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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.

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

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 `user` variable:

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. 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

• Arrow function should not return assignment. eslint no-return-assign

Last updated: Mar 7, 2024 Reading time · 4 min

# Table of Contents

• Split the assignment and return statement into 2 lines
• Solving the error in a React.js application
• Trying to return a comparison from a function
• Configuring the no-return-assign ESLint rule
• Disabling the no-return-assign ESLint rule globally
• Disabling the no-return-assign ESLint rule for a single line
• Disabling the no-return-assign ESLint rule for an entire file

# Arrow function should not return assignment. eslint no-return-assign

The ESLint error "Arrow function should not return assignment. eslint no-return-assign" is raised when you return an assignment from an arrow function.

To solve the error, wrap the assignment in curly braces, so it doesn't get returned from the arrow function.

• Return statement should not contain assignment. eslint no-return-assign

Here is an example of how the error occurs.

The issue in the example is that we're returning an assignment from an arrow function.

If you need to mutate a value that is located outside the function, use curly braces and then assign the value in the function's body.

The code sample above doesn't cause any warnings because the arrow function no longer returns the assignment.

Make sure you aren't returning an assignment explicitly as this also causes the error.

You can remove the return statement and assign the value to resolve the issue.

# Split the assignment and return statement into 2 lines

The error also occurs when you try to combine an assignment and a return statement into one.

To resolve the issue, assign a value to the variable on one line and use a return statement on the next.

The example above uses the Array.reduce method.

Notice that we first assign a value to the accumulator variable and then on the next line, return the variable.

Make sure you don't combine the two steps into a single line as it makes your code difficult to read.

# Solving the error in a React.js application

The error is often caused when using refs in older versions of React.js.

The following code causes the error.

To resolve the issue, wrap the assignment in curly braces, so it isn't returned from the arrow function.

Make sure you don't explicitly return the assignment as well.

# Trying to return a comparison from a function

If you meant to return a comparison from a function, use triple equals ( === ) and not a single equal sign.

Three equal signs === are used to compare values and a single equal = sign is used to assign a value to a variable.

# Configuring the no-return-assign ESLint rule

The no-return-assign ESLint rule has 2 values:

• except-parens (default) - disallow assignments unless they are enclosed in parentheses.
• always - disallow all assignments in return statements.

The following examples are all valid if you use the except-parens value.

If the rule's value is set to always , then you all assignments in return statements are disallowed.

# Disabling the no-return-assign ESLint rule globally

If you want to disable the no-return-assign ESLint rule globally, you have to edit your .eslintrc.js config file.

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

Make sure you don't have any trailing commas if you write your config in a JSON file.

# Disabling the no-return-assign ESLint rule for a single line

If you want to disable the no-return-assign rule for a single line, use the following comment.

Make sure to add the comment directly above the line that causes the warning.

# Disabling the no-return-assign ESLint rule for an entire file

If you want to disable the rule for an entire file, use the following comment instead.

Make sure to add the comment at the top of the file or at least above any functions that return assignments.

You can use the same approach to only disable the rule for a specific function.

The first ESLint comment disables the rule and the second comment enables it.

This is why the function on the last line causes the error - it is placed after the comment that enables the no-return-assign rule.

# Additional Resources

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

• React ESLint Error: X is missing in props validation
• eslint is not recognized as an internal or external command
• ESLint: Unexpected lexical declaration in case block [Fixed]
• ESLint error Unary operator '++' used no-plusplus [Solved]
• ESLint Prefer default export import/prefer-default-export
• Assignment to property of function parameter no-param-reassign
• Expected parentheses around arrow function argument arrow-parens
• ESLint: A form label must be associated with a control

Borislav Hadzhiev

Web Developer

Copyright © 2024 Borislav Hadzhiev

JavaScript: Don’t Reassign Your Function Arguments

UPDATE : The point of this post is to raise awareness that reassigning the value of an argument variable mutates the arguments object. The code example is contrived and exists solely to help illustrate that behavior.

Did you know that a JavaScript function’s named parameter variables are synonyms for the corresponding elements in that function’s Arguments object?

I ran into this while experimenting with a function that was written to take either two or three arguments, providing a default for the first argument if only two are passed.

Strangely, all of the values in the result object are set to "green" . I was expecting to see

But when I set favoriteColor to "green" I was also changing arguments[0] to be "green" . The situation only got worse when I set name = arguments[0] effectively changing arguments[1] to be "green" as well.

