js property assignment

11 Properties: assignment vs. definition

• 11.1.1  Assignment
• 11.1.2  Definition
• 11.2.1  Assigning to a property
• 11.2.2  Defining a property
• 11.3.1  Only definition allows us to create a property with arbitrary attributes
• 11.3.2  The assignment operator does not change properties in prototypes
• 11.3.3  Assignment calls setters, definition doesn’t
• 11.3.4  Inherited read-only properties prevent creating own properties via assignment
• 11.4.1  The properties of an object literal are added via definition
• 11.4.2  The assignment operator = always uses assignment
• 11.4.3  Public class fields are added via definition
• 11.5  Further reading and sources of this chapter

There are two ways of creating or changing a property prop of an object obj :

• Assigning : obj.prop = true
• Defining : Object.defineProperty(obj, '', {value: true})

This chapter explains how they work.

For this chapter, you should be familiar with property attributes and property descriptors. If you aren’t, check out §9 “Property attributes: an introduction” .

11.1 Assignment vs. definition

11.1.1 assignment.

We use the assignment operator = to assign a value value to a property .prop of an object obj :

This operator works differently depending on what .prop looks like:

Changing properties: If there is an own data property .prop , assignment changes its value to value .

Invoking setters: If there is an own or inherited setter for .prop , assignment invokes that setter.

Creating properties: If there is no own data property .prop and no own or inherited setter for it, assignment creates a new own data property.

That is, the main purpose of assignment is making changes. That’s why it supports setters.

11.1.2 Definition

To define a property with the key propKey of an object obj , we use an operation such as the following method:

This method works differently depending on what the property looks like:

• Changing properties: If an own property with key propKey exists, defining changes its property attributes as specified by the property descriptor propDesc (if possible).
• Creating properties: Otherwise, defining creates an own property with the attributes specified by propDesc (if possible).

That is, the main purpose of definition is to create an own property (even if there is an inherited setter, which it ignores) and to change property attributes.

11.2 Assignment and definition in theory (optional)

In specification operations, property descriptors are not JavaScript objects but Records , a spec-internal data structure that has fields . The keys of fields are written in double brackets. For example, Desc.[[Configurable]] accesses the field .[[Configurable]] of Desc . These records are translated to and from JavaScript objects when interacting with the outside world.

11.2.1 Assigning to a property

The actual work of assigning to a property is handled via the following operation in the ECMAScript specification:

These are the parameters:

• O is the object that is currently being visited.
• P is the key of the property that we are assigning to.
• V is the value we are assigning.
• Receiver is the object where the assignment started.
• ownDesc is the descriptor of O[P] or null if that property doesn’t exist.

The return value is a boolean that indicates whether or not the operation succeeded. As explained later in this chapter , strict-mode assignment throws a TypeError if OrdinarySetWithOwnDescriptor() fails.

This is a high-level summary of the algorithm:

• It traverses the prototype chain of Receiver until it finds a property whose key is P . The traversal is done by calling OrdinarySetWithOwnDescriptor() recursively. During recursion, O changes and points to the object that is currently being visited, but Receiver stays the same.
• Depending on what the traversal finds, an own property is created in Receiver (where recursion started) or something else happens.

In more detail, this algorithm works as follows:

If O has a prototype parent , then we return parent.[[Set]](P, V, Receiver) . This continues our search. The method call usually ends up invoking OrdinarySetWithOwnDescriptor() recursively.

Otherwise, our search for P has failed and we set ownDesc as follows:

With this ownDesc , the next if statement will create an own property in Receiver .

