assignment to property of function parameter 'column'

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.

assignment to property of function parameter eslint no param reassign

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

HatchJS.com

Cracking the Shell of Mystery

How to Assign to the Property of a Function Parameter in JavaScript

Assignment to Property of Function Parameter

One of the most powerful features of JavaScript is the ability to assign values to the properties of function parameters. This can be used to create complex and dynamic code that can be easily modified.

In this article, we will take a closer look at assignment to property of function parameter. We will discuss what it is, how it works, and how it can be used to improve your code.

We will also provide some examples of how assignment to property of function parameter can be used in practice. By the end of this article, you will have a solid understanding of this important JavaScript concept.

Property	Value	Example
name	“John Doe”	function greet(name) { console.log(`Hello, ${name}`); } greet(“John Doe”);
age	25	function calculateAge(birthdate) { const today = new Date(); const age = today.getFullYear() – birthdate.getFullYear(); return age; } const age = calculateAge(new Date(“1997-01-01”)); console.log(age);

Property

Value

Example

name

“John Doe”

function greet(name) {
console.log(`Hello, ${name}`);
}

greet(“John Doe”);

age

function calculateAge(birthdate) {
const today = new Date();
const age = today.getFullYear() – birthdate.getFullYear();
return age;
}

const age = calculateAge(new Date(“1997-01-01”));
console.log(age);

In JavaScript, a function parameter is a variable that is declared inside the function’s parentheses. When a function is called, the value of the argument passed to the function is assigned to the function parameter.

For example, the following function takes a string argument and prints it to the console:

js function greet(name) { console.log(`Hello, ${name}`); }

greet(“world”); // prints “Hello, world”

In this example, the `name` parameter is assigned the value of the `”world”` argument.

Assignment to property of function parameter

Assignment to property of function parameter is a JavaScript feature that allows you to assign a value to a property of a function parameter. This can be useful for initializing the value of a parameter or for passing a reference to an object.

For example, the following code assigns the value `”hello”` to the `name` property of the `greet` function parameter:

js function greet(name) { name.value = “hello”; }

greet({ value: “world” }); // prints “hello”

In this example, the `name` parameter is a JavaScript object. The `value` property of the `name` object is assigned the value of the `”hello”` argument.

When to use assignment to property of function parameter?

You should use assignment to property of function parameter when you need to:

Initialize the value of a parameter
Pass a reference to an object

Avoid creating a new object

Initializing the value of a parameter

You can use assignment to property of function parameter to initialize the value of a parameter. For example, the following code initializes the `name` property of the `greet` function parameter to the value of the `”world”` argument:

js function greet(name) { name.value = “world”; }

Passing a reference to an object

You can use assignment to property of function parameter to pass a reference to an object. For example, the following code passes a reference to the `person` object to the `greet` function:

js function greet(person) { console.log(`Hello, ${person.name}`); }

const person = { name: “John Doe” };

greet(person); // prints “Hello, John Doe”

You can use assignment to property of function parameter to avoid creating a new object. For example, the following code uses assignment to property of function parameter to avoid creating a new object for the `name` parameter:

greet(“John Doe”); // prints “Hello, John Doe”

In this example, the `name` parameter is a string literal. The `name` property of the `name` parameter is assigned the value of the `”John Doe”` string literal. This avoids creating a new object for the `name` parameter.

Assignment to property of function parameter is a JavaScript feature that can be used to initialize the value of a parameter, pass a reference to an object, and avoid creating a new object. It is a powerful feature that can be used to improve the performance and readability of your code.

Additional resources

[MDN: Assignment to property of function parameter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Assignment_to_property_of_function_parameter)
[Stack Overflow: When to use assignment to property of function parameter?](https://stackoverflow.com/questions/1435573/when-to-use-assignment-to-property-of-function-parameter)
[Codecademy: Assignment to property of function parameter](https://www.codecademy.com/learn/javascript/lessons/assignment-to-property-of-function-parameter)

3. How to use assignment to property of function parameter?

To use assignment to property of function parameter, you can simply assign a value to the property of the function parameter. For example, the following code assigns the value `”hello”` to the `name` property of the `greet` function parameter:

In this example, the `greet` function is called with the argument `”world”`. The `name` property of the `greet` function parameter is then assigned the value `”hello”`. When the `greet` function is called, the value of the `name` property is used to print the message `”Hello, world”`.