I had not realized that named arguments are synonyms for the elements of the Arguments object. I found a good explanation on Rx4AJAX:

The numbered properties of the Arguments Object are synonymous with the local variables that hold the named function parameters. They both reference the same address in the stack. If the function body has code that changes the value of a parameter either via a name reference or the arguments[] array reference, both referenced values will reflect the same value.

Regardless of the language, it is generally not a good idea to reassign the parameter variables inside a function. In JavaScript it turns out to be a really bad idea.

Additional information:

• Check out this jsFiddle to experiment with the code snippet above.
• A comment on this StackOverflow question describes this “magical behavior” of JavaScript.
• JavaScript Garden describes this behavior in its section on the arguments object.

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So the arguments object isn’t a “real” array, so you run into problems when you treat it as such.

Here’s a working example where you turn the arguments object into an array with Array.prototype.slice()

http://jsfiddle.net/wookiehangover/yZPj8/4/

This is a pretty common beginner’s mistake and is covered in most advanced books, such as Javascript Patterns or High Performance Javascript.

Here’s a good resource about how the arguments object works: https://developer.mozilla.org/en/JavaScript/Reference/functions_and_function_scope/arguments

If you slice the Arguments, the get aan array, which is not “live”. This way, you can reassign the arguments without any problems. http://jsfiddle.net/Avorin/yZPj8/6/

When I want to pass a varying number of parameters to a function, I either use a predefined object or an object literal myself to begin with (I presume this example function is simplified).

You can also clutter up the function calls with things like makePerson(null, “Joe”, 18) and test for nulls, too, instead of array lengths.

This is the solution I found, using this. instead of an args array. I’m not sure which solution is better.

http://jsfiddle.net/Q2LMT/

Or simply refer to the arguments by name when changing their values.

This article roughly says:

When you misuse the arguments object, unexpected results happen.

The solution: don’t misuse the arguments object. Leave the param list empty and use your logic to fill out variable names if you need that

This is why I love working with Rails… most Rails functions take hashes as arguments, so you can your real arguments in in any order, and it guarantees code verbosity. Example:

button_to ‘Add to Cart’, line_items_path(:product_id => product), :remote => true

where :remote=>true is the third argument, a hash, and contains all optional parameters you could add (in this case, :method, :disabled, :confirm, and :remote).

var makePerson = function(favoriteColor, name, age) { if (arguments.length < 3) { favoriteColor = "green"; name = arguments[0]; age = arguments[1]; } return { name: name, age: age, favoriteColor: (arguments.length < 3 ? "green" : favoriteColor) }; };

How very Perl-ish of Javascript.

Ignore this blog post’s advice. It is perfectly fine to reassign function arguments in Javascript. If you just follow the convention of putting option arguments at the end of the argument list instead of the beginning, you avoid this problem all together and simplify your code:

var makePerson = function(name, age, favoriteColor) { favoriteColor = favoriteColor || “green”; return { name: name, age: age, favoriteColor: favoriteColor }; };

Who makes the first argument optional? Seriously? There are numerous things wrong with your code.

What a terrible programming language.

Larry Clapp, this isn’t perlish at all. In Perl you do named parameters through local variables. They’re duplicated not ref-copied.

use strict; use warnings;

my $makePerson = sub { my ( $favoriteColor, $name, $age ) = @_;

if ( @_ $name , age => $age , favoriteColor => $favoriteColor }

use Data::Dumper; die Dumper $makePerson->(‘Joe’, 18);

What you’re confusing is Perl’s special array variable `@_` which is used to store references to the parameters from the caller, making them accessible in the callee. So the sub implementation themselves are pass-by-reference, but the assignment itself requires a total copy. Not to say you couldn’t achieve the same effect with Perl if you *really wanted too*, but it requires a ton of non-accidental (contrived) work.

my $makePerson = sub { my ( $favoriteColor, $name, $age ) = ( \$_[0], \$_[1], \$_[2] ); #my ( $favoriteColor, $name, $age ) = @_;

if ( length @_ $$name , age => $$age , favoriteColor => $$favoriteColor }

use Data::Dumper; my $name = ‘Joe’; my $age = 18; die Dumper $makePerson->($name, $age);

How about just using a configuration object?

var person = makePerson({name:”Joe”, age:18})

Inside the function look for the property you want to default.

JavaScript reveals more and more of its awful design. NaN != NaN ?????

the problem isn’t with using arguments , the problem is with your use of it.

Writing the code: function example (x, y, z) { x = 1; y = arguments[0]; z = arguments[1]; }

will make every value 1 because I wasn’t very careful about the order of my actions.