• If ownDesc.[[Writable]] is false , return false . This means that any non-writable property P (own or inherited!) prevents assignment.
• The current object O and the current property descriptor ownDesc on one hand.
• The original object Receiver and the original property descriptor existingDescriptor on the other hand.
• (If we get here, then we are still at the beginning of the prototype chain – we only recurse if Receiver does not have a property P .)
• If existingDescriptor specifies an accessor, return false .
• If existingDescriptor.[[Writable]] is false , return false .
• Return Receiver.[[DefineOwnProperty]](P, { [[Value]]: V }) . This internal method performs definition, which we use to change the value of property Receiver[P] . The definition algorithm is described in the next subsection.
• (If we get here, then Receiver does not have an own property with key P .)
• Return CreateDataProperty(Receiver, P, V) . ( This operation creates an own data property in its first argument.)
• (If we get here, then ownDesc describes an accessor property that is own or inherited.)
• Let setter be ownDesc.[[Set]] .
• If setter is undefined , return false .
• Perform Call(setter, Receiver, «V») . Call() invokes the function object setter with this set to Receiver and the single parameter V (French quotes «» are used for lists in the specification).

Return true .

11.2.1.1 How do we get from an assignment to OrdinarySetWithOwnDescriptor() ?

Evaluating an assignment without destructuring involves the following steps:

• In the spec, evaluation starts in the section on the runtime semantics of AssignmentExpression . This section handles providing names for anonymous functions, destructuring, and more.
• If there is no destructuring pattern, then PutValue() is used to make the assignment.
• For property assignments, PutValue() invokes the internal method .[[Set]]() .
• For ordinary objects, .[[Set]]() calls OrdinarySet() (which calls OrdinarySetWithOwnDescriptor() ) and returns the result.

Notably, PutValue() throws a TypeError in strict mode if the result of .[[Set]]() is false .

11.2.2 Defining a property

The actual work of defining a property is handled via the following operation in the ECMAScript specification:

The parameters are:

• The object O where we want to define a property. There is a special validation-only mode where O is undefined . We are ignoring this mode here.
• The property key P of the property we want to define.
• extensible indicates if O is extensible.
• Desc is a property descriptor specifying the attributes we want the property to have.
• current contains the property descriptor of an own property O[P] if it exists. Otherwise, current is undefined .

The result of the operation is a boolean that indicates if it succeeded. Failure can have different consequences. Some callers ignore the result. Others, such as Object.defineProperty() , throw an exception if the result is false .

This is a summary of the algorithm:

If current is undefined , then property P does not currently exist and must be created.

• If extensible is false , return false indicating that the property could not be added.
• Otherwise, check Desc and create either a data property or an accessor property.

If Desc doesn’t have any fields, return true indicating that the operation succeeded (because no changes had to be made).

If current.[[Configurable]] is false :

• ( Desc is not allowed to change attributes other than value .)
• If Desc.[[Configurable]] exists, it must have the same value as current.[[Configurable]] . If not, return false .
• Same check: Desc.[[Enumerable]]

Next, we validate the property descriptor Desc : Can the attributes described by current be changed to the values specified by Desc ? If not, return false . If yes, go on.

• If the descriptor is generic (with no attributes specific to data properties or accessor properties), then validation is successful and we can move on.
• The current property must be configurable (otherwise its attributes can’t be changed as necessary). If not, false is returned.
• Change the current property from a data property to an accessor property or vice versa. When doing so, the values of .[[Configurable]] and .[[Enumerable]] are preserved, all other attributes get default values ( undefined for object-valued attributes, false for boolean-valued attributes).
• (Due to current.[[Configurable]] being false , Desc.[[Configurable]] and Desc.[[Enumerable]] were already checked previously and have the correct values.)
• If Desc.[[Writable]] exists and is true , then return false .
• If Desc.[[Value]] exists and does not have the same value as current.[[Value]] , then return false .
• There is nothing more to do. Return true indicating that the algorithm succeeded.
• (Note that normally, we can’t change any attributes of a non-configurable property other than its value. The one exception to this rule is that we can always go from writable to non-writable. This algorithm handles this exception correctly.)
• If Desc.[[Set]] exists, it must have the same value as current.[[Set]] . If not, return false .
• Same check: Desc.[[Get]]

Set the attributes of the property with key P to the values specified by Desc . Due to validation, we can be sure that all of the changes are allowed.

