ECMA-262 (12th Edition) ECMAScript 2021 Language Specification - page 23

The abstract operation SetImmutablePrototype takes arguments

and

. It performs the following steps when called:

: Either

(

(

) is Null.

2. Let

current

be ?

.[[GetPrototypeOf]]().

3. If

internal method invariants defined in

current

) is

true

, return

true

4. Return

false

A proxy object is an

exotic object

whose essential internal methods are partially implemented using ECMAScript code.

Every proxy object has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the
proxy's

handler object

, or

null

. Methods (see

Table 32

) of a handler object may be used to augment the implementation

for one or more of the proxy object's internal methods. Every proxy object also has an internal slot called
[[ProxyTarget]] whose value is either an object or the

null

value. This object is called the proxy's

target object

An object is a

Proxy exotic object

if its essential internal methods (including [[Call]] and [[Construct]], if applicable) use

the definitions in this section. These internal methods are installed in

ProxyCreate

10.4.7.2 SetImmutablePrototype (

)

10.5 Proxy Object Internal Methods and Internal Slots

260

Table 32: Proxy Handler Methods

Internal Method

Handler Method

[[GetPrototypeOf]]

getPrototypeOf

[[SetPrototypeOf]]

setPrototypeOf

[[IsExtensible]]

isExtensible

[[PreventExtensions]]

preventExtensions

[[GetOwnProperty]]

getOwnPropertyDescriptor

[[DefineOwnProperty]]

defineProperty

[[HasProperty]]

has

[[Get]]

get

[[Set]]

set

[[Delete]]

deleteProperty

[[OwnPropertyKeys]]

ownKeys

[[Call]]

apply

[[Construct]]

construct

When a handler method is called to provide the implementation of a proxy object internal method, the handler
method is passed the proxy's target object as a parameter. A proxy's handler object does not necessarily have a method
corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation
of the corresponding internal method on the proxy's target object if the handler object does not have a method
corresponding to the internal trap.

The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a proxy object are always initialized when the object is
created and typically may not be modified. Some proxy objects are created in a manner that permits them to be
subsequently

revoked

. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are set to

null

causing subsequent invocations of internal methods on that proxy object to throw a

TypeError

exception.

Because proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript code,
it is possible to define a proxy object whose handler methods violates the invariants defined in

6.1.7.3

. Some of the

6.1.7.3

are essential integrity invariants. These invariants are explicitly enforced

by the proxy object internal methods specified in this section. An ECMAScript implementation must be robust in the
presence of all possible invariant violations.

In the following algorithm descriptions, assume

is an ECMAScript proxy object,

is a property key value,

is any

and

Desc

is a

Property Descriptor

record.

The [[GetPrototypeOf]] internal method of a

takes no arguments. It performs the following steps

10.5.1 [[GetPrototypeOf]] ( )

261

when called:

1. Let

handler

.[[ProxyHandler]].

2. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

4. Let

target

.[[ProxyTarget]].

5. Let

trap

be ?

handler

"getPrototypeOf"

6. If

trap

undefined

, then

a. Return ?

target

.[[GetPrototypeOf]]().

7. Let

handlerProto

be ?

trap

handler

, «

target

»).

8. If

(

handlerProto

) is neither Object nor Null, throw a

TypeError

exception.

9. Let

extensibleTarget

(

target

10. If

extensibleTarget

true

, return

handlerProto

11. Let

targetProto

be ?

target

.[[GetPrototypeOf]]().

12. If

handlerProto

targetProto

) is

false

, throw a

TypeError

exception.

13. Return

handlerProto

NOTE

The [[SetPrototypeOf]] internal method of a

takes argument

(an Object or

null

). It performs

the following steps when called:

: Either

(

(

) is Null.

2. Let

handler

.[[ProxyHandler]].

3. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

5. Let

target

.[[ProxyTarget]].

6. Let

trap

be ?

handler

"setPrototypeOf"

7. If

trap

undefined

, then

a. Return ?

target

.[[SetPrototypeOf]](

8. Let

booleanTrapResult

be !

trap

handler

, «

target

»)).

9. If

booleanTrapResult

false

, return

false

10. Let

extensibleTarget

(

target

11. If

extensibleTarget

true

, return

true

12. Let

targetProto

be ?

target

.[[GetPrototypeOf]]().