As the article you quoted states, the variables x, y, and z are synonymous with arguments [0], [1], and [2] respectively, so if I called example(3,4) all I would be doing in my function is assigning 3 to x and 4 to y with the function call, then assigning 1 to x, the value of x to y, then the value of y to z. All of my values would be the same (and 1) afterwards.

You do the same thing in your code. You pass in (favoriteColor: Joe, name: 18) and then set the favoriteColor to “green” before taking the value of “green” and pasting it on to the name, then taking the new value of name (also “green”) and pasting it in to the value of age. If you had instead written that code in the reverse order, it would have worked as you had initially expected.

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There are special edge cases like Array.prototype.reduce where assign to accumulator argument passed between iterations is only good practice:

const aggregate = someArray.reduce((acc, item) => { acc[item.prop] = (acc[item.prop] || 0) + 1; }, {} /* this is initial state */);

Choose Parameters or Arguments….but using both is asking for trouble.

If you are using Parameters defined in the Function signature, then you have no need to refer to the arguments information.

If you plan on using arguments, then do not define Parameters.

Mixing the two, is asking for problems and the reason for the overall purpose of this post.

Comments are closed.

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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 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

JS Tutorial

Js versions, js functions, js html dom, js browser bom, js web apis, js vs jquery, js graphics, js examples, js references, javascript function parameters.

A JavaScript function does not perform any checking on parameter values (arguments).

Function Parameters and Arguments

Earlier in this tutorial, you learned that functions can have parameters :

Function parameters are the names listed in the function definition.

Function arguments are the real values passed to (and received by) the function.

Parameter Rules

JavaScript function definitions do not specify data types for parameters.

JavaScript functions do not perform type checking on the passed arguments.

JavaScript functions do not check the number of arguments received.

Default Parameters

If a function is called with missing arguments (less than declared), the missing values are set to undefined .

Sometimes this is acceptable, but sometimes it is better to assign a default value to the parameter:

Default Parameter Values

ES6 allows function parameters to have default values.

If y is not passed or undefined, then y = 10.

Function Rest Parameter

The rest parameter (...) allows a function to treat an indefinite number of arguments as an array:

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The Arguments Object

JavaScript functions have a built-in object called the arguments object.

The argument object contains an array of the arguments used when the function was called (invoked).

This way you can simply use a function to find (for instance) the highest value in a list of numbers:

Or create a function to sum all input values:

If a function is called with too many arguments (more than declared), these arguments can be reached using the arguments object .

Arguments are Passed by Value

The parameters, in a function call, are the function's arguments.

JavaScript arguments are passed by value : The function only gets to know the values, not the argument's locations.

If a function changes an argument's value, it does not change the parameter's original value.

Changes to arguments are not visible (reflected) outside the function.

Objects are Passed by Reference

In JavaScript, object references are values.

Because of this, objects will behave like they are passed by reference:

If a function changes an object property, it changes the original value.

Changes to object properties are visible (reflected) outside the function.

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Default parameters

Default function parameters allow named parameters to be initialized with default values if no value or undefined is passed.

Description

In JavaScript, function parameters default to undefined . However, it's often useful to set a different default value. This is where default parameters can help.

In the following example, if no value is provided for b when multiply is called, b 's value would be undefined when evaluating a * b and multiply would return NaN .

In the past, the general strategy for setting defaults was to test parameter values in the function body and assign a value if they are undefined . In the following example, b is set to 1 if multiply is called with only one argument:

With default parameters, checks in the function body are no longer necessary. Now, you can assign 1 as the default value for b in the function head:

Parameters are still set left-to-right, overwriting default parameters even if there are later parameters without defaults.

Note: The first default parameter and all parameters after it will not contribute to the function's length .

The default parameter initializers live in their own scope, which is a parent of the scope created for the function body.

This means that earlier parameters can be referred to in the initializers of later parameters. However, functions and variables declared in the function body cannot be referred to from default value parameter initializers; attempting to do so throws a run-time ReferenceError . This also includes var -declared variables in the function body.

For example, the following function will throw a ReferenceError when invoked, because the default parameter value does not have access to the child scope of the function body:

This function will print the value of the parameter a , because the variable var a is hoisted only to the top of the scope created for the function body, not the parent scope created for the parameter list, so its value is not visible to b .

The default parameter allows any expression, but you cannot use await or yield that would pause the evaluation of the default expression. The parameter must be initialized synchronously .