11.3 Definition and assignment in practice

This section describes some consequences of how property definition and assignment work.

11.3.1 Only definition allows us to create a property with arbitrary attributes

If we create an own property via assignment, it always creates properties whose attributes writable , enumerable , and configurable are all true .

Therefore, if we want to specify arbitrary attributes, we must use definition.

And while we can create getters and setters inside object literals, we can’t add them later via assignment. Here, too, we need definition.

11.3.2 The assignment operator does not change properties in prototypes

Let us consider the following setup, where obj inherits the property prop from proto .

We can’t (destructively) change proto.prop by assigning to obj.prop . Doing so creates a new own property:

The rationale for this behavior is as follows: Prototypes can have properties whose values are shared by all of their descendants. If we want to change such a property in only one descendant, we must do so non-destructively, via overriding. Then the change does not affect the other descendants.

11.3.3 Assignment calls setters, definition doesn’t

What is the difference between defining the property .prop of obj versus assigning to it?

If we define, then our intention is to either create or change an own (non-inherited) property of obj . Therefore, definition ignores the inherited setter for .prop in the following example:

If, instead, we assign to .prop , then our intention is often to change something that already exists and that change should be handled by the setter:

11.3.4 Inherited read-only properties prevent creating own properties via assignment

What happens if .prop is read-only in a prototype?

In any object that inherits the read-only .prop from proto , we can’t use assignment to create an own property with the same key – for example:

Why can’t we assign? The rationale is that overriding an inherited property by creating an own property can be seen as non-destructively changing the inherited property. Arguably, if a property is non-writable, we shouldn’t be able to do that.

However, defining .prop still works and lets us override:

Accessor properties that don’t have a setter are also considered to be read-only:

The fact that read-only properties prevent assignment earlier in the prototype chain, has been given the name override mistake :

• It was introduced in ECMAScript 5.1.
• On one hand, this behavior is consistent with how prototypal inheritance and setters work. (So, arguably, it is not a mistake.)
• On the other hand, with the behavior, deep-freezing the global object causes unwanted side-effects.
• There was an attempt to change the behavior, but that broke the library Lodash and was abandoned ( pull request on GitHub ).
• Pull request on GitHub
• Wiki page on ECMAScript.org ( archived )

11.4 Which language constructs use definition, which assignment?

In this section, we examine where the language uses definition and where it uses assignment. We detect which operation is used by tracking whether or not inherited setters are called. See §11.3.3 “Assignment calls setters, definition doesn’t” for more information.

11.4.1 The properties of an object literal are added via definition

When we create properties via an object literal, JavaScript always uses definition (and therefore never calls inherited setters):

11.4.2 The assignment operator = always uses assignment

The assignment operator = always uses assignment to create or change properties.

11.4.3 Public class fields are added via definition

Alas, even though public class fields have the same syntax as assignment, they do not use assignment to create properties, they use definition (like properties in object literals):

11.5 Further reading and sources of this chapter

Section “Prototype chains” in “JavaScript for impatient programmers”

Email by Allen Wirfs-Brock to the es-discuss mailing list : “The distinction between assignment and definition […] was not very important when all ES had was data properties and there was no way for ES code to manipulate property attributes.” [That changed with ECMAScript 5.]

Shorthand Property and Method Names in JavaScript | ES6

ES6 introduced two new features to make objects more concise - Shorthand Properties and Shorthand Method Names.

Shorthand Properties

With Shorthand Properties, whenever you have a variable which is the same name as a property on an object, when constructing the object, you can omit the property name.

What that means is that code that used to look like this,

can now look like this.

Shorthand Method Names

Now, what if one of those properties was a function?

A function that is a property on an object is called a method. With ES6's Shorthand Method Names, you can omit the function keyword completely. What that means is that code that used to look like this,

can now look like this

Both Shorthand Properties and Shorthand Method Names are just syntactic sugar over the previous ways we used to add properties to an object. However, because they're such common tasks, even the smallest improvements eventually add up.