Assignment to property of function parameter can be used to initialize the value of a parameter, pass a reference to an object, or avoid creating a new object.

You can use assignment to property of function parameter to initialize the value of a parameter. For example, the following code initializes the value of the `name` property of the `greet` function parameter to the value of the `name` variable:

js function greet(name) { name = “world”; console.log(`Hello, ${name}`); }

In this example, the `name` variable is assigned the value `”world”` before the `greet` function is called. The `name` property of the `greet` function parameter is then assigned the value of the `name` variable. When the `greet` function is called, the value of the `name` property is used to print the message `”Hello, world”`.

You can use assignment to property of function parameter to pass a reference to an object. For example, the following code passes a reference to the `user` object to the `greet` function:

js function greet(user) { console.log(`Hello, ${user.name}`); }

const user = { name: “John Doe”, };

greet(user); // prints “Hello, John Doe”

In this example, the `user` object is passed to the `greet` function as a parameter. The `greet` function then uses the `name` property of the `user` object to print the message `”Hello, John Doe”`.

Avoiding creating a new object

In this example, the `user` variable is assigned the value of the `user` object. The `greet` function then uses the `name` property of the `user` variable to print the message `”Hello, John Doe”`.

By using assignment to property of function parameter, you can avoid creating a new object for the `user` variable. This can improve the performance of your code and reduce the amount of memory that is used.

4. Pitfalls of assignment to property of function parameter

There are a few pitfalls to be aware of when using assignment to property of function parameter:

The value of the property may be overwritten. If you assign a value to the property of a function parameter, the value of the property may be overwritten by the next time the function is called. For example, the following code assigns the value `”hello”` to the `name` property of the `greet` function parameter. The next time the `greet` function is called, the value of the `name` property will be overwritten by the value of the `name` argument.

js function greet(name) { name = “hello”; console.log(`Hello, ${name}`); }

greet(“world”); // prints “Hello, hello” greet(“hello”); // prints “Hello, hello”

A: Assignment to property of function parameter occurs when you assign a value to a property of a function parameter. This can be done by using the dot operator (.) to access the property, or by using the bracket operator ([]) to index into the property.

For example, the following code assigns the value “10” to the `x` property of the `foo()` function’s parameter `y`:

const foo = (y) => { y.x = 10; };

foo({ x: 5 }); // { x: 10 }

Q: Why is assignment to property of function parameter dangerous?

A: Assignment to property of function parameter can be dangerous because it can change the value of the property in the calling scope. This can lead to unexpected behavior and errors.

For example, the following code changes the value of the `x` property of the global variable `a`:

foo({ x: 5 }); // a.x is now 10

This behavior can be difficult to debug, as it may not be obvious that the change to the `x` property is being caused by the `foo()` function.

Q: How can I avoid assignment to property of function parameter?

There are a few ways to avoid assignment to property of function parameter. One way is to use the `const` keyword to declare the function parameter as a constant. This will prevent the value of the parameter from being changed.

Another way to avoid assignment to property of function parameter is to use the `readonly` keyword to declare the function parameter as read-only. This will prevent the value of the parameter from being changed, even by assignment to a property of the parameter.

Finally, you can also use the `Object.freeze()` method to freeze the object that is passed as the function parameter. This will prevent any changes to the object, including changes to the values of its properties.

Q: What are the best practices for assignment to property of function parameter?

The best practices for assignment to property of function parameter are as follows:

Use the `const` keyword to declare function parameters as constants.
Use the `readonly` keyword to declare function parameters as read-only.
Use the `Object.freeze()` method to freeze objects that are passed as function parameters.

Here are some key takeaways from this article:

Assigning to the property of a function parameter can change the value of the original variable.
This can lead to unexpected behavior and security vulnerabilities.
To avoid this problem, use the `const` keyword or pass arguments by reference.

By following these tips, you can write more secure and reliable JavaScript code.

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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 when not in strict mode (see When Not To Use It below). 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.

Strict mode code doesn’t sync indices of the arguments object with each parameter binding. Therefore, this rule is not necessary to protect against arguments object mutation in ESM modules or other strict mode functions.

This rule was introduced in ESLint v0.18.0.

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:

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.

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.

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.

TypeError: invalid assignment to const "x"

The JavaScript exception "invalid assignment to const" occurs when it was attempted to alter a constant value. JavaScript const declarations can't be re-assigned or redeclared.