Note: Because the default parameter is evaluated when the function is called, not when the function is defined, the validity of the await and yield operators depends on the function itself, not its surrounding function. For example, if the current function is not async , await will be parsed as an identifier and follow normal identifier syntax rules , even when this function is nested in an async function.

Passing undefined vs. other falsy values

In the second call in this example, even if the first argument is set explicitly to undefined (though not null or other falsy values), the value of the num argument is still the default.

Evaluated at call time

The default argument is evaluated at call time . Unlike with Python (for example), a new object is created each time the function is called.

This even applies to functions and variables:

Earlier parameters are available to later default parameters

Parameters defined earlier (to the left) are available to later default parameters:

This functionality can be approximated like this, which demonstrates how many edge cases are handled:

Destructured parameter with default value assignment

You can use default value assignment with the destructuring assignment syntax.

A common way of doing that is to set an empty object/array as the default value for the destructured parameter; for example: [x = 1, y = 2] = [] . This makes it possible to pass nothing to the function and still have those values prefilled:

Specifications

Browser compatibility

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

• Functions guide
• Rest parameters
• Nullish coalescing operator ( ?? )

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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.

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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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Destructuring assignment

The two most used data structures in JavaScript are Object and Array .

• Objects allow us to create a single entity that stores data items by key.
• Arrays allow us to gather data items into an ordered list.

However, when we pass these to a function, we may not need all of it. The function might only require certain elements or properties.

Destructuring assignment is a special syntax that allows us to “unpack” arrays or objects into a bunch of variables, as sometimes that’s more convenient.

Destructuring also works well with complex functions that have a lot of parameters, default values, and so on. Soon we’ll see that.

Array destructuring

Here’s an example of how an array is destructured into variables:

Now we can work with variables instead of array members.

It looks great when combined with split or other array-returning methods:

As you can see, the syntax is simple. There are several peculiar details though. Let’s see more examples to understand it better.

It’s called “destructuring assignment,” because it “destructurizes” by copying items into variables. However, the array itself is not modified.

It’s just a shorter way to write:

Unwanted elements of the array can also be thrown away via an extra comma:

In the code above, the second element of the array is skipped, the third one is assigned to title , and the rest of the array items are also skipped (as there are no variables for them).

…Actually, we can use it with any iterable, not only arrays:

That works, because internally a destructuring assignment works by iterating over the right value. It’s a kind of syntax sugar for calling for..of over the value to the right of = and assigning the values.

We can use any “assignables” on the left side.

For instance, an object property:

In the previous chapter, we saw the Object.entries(obj) method.

We can use it with destructuring to loop over the keys-and-values of an object:

The similar code for a Map is simpler, as it’s iterable:

There’s a well-known trick for swapping values of two variables using a destructuring assignment:

Here we create a temporary array of two variables and immediately destructure it in swapped order.

We can swap more than two variables this way.

The rest ‘…’

Usually, if the array is longer than the list at the left, the “extra” items are omitted.

For example, here only two items are taken, and the rest is just ignored:

If we’d like also to gather all that follows – we can add one more parameter that gets “the rest” using three dots "..." :

The value of rest is the array of the remaining array elements.

We can use any other variable name in place of rest , just make sure it has three dots before it and goes last in the destructuring assignment.

Default values

If the array is shorter than the list of variables on the left, there will be no errors. Absent values are considered undefined:

If we want a “default” value to replace the missing one, we can provide it using = :

Default values can be more complex expressions or even function calls. They are evaluated only if the value is not provided.

For instance, here we use the prompt function for two defaults:

Please note: the prompt will run only for the missing value ( surname ).

Object destructuring

The destructuring assignment also works with objects.

The basic syntax is:

We should have an existing object on the right side, that we want to split into variables. The left side contains an object-like “pattern” for corresponding properties. In the simplest case, that’s a list of variable names in {...} .

For instance:

Properties options.title , options.width and options.height are assigned to the corresponding variables.

The order does not matter. This works too:

The pattern on the left side may be more complex and specify the mapping between properties and variables.

If we want to assign a property to a variable with another name, for instance, make options.width go into the variable named w , then we can set the variable name using a colon:

The colon shows “what : goes where”. In the example above the property width goes to w , property height goes to h , and title is assigned to the same name.

For potentially missing properties we can set default values using "=" , like this:

Just like with arrays or function parameters, default values can be any expressions or even function calls. They will be evaluated if the value is not provided.