13. If

targetProto

) is

false

, throw a

TypeError

exception.

14. Return

true

[[GetPrototypeOf]] for proxy objects enforces the following invariants:

The result of [[GetPrototypeOf]] must be either an Object or

null

If the target object is not extensible, [[GetPrototypeOf]] applied to the proxy object must
return the same value as [[GetPrototypeOf]] applied to the proxy object's target object.

10.5.2 [[SetPrototypeOf]] (

)

262

NOTE

The [[IsExtensible]] internal method of a

takes no arguments. It performs the following steps

when called:

1. Let

handler

.[[ProxyHandler]].

2. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

4. Let

target

.[[ProxyTarget]].

5. Let

trap

be ?

handler

"isExtensible"

6. If

trap

undefined

, then

a. Return ?

(

target

7. Let

booleanTrapResult

be !

trap

handler

, «

target

»)).

8. Let

targetResult

(

target

9. If

booleanTrapResult

targetResult

) is

false

, throw a

TypeError

exception.

10. Return

booleanTrapResult

NOTE

The [[PreventExtensions]] internal method of a

takes no arguments. It performs the following

steps when called:

1. Let

handler

.[[ProxyHandler]].

2. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

4. Let

target

.[[ProxyTarget]].

5. Let

trap

be ?

handler

"preventExtensions"

6. If

trap

undefined

, then

a. Return ?

target

.[[PreventExtensions]]().

7. Let

booleanTrapResult

be !

trap

handler

, «

target

»)).

8. If

booleanTrapResult

true

, then

a. Let

extensibleTarget

(

target

b. If

extensibleTarget

true

, throw a

TypeError

exception.

9. Return

booleanTrapResult

[[SetPrototypeOf]] for proxy objects enforces the following invariants:

The result of [[SetPrototypeOf]] is a Boolean value.
If the target object is not extensible, the argument value must be the same as the result of
[[GetPrototypeOf]] applied to target object.

[[IsExtensible]] for proxy objects enforces the following invariants:

The result of [[IsExtensible]] is a Boolean value.
[[IsExtensible]] applied to the proxy object must return the same value as [[IsExtensible]]
applied to the proxy object's target object with the same argument.

10.5.3 [[IsExtensible]] ( )

10.5.4 [[PreventExtensions]] ( )

263

NOTE

The [[GetOwnProperty]] internal method of a

takes argument

(a property key). It performs the

following steps when called:

) is

true

2. Let

handler

.[[ProxyHandler]].

3. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

5. Let

target

.[[ProxyTarget]].

6. Let

trap

be ?

handler

"getOwnPropertyDescriptor"

7. If

trap

undefined

, then

a. Return ?

target

.[[GetOwnProperty]](

8. Let

trapResultObj

(

trap

handler

, «

target

»).

9. If

(

trapResultObj

) is neither Object nor Undefined, throw a

TypeError

exception.

10. Let

targetDesc

be ?

target

.[[GetOwnProperty]](

11. If

trapResultObj

undefined

, then

a. If

targetDesc

undefined

, return

undefined

b. If

targetDesc

.[[Configurable]] is

false

, throw a

TypeError

exception.

c. Let

extensibleTarget

(

target

d. If

extensibleTarget

false

, throw a

TypeError

exception.

e. Return

undefined

12. Let

extensibleTarget

(

target

13. Let

resultDesc

ToPropertyDescriptor

CompletePropertyDescriptor

trapResultObj

14. Call

IsCompatiblePropertyDescriptor

resultDesc

15. Let

valid

(

extensibleTarget

resultDesc

targetDesc

16. If

valid

false

, throw a

TypeError

exception.

17. If

resultDesc

.[[Configurable]] is

false

, then

a. If

targetDesc

undefined

targetDesc

.[[Configurable]] is

true

, then

i. Throw a

TypeError

exception.

b. If

resultDesc

has a [[Writable]] field and

resultDesc

.[[Writable]] is

false

, then

i. If

targetDesc

.[[Writable]] is

true

, throw a

TypeError

exception.

