javascript bracket assignment

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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Category Software Engineering

Published Feb 7, 2015

Updated Feb 9, 2015

Sections Article Purpose What Bracket Notation Is Practical Examples Succinct Update Listeners Variable Method Calls Object Properties Iteration Succinct Cache Check

Author Derek Knox

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JavaScript Bracket Notation - Practical Examples

Article purpose.

There are numerous examples online and in books that accurately describe the difference between dot notation and bracket notation in JavaScript. What I find lacking however are practical examples of the less common bracket notation. The purpose of this article is to provide practical examples of using bracket notation.

What Bracket Notation Is

Bracket notation enables a developer to access an object’s properties in JavaScript. It is an alternative to the more common dot notation. The detailed breakdown of each notation is littered across the internet; take your pick to learn more:

• Mozilla Developer Network
• Dot Notation and Square Bracket Notation in JavaScript
• JS dot-notation vs. bracket notation

Practical Examples

Bracket notation is a useful tool. The examples below provide practical applications of its use. If you have improvement ideas or additional application examples, just let me know ( @derekknox or derek [at] derekknox.com) and I’ll update the examples.

Succinct Update Listeners

The main idea behind this example is to determine which of two methods to call and call it in a succint and DRY fashion. When reading code, I commonly see an addEventListeners() method and a corresponding removeEventListeners() method when a single combined method could be created using bracket notation. In the updateListener() method below I'm specifically using 'addEventListener' and 'removeEventListener' , but you could instead use the 'on' and 'off' methods of jQuery if you were using that library.

This particular approach is abstract enough that it could be leveraged through a Utils class of some kind for all your elements' listeners. Also, it could easily be expanded to take numerous event types and corresponding handlers via arrays.

Variable Method Calls

This example expands on the Succinct Update Listeners example. The main difference is that there are more than two potential methods to call and the logic determining which to call can be random or more complex. The example below communicates the approach through an automated drawing program.

Object Properties Iteration

The implementation of this example is found in many places, but it can specifically be found in libraries like jQuery and in design patterns like the Mixin pattern. The main idea is that a developer may not know (or care) what properties exist on an object, but he/she needs access to them. By leveraging bracket notation, it is possible to access each property without knowing its name. The below example is sourced from Addy Osmani in his description of the Mixin pattern . Addy is creating a method called augment() which allows the properties of one object's prototype to be copied (via bracket notation) to another object's prototype. This is a form of inheritance.

Succinct Cache Check

This example is related to the Object Properties Iteration example above in its bracket notation use. The main idea here is that a developer can succintly gain reference to a previously created object via a cache. If there is no cached object, it is created and then cached. This is an approach that delays the creation of objects until needed (if ever). This approach works best when there is only a single instance of a particular object, but this is not required. The example below demonstrates a drawing program's requestBrush() method that attempts to return a cached brush. If there is no cached brush, one is created JIT and returned. Bracket notation is leveraged instead of an inferior and lengthy if/else chain or switch/case statement.

Other Ideas?

If you have improvement ideas or additional application examples, just let me know ( @derekknox or derek [at] derekknox.com) and I’ll update the examples.

HatchJS.com

Cracking the Shell of Mystery

Curly Braces in JavaScript: What They Are and How to Use Them

Curly Braces in JavaScript: A Comprehensive Guide

Curly braces are one of the most fundamental elements of JavaScript. They are used to denote the start and end of blocks of code, and they also play a role in defining functions, objects, and arrays. In this comprehensive guide, we will take a close look at curly braces in JavaScript, exploring their different uses and how they can be used to improve your code.

We will start by discussing the basics of curly braces, including how to use them to define blocks of code and how to nest them. We will then move on to look at how curly braces are used to define functions, objects, and arrays. Finally, we will discuss some of the common mistakes that people make with curly braces and how to avoid them.

By the end of this guide, you will have a solid understanding of curly braces in JavaScript and how to use them effectively. You will also be able to spot and correct common mistakes that people make with curly braces.

So let’s get started!

Curly braces are one of the most important symbols in JavaScript. They are used to define blocks of code, and they can also be used to group together related code statements.