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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 object properties, property management methods, javascript object.defineproperty().

The Object.defineProperty() method can be used to:

• Adding a new property to an object
• Changing property values
• Changing property metadata
• Changing object getters and setters

Adding a new Property

This example adds a new property to an object:

Changing a Property Value

This example changes a property value:

Property Attributes

All properties have a name. In addition they also have a value.

The value is one of the property's attributes.

Other attributes are: enumerable, configurable, and writable.

These attributes define how the property can be accessed (is it readable?, is it writable?)

In JavaScript, all attributes can be read, but only the value attribute can be changed (and only if the property is writable).

( ECMAScript 5 has methods for both getting and setting all property attributes)

Changing Meta Data

The following property meta data can be changed:

Getters and setters can also be changed:

This example makes language read-only:

This example makes language not enumerable:

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JavaScript getOwnProperyNames()

The Object.getOwnProperyNames() method can:

• List object properties

List all Object Properties

This example gets all properties of an object:

Object.getOwnProperyNames() will also list properties that is not enumerable:

JavaScript Object.keys()

The Object.keys() method can:

• List enumerable object properties

List Enumerable Object Properties

This example uses Object.keys() insted of Object.getOwnProperyNames() :

The getOwnPropertyNames() method returns all properties.

The Object.keys() method returns all enumerable properties.

If you define object properties without enumerable:false , the two methods will return the same.

Adding Getters and Setters

The Object.defineProperty() method can also be used to add Getters and Setters:

A Counter Example

Prototype properties.

JavaScript objects inherit the properties of their prototype.

The delete keyword does not delete inherited properties, but if you delete a prototype property, it will affect all objects inherited from the prototype.

Complete Object Reference

For a complete reference, go to our:

Complete JavaScript Object Reference .

The reference contains descriptions and examples of all Object Properties and Methods.

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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.

• If you have suggestions what to improve - please submit a GitHub issue or a pull request instead of commenting.
• If you can't understand something in the article – please elaborate.
• To insert few words of code, use the <code> tag, for several lines – wrap them in <pre> tag, for more than 10 lines – use a sandbox ( plnkr , jsbin , codepen …)

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Elektrostal (Q198419)

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• Pages with maps

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Current time by city

For example, New York

Current time by country

For example, Japan

Time difference

For example, London

For example, Dubai

Coordinates

For example, Hong Kong

For example, Delhi

For example, Sydney

Geographic coordinates of Elektrostal, Moscow Oblast, Russia

City coordinates

Coordinates of Elektrostal in decimal degrees

Coordinates of elektrostal in degrees and decimal minutes, utm coordinates of elektrostal, geographic coordinate systems.

WGS 84 coordinate reference system is the latest revision of the World Geodetic System, which is used in mapping and navigation, including GPS satellite navigation system (the Global Positioning System).

Geographic coordinates (latitude and longitude) define a position on the Earth’s surface. Coordinates are angular units. The canonical form of latitude and longitude representation uses degrees (°), minutes (′), and seconds (″). GPS systems widely use coordinates in degrees and decimal minutes, or in decimal degrees.

Latitude varies from −90° to 90°. The latitude of the Equator is 0°; the latitude of the South Pole is −90°; the latitude of the North Pole is 90°. Positive latitude values correspond to the geographic locations north of the Equator (abbrev. N). Negative latitude values correspond to the geographic locations south of the Equator (abbrev. S).

Longitude is counted from the prime meridian ( IERS Reference Meridian for WGS 84) and varies from −180° to 180°. Positive longitude values correspond to the geographic locations east of the prime meridian (abbrev. E). Negative longitude values correspond to the geographic locations west of the prime meridian (abbrev. W).

UTM or Universal Transverse Mercator coordinate system divides the Earth’s surface into 60 longitudinal zones. The coordinates of a location within each zone are defined as a planar coordinate pair related to the intersection of the equator and the zone’s central meridian, and measured in meters.

