JavaScript Lists

A random collection of different lists related to the JavaScript language.

Syntax

Primitives

Primitive	Literals
bigint	1n, 0xFn, 0o7n, 0b1n
boolean	true, false
null	null
number	1, 1.2, 0xF, 0o7, 0b1, 1e2, 1e-2
string	'string', "string", `string`
symbol	none
undefined	none (though accessible via global undefined)

Object Literals

Object	Literals
Object	{ key0: value0, key1: value1, keyN: valueN }
Array	[ value0, value1, valueN ]
RegExp	/expression/flags

Identifiers

Where name is the identifier being created and /* scope */ represents the scope where that identifier is available

Kind	Example
Catch error binding	try { } catch (name) { /* scope */ }
Class	/* scope */ class name {}
Named class name	(class name { /* scope */ })
Const	/* scope */ const name = value;
Function	/* scope */ function name () {}
Function parameter	(function (name /* scope */ ) { /* scope */ })
Named function expression name	(function name ( /* scope */ ) { /* scope */ })
Import	/* scope */ import name from 'path'
Label	name: { /* scope */ }
Let	/* scope */ let name;
Var	/* scope */ var name;

Note: Scopes within the parameter list refers to default parameter expressions.

Function Syntax Template

Square brackets ([]) represent optional keywords or names whereas angle brackets (<>) represents a required item. Not all combinations are compatible.

Kind	Syntax
Function Declaration or Expression	[async] function [*] [name] () {}
Arrow Function	[async] () => {}
Class Declaration or Expression	class [name] [extends <expression>] {}
Class Method	class { [static] [async] [get] [set] [*] [#]<name> () {} }
Object Method (Shorthand)	{ [async] [get] [set] [*] <name> () {} }

Creating Functions

Creating functions named name (or anonymous).

Kind	Example	Name	Constructable
Function Declaration	function name () {}	Explicit	Yes
Default Function Declaration	export default function () {}	Anonymous	Yes
Async Function Declaration	async function name () {}	Explicit	No
Generator Function Declaration	function * name () {}	Explicit	No
Async Generator Function Declaration	async function * name () {}	Explicit	No
Function Expression	(function () {})	Anonymous	Yes
Async Function Expression	(async function () {})	Anonymous	No
Generator Function Expression	(function * () {})	Anonymous	No
Async Generator Function Expression	(async function * () {})	Anonymous	No
Named Function Expression	(function name () {})	Explicit	Yes
Named Async Function Expression	(async function name () {})	Explicit	No
Named Generator Function Expression	(function * name () {})	Explicit	No
Named Async Generator Function Expression	(async function * name () {})	Explicit	No
Arrow Function	() => {}	Anonymous	No
Async Arrow Function	async () => {}	Anonymous	No
Class Declaration	class name { constructor () {} }	Explicit	Yes
Class Expression	(class { constructor () {} })	Anonymous	Yes
Named Class Expression	(class name { constructor () {} })	Explicit	Yes
Class Method	class { name () {} }	Explicit	No
Async Class Method	class { async name () {} }	Explicit	No
Class Method Generator	class { * name () {} }	Explicit	No
Async Class Method Generator	class { async * name() {} }	Explicit	No
Class Getter	class { get name() {} }	Explicit	No
Class Setter	class { set name(value) {} }	Explicit	No
Object Method (Shorthand)	({ name () {} })	Explicit	No
Async Object Method (Shorthand)	({ async name () {} })	Explicit	No
Object Method Generator (Shorthand)	({ * name () {} })	Explicit	No
Async Object Method Generator (Shorthand)	({ async * name() {} })	Explicit	No
Object Getter	({ get name() {} })	Explicit	No
Object Setter	({ set name(value) {} })	Explicit	No
Function Constructor	new Function(), Function()	Anonymous	Yes
AsyncFunction Constructor	new AsyncFunction(), AsyncFunction()	Anonymous	No
GeneratorFunction Constructor	new GeneratorFunction(), GeneratorFunction()	Anonymous	No
AsyncGeneratorFunction Constructor	new AsyncGeneratorFunction(), AsyncGeneratorFunction()	Anonymous	No

Note: Any expression context will work for expression variations, not just the grouping (()) used in the examples shown.

Note: The AsyncFunction, GeneratorFunction and AsyncGeneratorFunction constructors are not globally accessible but can be obtained from existing instances/declarations.

Calling Functions

Calling a function identified as name.

Kind	Call
Call	name()
Call	name.call()
Call	name.apply()
Template Literal Tag (Call)	name``
Construct	new name
Construct	new name()
Call	Refect.apply(name)
Construct	Reflect.construct(name)

Additional Ways to Make Calls

This lists more obscure ways of function calling which may require special function definitions.