In this tutorial, we will learn all about curly braces in JavaScript. We will cover the following topics:

What are curly braces in JavaScript?

• How to use curly braces in JavaScript

Common mistakes with curly braces

By the end of this tutorial, you will have a solid understanding of curly braces and how to use them effectively in your JavaScript code.

Curly braces are used to define blocks of code in JavaScript. A block of code is a group of statements that are executed together.

To define a block of code, you simply need to enclose the statements within curly braces. For example:

var x = 10; y = x + 10;

This code defines two statements:

• The first statement assigns the value 10 to the variable `x`.
• The second statement assigns the value of `x` + 10 to the variable `y`.

The two statements are executed together as a single unit.

How to use curly braces in JavaScript?

Curly braces are used in JavaScript for a variety of purposes. They can be used to define functions, loops, and conditional statements.

• To define a function, use the following syntax:

function myFunction() { // code to be executed when the function is called }

• To define a loop, use the following syntax:
• To define a conditional statement, use the following syntax:

if (condition) { // code to be executed if the condition is true } else { // code to be executed if the condition is false }

One of the most common mistakes with curly braces is forgetting to close them. This can cause your code to not work correctly.

For example, the following code will not work:

This code is missing the closing curly brace for the second statement. As a result, the second statement will not be executed.

Another common mistake with curly braces is using them incorrectly. For example, you cannot use curly braces to define a variable.

The following code is incorrect:

var x = 10; { var y = x + 10; }

This code attempts to define the variable `y` inside a curly brace block. However, this is not allowed.

Curly braces are an important part of JavaScript. They are used to define blocks of code, and they can also be used to group together related code statements.

By understanding how to use curly braces, you can write more efficient and readable JavaScript code.

Additional resources

• [JavaScript Tutorial: Curly Braces](https://www.w3schools.com/js/js_curly_braces.asp)
• [JavaScript Guide: Curly Braces](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/curly_braces)
• [Stack Overflow: Curly Braces in JavaScript](https://stackoverflow.com/questions/161037/curly-braces-in-javascript)

Curly Braces in JavaScript

Curly braces ({}) are used to define blocks of code in JavaScript. They are also used to group related statements together. For example, the following code defines a function that adds two numbers together:

function add(a, b) { return a + b; }

The curly braces in this code define the block of code that makes up the function definition. The statements inside the curly braces are executed when the function is called.

Curly braces are also used to group related statements together in conditional statements and loops. For example, the following code uses a conditional statement to check if a number is greater than zero:

if (number > 0) { console.log(“The number is greater than zero.”); }

The curly braces in this code group together the statements that are executed if the condition is true.

Curly braces are an important part of JavaScript syntax. They are used to define blocks of code and group related statements together. By using curly braces correctly, you can make your code more readable and easier to debug.

Common Mistakes when Using Curly Braces in JavaScript

There are a few common mistakes that people make when using curly braces in JavaScript. These mistakes can cause errors in your code and make it difficult to debug.

**One common mistake is to forget to close a curly brace.** This can cause errors in your code because the JavaScript interpreter will not know where the block of code ends. For example, the following code will cause an error:

The error in this code is that the curly brace after the `b` parameter is missing. This means that the JavaScript interpreter will not know where the function definition ends.

**Another common mistake is to use the wrong type of curly brace.** JavaScript uses curly braces to define blocks of code, so you should always use curly braces, not parentheses or brackets. For example, the following code will cause an error:

function add(a, b) { (return a + b); }

The error in this code is that the parentheses are used instead of curly braces. This means that the JavaScript interpreter will not know where the function definition ends.

**Finally, it is important to make sure that your curly braces are properly nested.** This means that each curly brace should be matched with another curly brace. For example, the following code is not properly nested:

function add(a, b) { if (a > b) { console.log(“a is greater than b”); } }

The error in this code is that the curly brace after the `b` parameter is not matched with another curly brace. This means that the JavaScript interpreter will not know where the conditional statement ends.

