eslint assignment to property of function parameter 'value'.(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
• Expected linebreaks to be 'LF' but found 'CRLF' linebreak-style
• Missing return type on function TypeScript ESLint error

Borislav Hadzhiev

Web Developer

Copyright © 2024 Borislav Hadzhiev

Understanding ESLint's `no-param-reassign` Rule for Clearer Functions

When you define a function, you can specify variables that the function will receive as input. These are called parameters. The no-param-reassign rule discourages directly changing the value of these parameters within the function.

There are two reasons for this:

• Clarity: If you modify a parameter inside the function, it can be confusing for someone reading the code to understand what the function does. They might expect the parameter value to stay the same throughout the function.
• Unexpected behavior: In JavaScript, there's a concept called the arguments object. This object holds the values of all the arguments passed to a function. If you change a parameter inside the function, it can also unintentionally modify the arguments object, leading to unexpected results.

Are there exceptions?

There are some cases where reassigning a parameter might be necessary. For instance, you might be working with an object and want to modify one of its properties. In these cases, ESLint provides ways to configure the no-param-reassign rule to allow such modifications while still catching unintended changes.

Strict mode and no-param-reassign

If your code is written in strict mode (a more secure way to write JavaScript), the no-param-reassign rule becomes less important. Strict mode already offers some protections against unintended modifications of the arguments object.

• You'll see an error message from ESLint when you try to directly reassign a parameter value within a function. This typically looks like no-param-reassign: Assignment to function parameter '<parameter_name>' .

Troubleshooting:

Configure no-param-reassign : ESLint allows some configuration for this rule. You can:

• Ignore Specific Parameters: If certain parameters are meant to be modified (like state in state management libraries), you can configure no-param-reassign to ignore those specific names.
• Allow Property Modifications: You can allow modifications to properties of objects passed as parameters while still disallowing complete reassignment.

Code Examples for no-param-reassign Rule

This code triggers a no-param-reassign error because it tries to change the value of the x parameter directly:

Fix: Working with the Parameter's Value

Here, we avoid the error by using the original parameter value ( num ) within the function:

Error: Modifying an Object Property

This code attempts to modify a property ( name ) of the object passed as a parameter, which can also trigger the error in some configurations:

Fix: Ignoring Property Modifications (Configuration)

If modifying object properties is intended, you can configure no-param-reassign to allow it:

Allowed Scenario: Working with Copies (Array)

This code demonstrates a case where creating a copy of the parameter (array) is necessary for modification:

Destructuring with Default Values:

• When defining the function, you can use destructuring with default values for parameters. This allows you to provide a fallback value if no argument is passed, avoiding the need to reassign later.

Early Return with Modified Copy:

• If you need to modify data passed as a parameter (like an object or array), consider creating a copy early on and working with the copy. Then, return the modified copy from the function.

Functional Programming Techniques:

Purpose:Controls which imports are allowed in your JavaScript code.Helps enforce coding standards and prevent potential issues

What is ESLint?ESLint is a popular tool for JavaScript developers that helps enforce coding style and identify potential errors

What it does: It checks your code for any variables that you use but haven't properly declared using var, let, or const

Having unused variables can clutter up your code and make it harder to read for you and other programmers. It also might indicate a bug if a variable was meant to be used but isn't. The no-unused-vars rule helps you find these unused variables and clean up your code

Here's a breakdown of how it works:What it catches: The rule flags any instance where you reference a variable, function

Here's what the rule considers "useless":Simply re-throwing the error: If your catch block just catches an error and then throws it again without any modification or handling

Here's how it works:Escapes in Strings: In JavaScript, backslashes (\) are used to escape characters that might otherwise have special meanings within strings

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 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

noParameterAssign

Diagnostic Category: lint/style/noParameterAssign

• JavaScript (and super languages)

Since : v1.0.0

• Same as: no-param-reassign

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.

Related links

• Disable a rule
• Configure the rule fix
• Rule options

Get the Reddit app

This subreddit is for anyone who wants to learn JavaScript or help others do so. Questions and posts about frontend development in general are welcome, as are all posts pertaining to JavaScript on the backend.

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.

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Fixing no-param-reassign Eslint issue in function

With below code, I am getting ESlint error Assignment to property of function parameter 'recurrence' at many places where I // eslint-disable-next-line no-param-reassign disabled the line. I dont want to use disable. How so solve this issue without using disable as legit way? Can anyone help me with this? THanks

• ecmascript-6

You can assign recurrence to a const variable in side the function and use that in side the function.

• So Now I have to replace recurrence.frequency with recurrenceValue.frequency every place where recurrence is used? –  Zeus Carl Commented Sep 23, 2020 at 6:50
• 1 yes exactly. or just a simply change the parameter name to something else and set const variable name as recurrence . so now you don't have to replace everywhere. –  Nazeer Commented Sep 23, 2020 at 8:00
• Thanks.... Can you help me here too? stackoverflow.com/questions/64023755/… –  Zeus Carl Commented Sep 23, 2020 at 8:17

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no-param-reassign: add option to ignore property assignment for whitelisted callers #9670

dzaman commented Nov 30, 2017

not-an-aardvark commented Nov 30, 2017 • edited Loading

Sorry, something went wrong.

clshortfuse commented Dec 14, 2017

zargold commented Dec 20, 2017 • edited Loading

clshortfuse commented Dec 20, 2017

not-an-aardvark commented Jan 19, 2018

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