18. Return

resultDesc

[[PreventExtensions]] for proxy objects enforces the following invariants:

The result of [[PreventExtensions]] is a Boolean value.
[[PreventExtensions]] applied to the proxy object only returns

true

if [[IsExtensible]]

applied to the proxy object's target object is

false

10.5.5 [[GetOwnProperty]] (

)

264

NOTE

The [[DefineOwnProperty]] internal method of a

takes arguments

(a property key) and

Desc

Property Descriptor

). It performs the following steps when called:

) is

true

2. Let

handler

.[[ProxyHandler]].

3. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

5. Let

target

.[[ProxyTarget]].

6. Let

trap

be ?

handler

"defineProperty"

7. If

trap

undefined

, then

a. Return ?

target

.[[DefineOwnProperty]](

Desc

8. Let

descObj

FromPropertyDescriptor

(

Desc

9. Let

booleanTrapResult

be !

trap

handler

, «

target

descObj

»)).

10. If

booleanTrapResult

false

, return

false

11. Let

targetDesc

be ?

target

.[[GetOwnProperty]](

12. Let

extensibleTarget

IsCompatiblePropertyDescriptor

(

target

13. If

Desc

has a [[Configurable]] field and if

Desc

.[[Configurable]] is

false

, then

a. Let

settingConfigFalse

true

14. Else, let

settingConfigFalse

false

15. If

targetDesc

undefined

, then

a. If

extensibleTarget

false

, throw a

TypeError

exception.

b. If

settingConfigFalse

true

, throw a

TypeError

exception.

16. Else,

a. If

extensibleTarget

Desc

targetDesc

) is

false

, throw a

TypeError

exception.

b. If

settingConfigFalse

true

and

targetDesc

.[[Configurable]] is

true

, throw a

TypeError

exception.

c. If

IsDataDescriptor

targetDesc

) is

true

targetDesc

.[[Configurable]] is

false

, and

targetDesc

.[[Writable]] is

true

, then

i. If

Desc

has a [[Writable]] field and

Desc

.[[Writable]] is

false

, throw a

TypeError

exception.

17. Return

true

[[GetOwnProperty]] for proxy objects enforces the following invariants:

The result of [[GetOwnProperty]] must be either an Object or

undefined

A property cannot be reported as non-existent, if it exists as a non-configurable own
property of the target object.
A property cannot be reported as non-existent, if the target object is not extensible, unless
it does not exist as an own property of the target object.
A property cannot be reported as existent, if the target object is not extensible, unless it
exists as an own property of the target object.
A property cannot be reported as non-configurable, unless it exists as a non-configurable
own property of the target object.
A property cannot be reported as both non-configurable and non-writable, unless it exists
as a non-configurable, non-writable own property of the target object.

10.5.6 [[DefineOwnProperty]] (

Desc

)

265

NOTE

The [[HasProperty]] internal method of a

takes argument

(a property key). It performs the

following steps when called:

) is

true

2. Let

handler

.[[ProxyHandler]].

3. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

5. Let

target

.[[ProxyTarget]].

6. Let

trap

be ?

handler

"has"

7. If

trap

undefined

, then

a. Return ?

target

.[[HasProperty]](

8. Let

booleanTrapResult

be !

trap

handler

, «

target

»)).

9. If

booleanTrapResult

false

, then

a. Let

targetDesc

be ?

target

.[[GetOwnProperty]](

b. If

targetDesc

is not

undefined

, then

i. If

targetDesc

.[[Configurable]] is

false

, throw a

TypeError

exception.

ii. Let

extensibleTarget

The [[Get]] internal method of a

(

target

iii. If

extensibleTarget

false

, throw a

TypeError

exception.

10. Return

booleanTrapResult

NOTE

of the property to the target object using [[DefineOwnProperty]] will not throw

takes arguments

(a property key) and

Receiver

(an

[[DefineOwnProperty]] for proxy objects enforces the following invariants:

The result of [[DefineOwnProperty]] is a Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding non-
configurable own property of the target object.
A non-configurable property cannot be non-writable, unless there exists a corresponding
non-configurable, non-writable own property of the target object.
If a property has a corresponding target object property then applying the

Property

Descriptor

an exception.

[[HasProperty]] for proxy objects enforces the following invariants:

The result of [[HasProperty]] is a Boolean value.
A property cannot be reported as non-existent, if it exists as a non-configurable own
property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of the target
object and the target object is not extensible.