Tips for Using Curly Braces in JavaScript

Here are a few tips for using curly braces in JavaScript:

• Always use the same number of curly braces on each side of a function name, loop keyword, or conditional statement. This will help to make your code more readable and easier to debug.
• Always use curly braces, not parentheses or brackets, to define blocks of code. This will help to prevent errors in your code.
• Make sure that your curly braces are properly nested. This will help to make your code more readable and easier to debug.

Q: What are curly braces in JavaScript?

A: Curly braces in JavaScript are used to define the scope of a block of code. They are also used to group together statements that are to be executed as a single unit.

Q: Why do we need curly braces in JavaScript?

A: Curly braces are needed in JavaScript to define the scope of a block of code. This is important because it helps to prevent errors and to make your code more readable.

Q: What is the difference between curly braces and parentheses in JavaScript?

A: Curly braces are used to define the scope of a block of code, while parentheses are used to group together arguments to a function.

Q: Can I use curly braces without parentheses in JavaScript?

A: Yes, you can use curly braces without parentheses in JavaScript. However, it is not recommended, as it can make your code more difficult to read and understand.

Q: What is the correct way to use curly braces in JavaScript?

A: The correct way to use curly braces in JavaScript is to enclose the code that you want to be executed as a single unit within the braces. For example:

function myFunction() { // Code to be executed }

Q: What are some common mistakes people make when using curly braces in JavaScript?

A: Some common mistakes people make when using curly braces in JavaScript include:

• Forgetting to close the braces
• Using the wrong type of brace (curly or square)
• Nesting braces incorrectly
• Using braces to group together statements that should not be grouped together

Q: How can I avoid making mistakes when using curly braces in JavaScript?

A: To avoid making mistakes when using curly braces in JavaScript, you can:

• Be careful to close the braces every time you open them
• Use the right type of brace (curly or square)
• Nest braces correctly
• Use braces only to group together statements that should be grouped together

Q: Where can I learn more about curly braces in JavaScript?

A: You can learn more about curly braces in JavaScript by reading the following resources:

• [The Mozilla Developer Network (MDN)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/curly_braces)
• [W3Schools](https://www.w3schools.com/js/js_controlflow_block.asp)

Here are some key takeaways regarding curly braces in JavaScript:

• Curly braces are used to define the scope of blocks of code.
• Curly braces can be used to group related statements together.
• Curly braces can be used to create functions, control statements, and classes.
• By understanding how curly braces work, you can write more efficient and readable JavaScript code.

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What is the use of curly brackets in the `var { … } = …` statements ?

Destructuring assignment allows us to assign the properties of an array or object to a bunch of variables that are sometimes very convenient and short. Consider the following illustration. Both of the below-mentioned methods are right and produce the same result.

Without Destructuring:

With Destructuring 

Object Destructuring Destructuring can be done on JavaScript objects. On the left side of the assignment operator, there is a pattern of variables in which the properties of an object are to be stored. The variable’s name must be the same as defined in the object. Let’s have a look at this concept from the following example. 

Note: Curly brackets ‘{ }’ are used to destructure the JavaScript Object properties.

Example:  

If we want the variables defined in the object should be assigned to the variables with other names then we can set it using a colon.

Array Destructuring: The elements of the array can also be destructured in the same way. The destructuring assignment can be used to assign the array values to a bunch of different variables in JavaScript. 

Note: Square brackets ‘[ ]’ are used to destructure the array elements.

Output:  

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JS: Dot and Bracket Notation

Learning goals.

• Use dot and bracket notation to access objects
• Compare expressions in dot and bracket notation and identify equivalent expressions
• Determine appropriate use cases for each notation
• Apply our knowledge of each notation to gain a deeper understanding about a familiar concept
• Object An unordered collection of related data in the form of key value pairs. JavaScript provides two notations for accessing object properties…
• Dot Notation A property in an object is accessed by giving the object’s name, followed by a period, followed by the property name (Example: user.name )
• Bracket Notation The object name is followed by a set of square brackets, with the property name specified inside the brackets via either a string (Example: user['name'] ) or a variable (Example: user[name] )

Watch the first 7 minutes of this video about dot and bracket notation.