What went wrong?

A constant is a value that cannot be altered by the program during normal execution. It cannot change through re-assignment, and it can't be redeclared. In JavaScript, constants are declared using the const keyword.

Invalid redeclaration

Assigning a value to the same constant name in the same block-scope will throw.

Fixing the error

There are multiple options to fix this error. Check what was intended to be achieved with the constant in question.

If you meant to declare another constant, pick another name and re-name. This constant name is already taken in this scope.

const, let or var?

Do not use const if you weren't meaning to declare a constant. Maybe you meant to declare a block-scoped variable with let or global variable with var .

Check if you are in the correct scope. Should this constant appear in this scope or was it meant to appear in a function, for example?

const and immutability

The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned. For instance, in case the content is an object, this means the object itself can still be altered. This means that you can't mutate the value stored in a variable:

But you can mutate the properties in a variable:

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Type Compatibility

Type compatibility in TypeScript is based on structural subtyping. Structural typing is a way of relating types based solely on their members. This is in contrast with nominal typing. Consider the following code:

In nominally-typed languages like C# or Java, the equivalent code would be an error because the Dog class does not explicitly describe itself as being an implementer of the Pet interface.

TypeScript’s structural type system was designed based on how JavaScript code is typically written. Because JavaScript widely uses anonymous objects like function expressions and object literals, it’s much more natural to represent the kinds of relationships found in JavaScript libraries with a structural type system instead of a nominal one.

A Note on Soundness

TypeScript’s type system allows certain operations that can’t be known at compile-time to be safe. When a type system has this property, it is said to not be “sound”. The places where TypeScript allows unsound behavior were carefully considered, and throughout this document we’ll explain where these happen and the motivating scenarios behind them.

Starting out

The basic rule for TypeScript’s structural type system is that x is compatible with y if y has at least the same members as x . For example consider the following code involving an interface named Pet which has a name property:

To check whether dog can be assigned to pet , the compiler checks each property of pet to find a corresponding compatible property in dog . In this case, dog must have a member called name that is a string. It does, so the assignment is allowed.

The same rule for assignment is used when checking function call arguments:

Note that dog has an extra owner property, but this does not create an error. Only members of the target type ( Pet in this case) are considered when checking for compatibility. This comparison process proceeds recursively, exploring the type of each member and sub-member.

Be aware, however, that object literals may only specify known properties . For example, because we have explicitly specified that dog is of type Pet , the following code is invalid:

Comparing two functions

While comparing primitive types and object types is relatively straightforward, the question of what kinds of functions should be considered compatible is a bit more involved. Let’s start with a basic example of two functions that differ only in their parameter lists:

To check if x is assignable to y , we first look at the parameter list. Each parameter in x must have a corresponding parameter in y with a compatible type. Note that the names of the parameters are not considered, only their types. In this case, every parameter of x has a corresponding compatible parameter in y , so the assignment is allowed.

The second assignment is an error, because y has a required second parameter that x does not have, so the assignment is disallowed.

You may be wondering why we allow ‘discarding’ parameters like in the example y = x . The reason for this assignment to be allowed is that ignoring extra function parameters is actually quite common in JavaScript. For example, Array#forEach provides three parameters to the callback function: the array element, its index, and the containing array. Nevertheless, it’s very useful to provide a callback that only uses the first parameter:

Now let’s look at how return types are treated, using two functions that differ only by their return type:

The type system enforces that the source function’s return type be a subtype of the target type’s return type.

Function Parameter Bivariance

When comparing the types of function parameters, assignment succeeds if either the source parameter is assignable to the target parameter, or vice versa. This is unsound because a caller might end up being given a function that takes a more specialized type, but invokes the function with a less specialized type. In practice, this sort of error is rare, and allowing this enables many common JavaScript patterns. A brief example:

You can have TypeScript raise errors when this happens via the compiler flag strictFunctionTypes .

Optional Parameters and Rest Parameters

When comparing functions for compatibility, optional and required parameters are interchangeable. Extra optional parameters of the source type are not an error, and optional parameters of the target type without corresponding parameters in the source type are not an error.

When a function has a rest parameter, it is treated as if it were an infinite series of optional parameters.

This is unsound from a type system perspective, but from a runtime point of view the idea of an optional parameter is generally not well-enforced since passing undefined in that position is equivalent for most functions.