10.5.7 [[HasProperty]] (

)

10.5.8 [[Get]] (

Receiver

)

266

). It performs the following steps when called:

) is

true

2. Let

handler

.[[ProxyHandler]].

3. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

5. Let

target

.[[ProxyTarget]].

6. Let

trap

be ?

handler

"get"

7. If

trap

undefined

, then

a. Return ?

target

.[[Get]](

Receiver

8. Let

trapResult

(

trap

handler

, «

target

Receiver

»).

9. Let

targetDesc

be ?

target

.[[GetOwnProperty]](

10. If

targetDesc

is not

undefined

and

targetDesc

.[[Configurable]] is

false

, then

a. If

IsDataDescriptor

targetDesc

) is

true

and

targetDesc

.[[Writable]] is

false

, then

i. If

The [[Set]] internal method of a

trapResult

targetDesc

.[[Value]]) is

false

, throw a

TypeError

exception.

b. If

IsAccessorDescriptor

(

targetDesc

) is

true

and

targetDesc

.[[Get]] is

undefined

, then

i. If

trapResult

is not

undefined

, throw a

TypeError

exception.

11. Return

trapResult

NOTE

). It performs the following steps when called:

) is

true

2. Let

handler

.[[ProxyHandler]].

3. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

5. Let

target

.[[ProxyTarget]].

6. Let

trap

be ?

handler

"set"

7. If

trap

undefined

, then

a. Return ?

target

.[[Set]](

Receiver

8. Let

booleanTrapResult

be !

trap

handler

, «

target

Receiver

»)).

9. If

booleanTrapResult

false

, return

false

10. Let

targetDesc

be ?

target

.[[GetOwnProperty]](

11. If

targetDesc

is not

undefined

and

targetDesc

.[[Configurable]] is

false

, then

a. If

IsDataDescriptor

targetDesc

) is

true

and

targetDesc

.[[Writable]] is

false

, then

i. If

property is a non-configurable own

targetDesc

.[[Value]]) is

false

, throw a

TypeError

exception.

[[Get]] for proxy objects enforces the following invariants:

The value reported for a property must be the same as the value of the corresponding
target object property if the target object property is a non-writable, non-configurable own

data property

The value reported for a property must be

undefined

if the corresponding target object

accessor property

that has

undefined

as its [[Get]]

attribute.

10.5.9 [[Set]] (

Receiver

)

267

b. If

IsAccessorDescriptor

(

targetDesc

) is

true

, then

i. If

targetDesc

.[[Set]] is

undefined

, throw a

TypeError

exception.

12. Return

true

NOTE

The [[Delete]] internal method of a

takes argument

(a property key). It performs the following

steps when called:

) is

true

2. Let

handler

.[[ProxyHandler]].

3. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

5. Let

target

.[[ProxyTarget]].

6. Let

trap

be ?

handler

"deleteProperty"

7. If

trap

undefined

, then

a. Return ?

target

.[[Delete]](

8. Let

booleanTrapResult

be !

trap

handler

, «

target

»)).

9. If

booleanTrapResult

false

, return

false

10. Let

targetDesc

be ?

target

.[[GetOwnProperty]](

11. If

targetDesc

undefined

, return

true

12. If

targetDesc

.[[Configurable]] is

false

, throw a

TypeError

exception.

13. Let

extensibleTarget

(

target

14. If

extensibleTarget

false

, throw a

TypeError

exception.

15. Return

true

NOTE

The [[OwnPropertyKeys]] internal method of a

takes no arguments. It performs the following

[[Set]] for proxy objects enforces the following invariants:

The result of [[Set]] is a Boolean value.
Cannot change the value of a property to be different from the value of the corresponding
target object property if the corresponding target object property is a non-writable, non-
configurable own

data property

Cannot set the value of a property if the corresponding target object property is a non-
configurable own

accessor property

that has

undefined

as its [[Set]] attribute.

[[Delete]] for proxy objects enforces the following invariants:

The result of [[Delete]] is a Boolean value.
A property cannot be reported as deleted, if it exists as a non-configurable own property
of the target object.
A property cannot be reported as deleted, if it exists as an own property of the target
object and the target object is non-extensible.