In the code below prompt asks for width , but not for title :

We also can combine both the colon and equality:

If we have a complex object with many properties, we can extract only what we need:

The rest pattern “…”

What if the object has more properties than we have variables? Can we take some and then assign the “rest” somewhere?

We can use the rest pattern, just like we did with arrays. It’s not supported by some older browsers (IE, use Babel to polyfill it), but works in modern ones.

It looks like this:

In the examples above variables were declared right in the assignment: let {…} = {…} . Of course, we could use existing variables too, without let . But there’s a catch.

This won’t work:

The problem is that JavaScript treats {...} in the main code flow (not inside another expression) as a code block. Such code blocks can be used to group statements, like this:

So here JavaScript assumes that we have a code block, that’s why there’s an error. We want destructuring instead.

To show JavaScript that it’s not a code block, we can wrap the expression in parentheses (...) :

Nested destructuring

If an object or an array contains other nested objects and arrays, we can use more complex left-side patterns to extract deeper portions.

In the code below options has another object in the property size and an array in the property items . The pattern on the left side of the assignment has the same structure to extract values from them:

All properties of options object except extra which is absent in the left part, are assigned to corresponding variables:

Finally, we have width , height , item1 , item2 and title from the default value.

Note that there are no variables for size and items , as we take their content instead.

Smart function parameters

There are times when a function has many parameters, most of which are optional. That’s especially true for user interfaces. Imagine a function that creates a menu. It may have a width, a height, a title, an item list and so on.

Here’s a bad way to write such a function:

In real-life, the problem is how to remember the order of arguments. Usually, IDEs try to help us, especially if the code is well-documented, but still… Another problem is how to call a function when most parameters are ok by default.

That’s ugly. And becomes unreadable when we deal with more parameters.

Destructuring comes to the rescue!

We can pass parameters as an object, and the function immediately destructurizes them into variables:

We can also use more complex destructuring with nested objects and colon mappings:

The full syntax is the same as for a destructuring assignment:

Then, for an object of parameters, there will be a variable varName for the property incomingProperty , with defaultValue by default.

Please note that such destructuring assumes that showMenu() does have an argument. If we want all values by default, then we should specify an empty object:

We can fix this by making {} the default value for the whole object of parameters:

In the code above, the whole arguments object is {} by default, so there’s always something to destructurize.

Destructuring assignment allows for instantly mapping an object or array onto many variables.

The full object syntax:

This means that property prop should go into the variable varName and, if no such property exists, then the default value should be used.

Object properties that have no mapping are copied to the rest object.

The full array syntax:

The first item goes to item1 ; the second goes into item2 , and all the rest makes the array rest .

It’s possible to extract data from nested arrays/objects, for that the left side must have the same structure as the right one.

We have an object:

Write the destructuring assignment that reads:

• name property into the variable name .
• years property into the variable age .
• isAdmin property into the variable isAdmin (false, if no such property)

Here’s an example of the values after your assignment:

The maximal salary

There is a salaries object:

Create the function topSalary(salaries) that returns the name of the top-paid person.

• If salaries is empty, it should return null .
• If there are multiple top-paid persons, return any of them.

P.S. Use Object.entries and destructuring to iterate over key/value pairs.

Open a sandbox with tests.

Open the solution with tests in a sandbox.

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How to solve this assignment to property of function parameter

Can anyone help me with this function too,in below function I am getting an error of

How can I solve this above error with below function? How can I re-write the function without changing the function logic? And solve the eslint error too.

Usage example - for the above function

• ecmascript-6

• what is the expected output? –  Mulan Commented Sep 23, 2020 at 14:42

you can assign passing variables to some other variables.

Snippet below.

const data = { executed:[ {_id: "5f23d394cd 480e300", field: "name", value: "Jolly", operator: "equal"}, {_id: "5f30d39f4cd8d0e301", field: "status", value: "EXPIRED", operator: "equal"}, {_id: "5f230d39001480e302", field: "grp", value: "874-3-11-4-56", operator: "equal"}, {_id: "59f4cd8d001480e303", field: "grp", value: "873-5-12-4-77", operator: "equal"} ], created_at: "2020-07-30T18:11:05.992Z", name: "Kind Find", _id: "1f230d39f4cd8d441480e2dd" } console.log( data.executed.reduce((carryValue, currentValue) => { const current = currentValue; const carry = carryValue; if (current.field === 'grp') { carry[current.field] = (carry[current.field] || []).concat(current.value); } else { carry[current.field] = carry[current.field] ? [carry[current.field], current.value] : current.value; } return carry; }, {}) );

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