The motivating example is the common pattern of a function that takes a callback and invokes it with some predictable (to the programmer) but unknown (to the type system) number of arguments:

Functions with overloads

When a function has overloads, each overload in the target type must be matched by a compatible signature on the source type. This ensures that the source function can be called in all the same cases as the target function.

Enums are compatible with numbers, and numbers are compatible with enums. Enum values from different enum types are considered incompatible. For example,

Classes work similarly to object literal types and interfaces with one exception: they have both a static and an instance type. When comparing two objects of a class type, only members of the instance are compared. Static members and constructors do not affect compatibility.

Private and protected members in classes

Private and protected members in a class affect their compatibility. When an instance of a class is checked for compatibility, if the target type contains a private member, then the source type must also contain a private member that originated from the same class. Likewise, the same applies for an instance with a protected member. This allows a class to be assignment compatible with its super class, but not with classes from a different inheritance hierarchy which otherwise have the same shape.

Because TypeScript is a structural type system, type parameters only affect the resulting type when consumed as part of the type of a member. For example,

In the above, x and y are compatible because their structures do not use the type argument in a differentiating way. Changing this example by adding a member to Empty<T> shows how this works:

In this way, a generic type that has its type arguments specified acts just like a non-generic type.

For generic types that do not have their type arguments specified, compatibility is checked by specifying any in place of all unspecified type arguments. The resulting types are then checked for compatibility, just as in the non-generic case.

For example,

Advanced Topics

Subtype vs assignment.

So far, we’ve used “compatible”, which is not a term defined in the language spec. In TypeScript, there are two kinds of compatibility: subtype and assignment. These differ only in that assignment extends subtype compatibility with rules to allow assignment to and from any , and to and from enum with corresponding numeric values.

Different places in the language use one of the two compatibility mechanisms, depending on the situation. For practical purposes, type compatibility is dictated by assignment compatibility, even in the cases of the implements and extends clauses.

any , unknown , object , void , undefined , null , and never assignability

The following table summarizes assignability between some abstract types. Rows indicate what each is assignable to, columns indicate what is assignable to them. A ” ✓ ” indicates a combination that is compatible only when strictNullChecks is off.

	any	unknown	object	void	undefined	null	never
any →		✓	✓	✓	✓	✓	✕
unknown →	✓		✕	✕	✕	✕	✕
object →	✓	✓		✕	✕	✕	✕
void →	✓	✓	✕		✕	✕	✕
undefined →	✓	✓	✓	✓		✓	✕
null →	✓	✓	✓	✓	✓		✕
never →	✓	✓	✓	✓	✓	✓

Reiterating The Basics :

Everything is assignable to itself.
any and unknown are the same in terms of what is assignable to them, different in that unknown is not assignable to anything except any .
unknown and never are like inverses of each other. Everything is assignable to unknown , never is assignable to everything. Nothing is assignable to never , unknown is not assignable to anything (except any ).
void is not assignable to or from anything, with the following exceptions: any , unknown , never , undefined , and null (if strictNullChecks is off, see table for details).
When strictNullChecks is off, null and undefined are similar to never : assignable to most types, most types are not assignable to them. They are assignable to each other.
When strictNullChecks is on, null and undefined behave more like void : not assignable to or from anything, except for any , unknown , and void ( undefined is always assignable to void ).

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How to use a nightly build of TypeScript

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Why assignment to property of function parameter is bad style. #1217

zhaoxiongfei commented Dec 21, 2016 • edited Loading

req, res, next) => { req.user = Guest; return next(); };

Often used, can not be avoided.
How should I do?

👍 4 reactions

ljharb commented Dec 21, 2016

Since express does require this, you currently can only disable the rule entirely, on a per-file basis, or on a per-line/per-function basis.

eslint has an option proposed to cover this:

Once that's enabled, we'll likely enable it by default to cover , , and .

👍 1 reaction

Sorry, something went wrong.

zhaoxiongfei commented Dec 27, 2016

Thank you sir.

ljharb commented Dec 27, 2016

(the question's answered, but reopening to track the eslint rule change)

felixsanz commented Dec 30, 2016 • edited Loading

You are not reassigning the parameter, you are just creating a property on the object.

Because of this, you can allow it today with .

ljharb commented Dec 30, 2016

Yes, but is something that should be strictly enforced everywhere, where a framework requires it, like express. The rule change would be for , but with exceptions for , etc.