• Independently, complete the exercise found on this repl .
• When you finish, use the zoom chat to DM me your answer to this question: Which of the following expressions is equivalent to car.brand ? A. car[brand] B. car['brand']

Equivalent Expressions

Objects are a key piece of working with JavaScript. In order for objects to be helpful for us, we have to be able to access the information inside of the objects. To do this, we can use dot notation or bracket notation .

Here are some examples of dot notation :

house.address student.gradeLevel school.classroom.teacher

Here are equivalent expressions in bracket notation :

house[‘address’] student[‘gradeLevel’] school[‘classroom’][‘teacher’]

Stop and Think

• What differences do you notice in the way each notation is written?

Key Point #1

You can write equivalent expressions using dot and bracket notation. For example: object.property is equivalent to object['property'] .

As you have seen in a couple of examples today, you can chain multiple properties onto an expression in order to dig in deeper on an object. Let’s take this object, for example:

If we wanted to access this user’s zip code using dot notation , we could write: user.contactInfo.address.zip

In order to access their zip using bracket notation , we would write: user[‘contactInfo’][‘address’][‘zip’]

You can also mix and match! We could write something like this and it would work: user.contactInfo[‘address’].zip

Notice how each block is formatted: user .contactInfo [‘address’] .zip

Paired Practice

• In a breakout room, complete the exercises found on this repl .
• When you finish, discuss this question with your partner: In your opinion, which notation is easier to read and write?
• Be prepared to discuss as a whole group after. Write down any questions that pop up along the way so we can discuss as a group!

Key Point #2

Whenever it is possible, we will default to using dot notation.

Bracket Notation & Variables

Dot notation only works if you have access to the exact property name. Dot notation interprets the expression literally. Meaning, it will look for the actual property that is passed in. Let’s take, for example, this code block :

If we ran the command below, we would get undefined. This is because the JavaScript interpreter is looking for a property that is literally called “lookupField” and it does not exist:

The same would happen in this case:

We can use bracket notation in our favor, by passing in the variable, like the example below. In this case, the interpreter will evaluate whats between the brackets, register lookupField as a variable and then pass in it’s value of ‘greeting’ to get the output of ‘hello’ :

If we reassigned the value of lookupField and then ran the same command as above, we’d get a new output:

Remember: a variable represents some other value, and that value could be reassigned/change over time. This means dot notation is not an option when using a variable to represent the object’s key we are trying to access because to use dot notation, we must be able to type out the exact letter-by-letter name the key. Bracket notation gives us the ability to use variables to access values in an object. This is especially helpful with the value of the variable changes.

Key Point #3

We must use bracket notation whenever we are accessing an object’s property using a variable or when the property’s key is a number or includes a symbol or is two words with a space .

Take a moment to read through this code:

What will be returned from the checkForFood function?

Applying What We’ve learned

Even if these concepts are new to you, you’ve actually been putting them into practice for awhile now! Let’s take a deeper look into something familiar to you: for loops .

Key Point #4

When we use dot notation, the JS interpreter looks in the object for a key that is an exact letter-by-letter literal match to whatever comes after the dot. When we use bracket notation, the JS interpreter evaluates everything between the brackets, then looks in the object for a key that matches whatever the code between the brackets evaluated to.

In your notebook, answer the following questions:

• How does the JavaScript interpreter handle dot and bracket notation differently?
• When should you use dot notation? Bracket notation?
• What is a limitation of using dot notation? How does bracket notation address this?

Homework (Optional and Spicy!)

• Complete the code challenges found on this repl . These are tough! Do what you can. Stuck? Look here .
• Answer the questions found in the JavaScript section of this codepen .

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Object initializer

An object initializer is a comma-delimited list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ( {} ). Objects can also be initialized using Object.create() or by invoking a constructor function with the new operator.

Description

An object initializer is an expression that describes the initialization of an Object . Objects consist of properties , which are used to describe an object. The values of object properties can either contain primitive data types or other objects.

Object literal syntax vs. JSON

The object literal syntax is not the same as the J ava S cript O bject N otation ( JSON ). Although they look similar, there are differences between them:

• JSON only permits property definition using the "property": value syntax. The property name must be double-quoted, and the definition cannot be a shorthand. Computed property names are not allowed either.
• JSON object property values can only be strings, numbers, true , false , null , arrays, or another JSON object. This means JSON cannot express methods or non-plain objects like Date or RegExp .
• In JSON, "__proto__" is a normal property key. In an object literal, it sets the object's prototype .