10.5.10 [[Delete]] (

)

10.5.11 [[OwnPropertyKeys]] ( )

268

steps when called:

1. Let

handler

.[[ProxyHandler]].

2. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

4. Let

target

.[[ProxyTarget]].

5. Let

trap

be ?

handler

"ownKeys"

6. If

trap

undefined

, then

a. Return ?

target

.[[OwnPropertyKeys]]().

7. Let

trapResultArray

be ?

trap

handler

, «

target

»).

8. Let

trapResult

CreateListFromArrayLike

(

trapResultArray

, « String, Symbol »).

9. If

trapResult

contains any duplicate entries, throw a

TypeError

exception.

10. Let

extensibleTarget

(

target

11. Let

targetKeys

be ?

target

.[[OwnPropertyKeys]]().

12.

targetKeys

is a

whose elements are only String and Symbol values.

13.

targetKeys

contains no duplicate entries.

14. Let

targetConfigurableKeys

be a new empty

15. Let

targetNonconfigurableKeys

be a new empty

16. For each element

key

targetKeys

, do

a. Let

desc

be ?

target

.[[GetOwnProperty]](

key

b. If

desc

is not

undefined

and

desc

.[[Configurable]] is

false

, then

i. Append

key

as an element of

targetNonconfigurableKeys

c. Else,

i. Append

key

as an element of

targetConfigurableKeys

17. If

extensibleTarget

true

and

targetNonconfigurableKeys

is empty, then

a. Return

trapResult

18. Let

uncheckedResultKeys

be a

whose elements are the elements of

trapResult

19. For each element

key

targetNonconfigurableKeys

, do

a. If

key

is not an element of

uncheckedResultKeys

, throw a

TypeError

exception.

b. Remove

key

from

uncheckedResultKeys

20. If

extensibleTarget

true

, return

trapResult

21. For each element

key

targetConfigurableKeys

, do

a. If

key

is not an element of

uncheckedResultKeys

, throw a

TypeError

exception.

b. Remove

key

from

uncheckedResultKeys

22. If

uncheckedResultKeys

is not empty, throw a

TypeError

exception.

23. Return

trapResult

NOTE

[[OwnPropertyKeys]] for proxy objects enforces the following invariants:

The result of [[OwnPropertyKeys]] is a

The returned

contains no duplicate entries.

The Type of each result

element is either String or Symbol.

The result

must contain the keys of all non-configurable own properties of the target

object.
If the target object is not extensible, then the result

must contain all the keys of the

own properties of the target object and no other values.

269

The [[Call]] internal method of a

takes arguments

thisArgument

(an

)

and

argumentsList

of ECMAScript language values). It performs the following steps when called:

1. Let

handler

.[[ProxyHandler]].

2. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

4. Let

target

.[[ProxyTarget]].

5. Let

trap

be ?

handler

"apply"

6. If

trap

undefined

, then

a. Return ?

(

target

thisArgument

argumentsList

7. Let

argArray

be !

CreateArrayFromList

(

argumentsList

8. Return ?

(

trap

handler

, «

target

thisArgument

argArray

»).

NOTE

The [[Construct]] internal method of a

takes arguments

argumentsList

of ECMAScript

language values) and

newTarget

constructor

). It performs the following steps when called:

1. Let

handler

.[[ProxyHandler]].

2. If

handler

null

, throw a

TypeError

exception.

handler

) is Object.

4. Let

target

.[[ProxyTarget]].

IsConstructor

(

target

) is

true

6. Let

trap

be ?

handler

"construct"

7. If

trap

undefined

, then

a. Return ?

Construct

(

target

argumentsList

newTarget

8. Let

argArray

be !

CreateArrayFromList

(

argumentsList

9. Let

newObj

(

trap

handler

, «

target

argArray

newTarget

»).

10. If

(

newObj

) is not Object, throw a

TypeError

exception.

11. Return

newObj

NOTE 1

NOTE 2

The abstract operation ProxyCreate takes arguments

target

and

handler

. It is used to specify the creation of new Proxy

only has a [[Call]] internal method if the initial value of its [[ProxyTarget]]

internal slot is an object that has a [[Call]] internal method.

only has a [[Construct]] internal method if the initial value of its

[[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.