👍 3 reactions
👎 1 reaction

christianbundy commented Mar 2, 2017

Just a quick update, this has been merged and will be released in the next version of ESLint (which I believe is tomorrow):

shanev commented Mar 7, 2017

Another example is in Koa routing:

.get('/', (ctx) => { ctx.status = 200; });

👍 5 reactions

ljharb commented Mar 16, 2017

The next release of eslint-config-airbnb-base, and likely eslint-config-airbnb, will include this loosening of no-param-reassign.

🎉 5 reactions

jbruni commented Nov 7, 2017

as " " is ok.

as " " means enforces immutability paradigm. It makes a profound design choice for us.

ljharb commented Nov 7, 2017

Airbnb's config makes many profound design choices for you; that's the point. If you don't agree with them, you can easily override rules and/or fork the guide.

kholiavko-roman commented Feb 14, 2019 • edited Loading

Guys does anybody knows how to set up eslit for showing error if I modify value from param array ?
Here is example:

checkIsValid = (id, array) => { for (let i = 0; i < array.length; i++) { const item = array[i]; if (item.param) { item.value = null; item.payment = null; } } return array; };

ljharb commented Feb 14, 2019

that’s pushing the limits of static analysis; you may want to try eslint-plugin-immutable, but it’s not very reliable due to the way the language works.

aboyton commented Feb 15, 2019

Or try TypeScript and something like .

kholiavko-roman commented Feb 15, 2019

Thanks for reply. I think I will try eslint-plugin-immutable for the first.

Successfully merging a pull request may close this issue.

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

assignment to property of function parameter 'column'

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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I want to assign function and parameters to results

The above images show a table row then a child row which get results from ajax. I'm printing whole table instead of json because i want to assign function to each of the numbers then show specific results (more details).

Param1 will be manager id and Param2 will be status like which counter details to show, like in the image Active has count of 203, so when i click on 203 it pass parameters to function then send another ajax call to show more details in modal.

So how to make work with json with function/parameters ?

You can add that with columns.render

Its not in data table, it is in child row. columns.render work with child row ?

No columns.render does not work with Child Detail rows. If you are following the structure in the example the format() function is used to build the HTML for the child row. Datatables has no visibility into what you put in the HTML. You should be able to place any valid HTML you want. If you are having problems please post a link to your page or a test case so we can take a look. https://datatables.net/manual/tech-notes/10#How-to-provide-a-test-case

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Assignment to property of function parameter 'item'

i have that lint error: Assignment to property of function parameter 'item'

What is the correct way to remove this error?

does this help? stackoverflow.com/questions/35637770/… – Alan Yu Commented Nov 16, 2021 at 13:44
2 You are assigning to a property of the parameter. You could use .map and create a new array of new arrays, or disable the rule for this specific functionality. – jonrsharpe Commented Nov 16, 2021 at 13:46
2 Does getData return JSON? Where do you parse that JSON? Also, what's the structure of each item ? Can you actually set the index like that? You should probably be using map rather than forEach for this operation. – Andy Commented Nov 16, 2021 at 13:47
[ [ "07/10/2020", 0.04, 0.05, 0.04 ], [ "07/10/2020", 0.04, 0.06, 0.04 ] ] @Andy thats the return of getData – Giuseppe Fantoni Commented Nov 16, 2021 at 14:04
I try to use the map but I can't use it well – Giuseppe Fantoni Commented Nov 16, 2021 at 14:05

Here's an example with map .

const response = [ [ "07/10/2020", 0.04, 0.05, 0.04 ], [ "07/10/2020", 0.04, 0.06, 0.04 ] ]; // Iterate over the response const data = response.map(item => { // For each item destrucuture the data from // the rest of the elements const [date, ...rest] = item; // Return a new array which has the updated // date, and the other elements then spread out return [ moment(new Date(date)).format('DD/MMM/YY'), ...rest ]; }); console.log(data); <script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.29.1/moment.min.js"></script>