Kind	Definition	Call
Getter	obj = { get name () {} }	obj.name
Setter	obj = { set name (value) {} }	obj.name = null
Thenable	obj = { then () {} }	await obj
Iterator	obj = { [Symbol.iterator] () {} }	[...obj]
Iterator	obj = { [Symbol.iterator] () {} }	for (let i of obj);
Asynchronous Iterator	obj = { [Symbol.asyncIterator] () {} }	for await (let i of obj);
To Primitive	obj = { toString () {} }	Object[obj]
To Primitive	obj = { valueOf () {} }	+obj
To Primitive	obj = { [Symbol.toPrimitive] () {} }	+obj
Has Instance	obj = { [Symbol.hasInstance] () {} }	null instanceof obj

Use of Braces in Syntax

Use parentheses () for:

• grouping
• function parameter lists
• function calling
• if conditions
• while conditions
• for setup (for...in, for...of, for await...of)
• switch expressions
• with expression (deprecated)
• catch exception identifier
• super() call syntax
• dynamic import() syntax

Use square braces [] for:

• property access
• array literals
• array destructuring
• computed properties

Use curly braces {} for:

• code blocks
• try catch blocks
• function bodies
• class bodies
• class static blocks
• object literals
• object destructuring
• string interpolation
• unicode
• named imports
• named exports

Each of these also have special meaning when used in regular expressions:

• (): assertions, groups
• []: character classes,
• {}: character classes, quantifiers, unicode

ASI Considerations

There can be ambiguity between operations when starting a line with the following characters where ASI (Automatic Semicolon Insertion) will not insert a semicolon to terminate the previous line's expression.

Line Starts With	Semicolon Before	No Semicolon Before
[	Array literal	Array access
(	Grouping	Function call
`	Template literal	Tagged template literal
+	Unary +	Addition
-	Unary -	Subtraction
*	Generator method	Multiplication (of preceding field initializer)
/	RegExp literal	Division

Types

Primitive Objects

Object Type	Constructor	Conversion
BigInt	No	Yes
Boolean	Yes	Yes
Number	Yes	Yes
String	Yes	Yes
Symbol	No	No

BigInt vs. Number

Where value in the examples represents a BigInt or Number value.

	BigInt	Number
Constructor	No	Yes
Exponential Notation	No	Yes
Supports Decimal Values	No	Yes
Supported in JSON	No	Yes
typeof value	'bigint'	'number'
Object.is(value, -value) where value is 0	true	false
1 + value (or other arithmetic operatation)	Error	Number
1n + value (or other arithmetic operatation)	BigInt	Error
1 ^ value (or other bitwise operatation)	Error	Number
1n ^ value (or other bitwise operatation)	BigInt	Error
+value	Error	Number
Math.round(value) (or other Math operation)	Error	Number
Array index in methods (e.g. array.slice(value))	Error	Yes

Default Property Descriptors

Operation	Example	Default Descriptor
Assignment	object.prop = value	configurable: true enumerable: true writable: true
Global var Declaration	var prop = value	configurable: false enumerable: true writable: true
Class Field	class { prop = value }	configurable: true enumerable: true writable: true
Class Method	class { prop () {} }	configurable: true enumerable: false writable: true
Define Property	Object.defineProperty(object, 'prop', { value })	configurable: false enumerable: false writable: false
Imports (prop in Module)	import * as Module from './module.js'	configurable: false enumerable: true writable: true

Every assign operation as well as many of the other define operations (those that don't explicitly say "define") will use the same Assignment descriptor configuration.

Null Prototypes

Some native objects in JavaScript do not inherit from the standard Object.prototype, instead having a null prototype.

• Object.prototype
• Array.prototype[Symbol.unscopables]
• Objects returned by Object.groupBy()
• Objects returned by JSON.rawJSON()
• Regexp groups ("string".match(/(?<g>)/).groups)
• Module objects (module in import * as module from "module.js")
• import.meta
• Proxy objects (observable in debugger only)

To Primitive Precedence

Different methods get prioritization when converting objects to primitives. If present, a Symbol.toPrimitive will always be used. If not, conversion will rely on toString() and valueOf(), using the other as a fallback if the first attempted fails to produce a primitive. Which is called first depends on the "hint" of the operation. This can have one of 3 values: "string", "number", or "default". "string" prefers toString() while "number" and "default" prefer valueOf(). If Symbol.toPrimitive is available, the hint will be passed in as an argument.

Where value is an ordinary object.