Elevation above sea level is a measure of a geographic location’s height. We are using the global digital elevation model GTOPO30 .

Elektrostal , Moscow Oblast, Russia

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Typescript Object destructuring results in "Property assignment expected."

I am transitioning a project from Babel to Typescript and receive the following compiler error:

from code that looks like this:

This code previously worked fine under Babel, but causes the error above when attempting to compile via Typescript. Is object destructuring different in Typescript?

• ecmascript-2016
• destructuring

• 2 shouldn't it be var { auth } = this.props; ? –  Icepickle Jan 19, 2016 at 15:22
• 1 Isn't this ES7 syntax? –  MinusFour Jan 19, 2016 at 15:27

The feature you are looking for is Object spread/rest operators (proposed for ES7). It looks like it's planned but not yet implemented:

We want to wait for the proposal to reach Stage 3 before addressing this.

More info here .

Edit : The proposal is in stage-3. We'll likely see it drafted on ES2018 (ES9). Support has been added to TypeScript as well (starting from 2.1).

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JavaScript reference

The JavaScript reference serves as a repository of facts about the JavaScript language. The entire language is described here in detail. As you write JavaScript code, you'll refer to these pages often (thus the title "JavaScript reference").

The JavaScript language is intended to be used within some larger environment, be it a browser, server-side scripts, or similar. For the most part, this reference attempts to be environment-agnostic and does not target a web browser environment.

If you are new to JavaScript, start with the guide . Once you have a firm grasp of the fundamentals, you can use the reference to get more details on individual objects and language constructs.

JavaScript standard built-in objects , along with their methods and properties.

Value properties

Function properties.

• parseFloat()
• decodeURI()
• decodeURIComponent()
• encodeURI()
• encodeURIComponent()
• escape() Deprecated
• unescape() Deprecated

Fundamental objects

Error objects.

• AggregateError
• ReferenceError
• SyntaxError
• InternalError Non-standard

Numbers and dates

Text processing, indexed collections.

• Uint8ClampedArray
• Uint16Array
• Uint32Array
• BigInt64Array
• BigUint64Array
• Float16Array
• Float32Array
• Float64Array

Keyed collections

Structured data.

• ArrayBuffer
• SharedArrayBuffer

Managing memory

• FinalizationRegistry

Control abstraction objects

• AsyncIterator
• GeneratorFunction
• AsyncGeneratorFunction
• AsyncGenerator
• AsyncFunction

Internationalization

• Intl.Collator
• Intl.DateTimeFormat
• Intl.DisplayNames
• Intl.DurationFormat
• Intl.ListFormat
• Intl.Locale
• Intl.NumberFormat
• Intl.PluralRules
• Intl.RelativeTimeFormat
• Intl.Segmenter

JavaScript statements and declarations

Control flow

• try...catch

Declaring variables

Functions and classes.

• async function
• async function*
• for await...of
• Expression statement
• with Deprecated

Expressions and operators

JavaScript expressions and operators .

Primary expressions

Left-hand-side expressions.

• Property accessors
• import.meta

Increment and decrement

Unary operators, arithmetic operators, relational operators.

• < (Less than)
• > (Greater than)

Equality operators

Bitwise shift operators.

• >>>

Binary bitwise operators

Binary logical operators, conditional (ternary) operator.

• (condition ? ifTrue : ifFalse)

Assignment operators

• >>>=
• &&=
• [a, b] = arr , { a, b } = obj

Yield operators

Spread syntax, comma operator.

JavaScript functions.

• Arrow Functions
• Default parameters
• Rest parameters
• Method definitions

JavaScript classes.

• constructor
• Private properties
• Public class fields
• Static initialization blocks

Additional reference pages

• Lexical grammar
• Data types and data structures
• Iteration protocols
• Trailing commas
• Strict mode
• Deprecated features