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COMMENTS

Assignment to property of function parameter (no-param-reassign)
10. This is a common ESLint issue that appears frequently on old codebase. You have modified the result variable which was passed as parameter. This behavior is prohibited by the rule. To resolve it, copy the argument to a temporary variable and work on it instead: export const fn = article => article.categoryValueDtoSet.reduce((res, item) => {.
Assignment to property of function parameter no-param-reassign
function createEmployee(emp) { // ⛔️ Assignment to property of function parameter 'emp'. eslint no-param-reassign. emp.name = 'bobby hadz'; emp.salary = 500; return emp; } 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.
How to Assign to the Property of a Function Parameter in JavaScript
You can use assignment to property of function parameter to initialize the value of a parameter. For example, the following code initializes the `name` property of the `greet` function parameter to the value of the `"world"` argument: js. function greet (name) {. name.value = "world"; }
no-param-reassign
If you want to allow assignment to function parameters, then you can safely disable this rule. Strict mode code doesn't sync indices of the arguments object with each parameter binding. Therefore, this rule is not necessary to protect against arguments object mutation in ESM modules or other strict mode functions. Version
No-param-reassign
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 ...
no-param-reassign
Rule Details. This rule aims to prevent unintended behavior caused by modification or reassignment of function parameters. Examples of incorrect code for this rule: /*eslint no-param-reassign: "error"*/ function foo (bar) {. bar = 13; } function foo (bar) {. bar++;
no-param-reassign
Options. 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.. props ...
TypeError: invalid assignment to const "x"
The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned. For instance, in case the content is an object, this means the object itself can still be altered. This means that you can't mutate the value stored in a variable: js.
TypeScript: Documentation
Function Parameter Bivariance. When comparing the types of function parameters, assignment succeeds if either the source parameter is assignable to the target parameter, or vice versa. This is unsound because a caller might end up being given a function that takes a more specialized type, but invokes the function with a less specialized type.
Why eslint throws "Assignment to property of function parameter
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;
Why assignment to property of function parameter is bad style
Why assignment to property of function parameter is bad style. #1217. zhaoxiongfei opened this issue Dec 21, 2016 · 14 comments · Fixed by #1325. Labels. needs eslint rule change/addition semver-patch: loosen/fix/document rules. Comments. Copy link
优雅解决： assignment to property of function parameter 'state'
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。. obj.key = 1; // 可能对象本身就用key的 ...
Assignment could be replaced with operator assignment
Assignment could be replaced with operator assignment. Reports an assignment operation that can be replaced by an operator assignment to make your code shorter and probably clearer. Example: x = x + 3; x = x / 3; After the quick fix is applied the result looks like: x += 3; x /= 3; Locating this inspection By ID
Is it possible to pass a property as a method parameter?
First thing is passing the parameters. You can pass a property as a by-value parameter, because a value is copied from the actual parameter anyway, and a compiler can do it with the property. In contract, out or ref parameter-passing method deal with the reference to the object, which of course cannot be done with the property just because this ...
I want to assign function and parameters to results
I want to assign function and parameters to results. The above images show a table row then a child row which get results from ajax. I'm printing whole table instead of json because i want to assign function to each of the numbers then show specific results (more details). Param1 will be manager id and Param2 will be status like which counter ...
How to solve this assignment to property of function parameter
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Pass parameter's values to a function from marked records
Pass parameter's values to a function from marked records 4 hours ago In my project I have a table created from a POST call to a REST service. ... I want to call the function, pass the value of the key column in the record selected to the function and repopulate the table with the new payload coming back from the REST service. Below is the ...
javascript
You are assigning to a property of the parameter. You could use .map and create a new array of new arrays, or disable the rule for this specific functionality. - jonrsharpe

# Table of Contents

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

# 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

# Additional Resources

Borislav Hadzhiev

How to Assign to the Property of a Function Parameter in JavaScript

3. How to use assignment to property of function parameter?

4. Pitfalls of assignment to property of function parameter

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Why assignment to property of function parameter is bad style. #1217

zhaoxiongfei commented Dec 21, 2016 • edited Loading

ljharb commented Dec 21, 2016

zhaoxiongfei commented Dec 27, 2016

ljharb commented Dec 27, 2016

felixsanz commented Dec 30, 2016 • edited Loading

ljharb commented Dec 30, 2016

christianbundy commented Mar 2, 2017

shanev commented Mar 7, 2017

ljharb commented Mar 16, 2017

jbruni commented Nov 7, 2017

ljharb commented Nov 7, 2017

kholiavko-roman commented Feb 14, 2019 • edited Loading

ljharb commented Feb 14, 2019

aboyton commented Feb 15, 2019

kholiavko-roman commented Feb 15, 2019

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

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Is it possible to pass a property as a method parameter?

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Add your solution here

I want to assign function and parameters to results

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