Operation	Primary	Hint	Secondary	Backup
'' == value	Symbol.toPrimitive	"default"	valueOf	toString
'' + value	Symbol.toPrimitive	"default"	valueOf	toString
new Date(value)	Symbol.toPrimitive	"default"	valueOf	toString
1 > value (or other relational operation)	Symbol.toPrimitive	"number"	valueOf	toString
1 * value (or other arithmetic operation)	Symbol.toPrimitive	"number"	valueOf	toString
1 ^ value (or other bitwise operation)	Symbol.toPrimitive	"number"	valueOf	toString
+value (or other unary operation)	Symbol.toPrimitive	"number"	valueOf	toString
Math.round(value) (or other Math operation)	Symbol.toPrimitive	"number"	valueOf	toString
Number(value)	Symbol.toPrimitive	"number"	valueOf	toString
BigInt(value)	Symbol.toPrimitive	"number"	valueOf	toString
`${value}`	Symbol.toPrimitive	"string"	toString	valueOf
String(value)	Symbol.toPrimitive	"string"	toString	valueOf
Symbol(value)	Symbol.toPrimitive	"string"	toString	valueOf
Object[value]	Symbol.toPrimitive	"string"	toString	valueOf
value in object	Symbol.toPrimitive	"string"	toString	valueOf

Assign vs. Define

Assign sets a property through getter/setters (if they exist) while define will not.

Operation	Example	Method
Assignment Operator	object.prop = value	Assign
Array Methods	object.push(value) object.unshift(value) etc.	Assign
Object.assign	Object.assign(object, { prop: value })	Assign
Reflect.set	Reflect.set(object, 'prop', value)	Assign
Array Literal	[ value ]	Define
Array Spread	[ ...[ value ] ]	Define
Class Declaration Fields	class { prop = value }	Define
Class Declaration Methods	class { prop() {} }	Define
JSON.parse	JSON.parse('{"prop":value}')	Define
Object Literal	{ prop: value }	Define
Object Spread	{ ...{ prop: value } }	Define
Object.create	Object.create(object, { prop: { value } })	Define
Object.defineProperty	Object.defineProperty(object, 'prop', { value })	Define
Object.fromEntries	Object.fromEntries([['prop', value]])	Define

Loops

Statements

Statement	Loops Over
do...while	User-defined condition
for	User-defined condition
for...in	Objects
for...of	Iterables
for await...of	Async Iterables
while	User-defined condition

Collection Methods

Method	Return Value	Early Exit	In Array	In Map	In Set
every	Boolean	Yes	Yes	No	No
some	Boolean	Yes	Yes	No	No
filter	any	No	Yes	No	No
find	any	Yes	Yes	No	No
findIndex	Number	Yes	Yes	No	No
flatMap	Array	No	Yes	No	No
forEach	none	No	Yes	Yes	Yes
map	Array	No	Yes	No	No
reduce	any	No	Yes	No	No
reduceRight	any	No	Yes	No	No

Metadata Collections

Metadata collections can be created for objects and iterated over.

For	Operation	Returns
Array/Map/Set	value.entries()	Iterable
Array/Map/Set	value.keys()	Iterable
Array/Map/Set	value.values()	Iterable
non-null	Object.entries(value)	Array
non-null	Object.keys(value)	Array
non-null	Object.values(value)	Array
non-null	Object.getOwnPropertyDescriptors(value)	Object
non-null	Object.getOwnPropertyNames(value)	Array
non-null	Object.getOwnPropertySymbols(value)	Array
Object	Reflect.ownKeys(object)	Array

Object Iteration Values

When iterating over an object, what values are used for that iteration.

Operation	String Keys	Symbol Keys	Own	Inherited	Non-enumerable
for...in	Yes	No	Yes	Yes	No
JSON.stringify	Yes	No	Yes	No	No
Object.entries	Yes	No	Yes	No	No
Object.keys	Yes	No	Yes	No	No
Object.values	Yes	No	Yes	No	No
Object.assign	Yes	Yes	Yes	No	No
Object.defineProperties	Yes	Yes	Yes	No	No
Object.getOwnPropertyDescriptors	Yes	Yes	Yes	No	Yes
Object.getOwnPropertyNames	Yes	No	Yes	No	Yes
Object.getOwnPropertySymbols	No	Yes	Yes	No	Yes
Reflect.ownKeys	Yes	Yes	Yes	No	Yes
Object Spread	Yes	Yes	Yes	No	No

Deprecated

These were older lists entries that no longer apply.

Loops

Iteration Order

A specification-defined order known as [[OwnPropertyKeys]] is set for some operations. Other operations (like for...in) do not have to follow this order and are implementation-dependent. [[OwnPropertyKeys]] ordering uses:

1. Integer keys between 0 and 2^53-1 (inclusive) in ascending order
2. Other non-symbol keys in creation order
3. Symbol keys in creation order

Operation	Ordering
for...in	Not guaranteed
JSON.stringify	Not guaranteed
JSON.parse	Not guaranteed
Object.entries	Not guaranteed
Object.keys	Not guaranteed
Object.values	Not guaranteed
Object.assign	Well-defined
Object.defineProperties	Well-defined
Object.getOwnPropertyDescriptors	Well-defined
Object.getOwnPropertyNames	Well-defined
Object.getOwnPropertySymbols	Well-defined
Reflect.ownKeys	Well-defined

Where well-defined refers to [[OwnPropertyKeys]] ordering.

:put_litter_in_its_place:   Deprecation: As of ES2020's for-in mechanics, iteration order is now well defined for the places where before it was not guaranteed.