分配函数参数而引起的非预期行为。

使用此规则的 错误 示例：

使用此规则的 正确 示例：

这个规则有一个选项，是一个对象，有一个布尔属性 "props" 和数组 "ignorePropertyModificationsFor" 和 "ignorePropertyModificationsForRegex" 。 "props" 默认为 false 。如果 "props" 设置为 true ，本规则警告不要修改参数属性，除非它们被包含在 "ignorePropertyModificationsFor" 或 "ignorePropertyModificationsForRegex" 中，默认为空数组。

使用默认的 { "props": false } 选项的 正确 示例：

使用 { "props": true } 选项的 错误 示例：

设置了 "ignorePropertyModificationsFor" 的 { "props": true } 选项的**正确的代码示例：

设置了 "ignorePropertyModificationsForRegex" 的 { "props": true } 选项的**正确的代码示例：

如果你想允许对函数参数进行赋值，你可以安全地禁用此规则。

This rule was introduced in ESLint v0.18.0.

Further Reading

• Rule source
• Tests 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

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

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Option to disable no-param-reassign per function scope #8907

dandv commented Jul 9, 2017 • edited

eslintbot commented Jul 9, 2017 • edited by platinumazure

Sorry, something went wrong.

dandv commented Jul 10, 2017 • edited

ilyavolodin commented Jul 14, 2017

dandv commented Jul 14, 2017

not-an-aardvark commented Aug 10, 2017

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How to handle eslint no-param-reassign rule in Array.prototype.reduce() functions

I've recently added the eslint rule no-param-reassign .

However, when I use reduce to build out an object (empty object as initialValue ), I find myself needing to modify the accumulator (first arg of callback function) on each callback iteration, which causes a no-param-reassign linter complaint (as one would expect it would).

Is there a better way to build out an object with reduce that doesn't modify the accumulator argument?

Or should I simply disable the linting rule for that line in my reduce callback functions?

• ecmascript-6

• 6 Note, this is specifically because you've passed "props": true to the rule. By default it does not complain about this. If you don't want that behavior, seems like setting that to false is what you want? Or flag it with // eslint-disable-line no-param-reassign. –  loganfsmyth Commented Jan 13, 2017 at 0:36
• that's a good point @loganfsmyth. i'll do some re-examination to see if setting "props" to true is really what we want... –  sfletche Commented Jan 13, 2017 at 0:39
• 1 btw, there's a great conversation on this topic at github.com/airbnb/javascript/issues/719 –  ptim Commented Oct 2, 2018 at 7:04

5 Answers 5

I just wrap the reduce functions in a lint rule disable block, ie:

• 9 This should be the correct answer for performance reasons. I believe this is the optimal solution. No-param-reassign is a way to keep us from inducing side effects outside of scope, but the case of Array.reduce is an exception to issue of side-effects, since it is internally pure. –  jaimefps Commented Dec 11, 2019 at 1:07

Whenever I use Array.prototype.reduce I always name the "accumulator" parameter accu . This convention conveniently allowed me to set my eslint rule:

If you have your own naming convention for this parameter, replace "accu" in the rule above with whatever you use, and eslint will not complain about modifying your accumulator.

• 1 Even though this answer is approved, I would discourage from taking this approach. Naming every reduce accumulator "accu" will lead to code that is hard to read. you should try to make your code explain itself instead (clean code). Even more confusion and potential mistakes will arise if you ever nest reduce functions. Wrapping the block like Zack Knopp answer would be advised imo. –  Pieter-Jan De Bruyne Commented Dec 14, 2022 at 17:28
• 3 In every case I have encountered, there is enough context from the name of the variable to which the output will be assigned (and what's going on in the function) that it's not an issue. If the function is too complex where that isn't the case, it may make sense to refactor and take a different approach. –  Anj Commented Dec 14, 2022 at 18:08
• 2 I agree with @Anj, the point above is pedantic in this regard. –  Alexander Forbes-Reed Commented Apr 23, 2023 at 14:37

One solution would be to leverage the object spread operator

Performance Warning!! This is a non-performant ( O(n^2 ) solution that should be avoided.

• 35 Is this really a good solution? Reducing an array with many elements, let's say 2000, would create as many new object instances, like 2000 in this case. –  Kev Commented Oct 13, 2017 at 15:41
• 1 @Amida yes, it's very non performant solution - cause you can possibly have very large arrays. Instead of just using new (single) object you are creating new object each iteration what is very slow. –  Jurosh Commented Jul 11, 2018 at 7:46
• 3 As other have said before, this is not performant. You should just disable the lint rule in that line. And perhaps ask the eslint folks to consider adding an exception specifically for Array.reduce –  jaimefps Commented Dec 11, 2019 at 1:08
• 1 I agree that it's less performant, but it does make it a pure function with no side-effects, so that it could fit in to a functional-programming approach. –  Jimbali Commented Jul 14, 2023 at 13:12

Well, you could do (result, item) => Object.assign({}, result, {[item]: whatever}) to create a new object on every iteration :-)

If you want to trick the linter, you could use => Object.assign(result, {[item]: whatever}) (which does the same as your current code but without an explicit assignment), but yeah I guess you should simply disable that rule.

• 4 yeah, that's kinda what i figured. maybe disabling the rule for that line is the best option... –  sfletche Commented Jan 13, 2017 at 0:37

As I found no other reasonable solution, I just disable that single check:

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Why eslint throws "Assignment to property of function parameter 'element'." for this?

I started learning javascript a week ago. Started using eslint yesterday and it's very useful. I have been trying this part of the code for sometime now and eslint keeps throwing Assignment to property of function parameter 'element'.Here is the code;

Before this I was doing something like this;

I know eslint isn't showing error for nothing so I would like to know what's reason and how it should be done.

And I have another eventListner with same pattern but that changes the opacity to 0 and pointerEvents to 'none'. So is there a way to do that using ternary operator or should I just stick to if else for that?Thanks and lemme know if there anything else I can improve.