JSON is a strict subset of the object literal syntax, meaning that every valid JSON text can be parsed as an object literal, and would likely not cause syntax errors. The only exception is that the object literal syntax prohibits duplicate __proto__ keys, which does not apply to JSON.parse() . The latter treats __proto__ like a normal property and takes the last occurrence as the property's value. The only time when the object value they represent (a.k.a. their semantic) differ is also when the source contains the __proto__ key — for object literals, it sets the object's prototype; for JSON, it's a normal property.

Creating objects

An empty object with no properties can be created like this:

However, the advantage of the literal or initializer notation is, that you are able to quickly create objects with properties inside the curly braces. You notate a list of key: value pairs delimited by commas.

The following code creates an object with three properties and the keys are "foo" , "age" and "baz" . The values of these keys are a string "bar" , the number 42 , and another object.

Accessing properties

Once you have created an object, you might want to read or change them. Object properties can be accessed by using the dot notation or the bracket notation. (See property accessors for detailed information.)

Property definitions

We have already learned how to notate properties using the initializer syntax. Oftentimes, there are variables in your code that you would like to put into an object. You will see code like this:

There is a shorter notation available to achieve the same:

Duplicate property names

When using the same name for your properties, the second property will overwrite the first.

After ES2015, duplicate property names are allowed everywhere, including strict mode . You can also have duplicate property names in classes . The only exception is private properties , which must be unique in the class body.

• Method definitions

A property of an object can also refer to a function or a getter or setter method.

A shorthand notation is available, so that the keyword function is no longer necessary.

There is also a way to concisely define generator methods.

Which is equivalent to this ES5-like notation (but note that ECMAScript 5 has no generators):

For more information and examples about methods, see method definitions .

Computed property names

The object initializer syntax also supports computed property names. That allows you to put an expression in square brackets [] , that will be computed and used as the property name. This is reminiscent of the bracket notation of the property accessor syntax, which you may have used to read and set properties already.

Now you can use a similar syntax in object literals, too:

Spread properties

Object literals support the spread syntax . It copies own enumerable properties from a provided object onto a new object.

Shallow-cloning (excluding prototype ) or merging objects is now possible using a shorter syntax than Object.assign() .

Warning: Note that Object.assign() triggers setters , whereas the spread syntax doesn't!

Prototype setter

A property definition of the form __proto__: value or "__proto__": value does not create a property with the name __proto__ . Instead, if the provided value is an object or null , it points the [[Prototype]] of the created object to that value. (If the value is not an object or null , the object is not changed.)

Note that the __proto__ key is standardized syntax, in contrast to the non-standard and non-performant Object.prototype.__proto__ accessors. It sets the [[Prototype]] during object creation, similar to Object.create — instead of mutating the prototype chain.

Only a single prototype setter is permitted in an object literal. Multiple prototype setters are a syntax error.

Property definitions that do not use "colon" notation are not prototype setters. They are property definitions that behave identically to similar definitions using any other name.

Specifications

Browser compatibility

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

• Property accessors
• Lexical grammar

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what is the point of parentheses around assignment expression in javascript [duplicate]

I read the following code here:

What is the difference between that and the following:

Considering that they have the same outcome.

P.S. The point is the parentheses around the assignment expression

Your second example shouldn't work. You should see an error.

The reason is that the curly brackets { a, b } are considered a block not an object literal.

When surounded by parentheses it's considered an object literal

Note: The parentheses ( ... ) around the assignment statement are required when using object literal destructuring assignment without a declaration. {a, b} = {a: 1, b: 2} is not valid stand-alone syntax, as the {a, b} on the left-hand side is considered a block and not an object literal. However, ({a, b} = {a: 1, b: 2}) is valid, as is const {a, b} = {a: 1, b: 2} Your ( ... ) expression needs to be preceded by a semicolon or it may be used to execute a function on the previous line.

Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#assignment_without_declaration

Not the answer you're looking for? Browse other questions tagged javascript or ask your own question .

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