[[Construct]] for proxy objects enforces the following invariants:

The result of [[Construct]] must be an Object.

10.5.12 [[Call]] (

thisArgument

argumentsList

)

10.5.13 [[Construct]] (

argumentsList

newTarget

)

10.5.14 ProxyCreate (

target

handler

)

270

exotic objects. It performs the following steps when called:

1. If

(

target

) is not Object, throw a

TypeError

exception.

2. If

(

handler

) is not Object, throw a

TypeError

exception.

3. Let

be !

MakeBasicObject

(« [[ProxyHandler]], [[ProxyTarget]] »).

4. Set

's essential internal methods, except for [[Call]] and [[Construct]], to the definitions specified in

10.5

5. If

IsCallable

target

) is

true

, then

a. Set

.[[Call]] as specified in

10.5.12

b. If

IsConstructor

target

) is

true

, then

i. Set

.[[Construct]] as specified in

10.5.13

6. Set

.[[ProxyTarget]] to

target

7. Set

.[[ProxyHandler]] to

handler

8. Return

SourceCharacter

any Unicode code point

ECMAScript code is expressed using Unicode. ECMAScript source text is a sequence of code points. All Unicode code
point values from U+0000 to U+10FFFF, including surrogate code points, may occur in source text where permitted by
the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not
relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript
implementation processes the source text as if it was an equivalent sequence of

SourceCharacter

values, each

SourceCharacter

being a Unicode code point. Conforming ECMAScript implementations are not required to perform

any normalization of source text, or behave as though they were performing normalization of source text.

The components of a combining character sequence are treated as individual Unicode code points even though a user
might think of the whole sequence as a single character.

11 ECMAScript Language: Source Code

11.1 Source Text

Syntax

271

NOTE

The abstract operation UTF16EncodeCodePoint takes argument

(a Unicode code point). It performs the following

steps when called:

be the code unit whose value is

: 0

≤

0x10FFFF.

2. If

≤

0xFFFF, return the String value consisting of the code unit whose value is

3. Let

cu1

floor

((

- 0x10000) / 0x400) + 0xD800.

4. Let

cu2

be the code unit whose value is ((

- 0x10000)

modulo

0x400) + 0xDC00.

5. Return the

string-concatenation

cu1

and

cu2

The abstract operation CodePointsToString takes argument

text

(a sequence of Unicode code points). It converts

text

into a String value, as described in

6.1.4

. It performs the following steps when called:

1. Let

result

be the empty String.

2. For each code point

text

, do

a. Set

result

to the

string-concatenation

result

and !

UTF16EncodeCodePoint

(

3. Return

result

The abstract operation UTF16SurrogatePairToCodePoint takes arguments

lead

(a code unit) and

trail

(a code unit). Two

code units that form a UTF-16

surrogate pair

are converted to a code point. It performs the following steps when

called:

lead

is a

leading surrogate

and

trail

is a

trailing surrogate

2. Let

be (

lead

- 0xD800)

0x400 + (

trail

- 0xDC00) + 0x10000.

In string literals, regular expression literals, template literals and identifiers, any Unicode code
point may also be expressed using Unicode escape sequences that explicitly express a code
point's numeric value. Within a comment, such an escape sequence is effectively ignored as part
of the comment.

ECMAScript differs from the Java programming language in the behaviour of Unicode escape
sequences. In a Java program, if the Unicode escape sequence

\u000A

, for example, occurs

within a single-line comment, it is interpreted as a line terminator (Unicode code point U+000A is
LINE FEED (LF)) and therefore the next code point is not part of the comment. Similarly, if the
Unicode escape sequence

\u000A

occurs within a string literal in a Java program, it is likewise

interpreted as a line terminator, which is not allowed within a string literal—one must write

instead of

\u000A

to cause a LINE FEED (LF) to be part of the String value of a string literal. In

an ECMAScript program, a Unicode escape sequence occurring within a comment is never
interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode
escape sequence occurring within a string literal in an ECMAScript program always contributes
to the literal and is never interpreted as a line terminator or as a code point that might terminate
the string literal.

11.1.1 Static Semantics: UTF16EncodeCodePoint (

)

11.1.2 Static Semantics: CodePointsToString (

text

)

11.1.3 Static Semantics: UTF16SurrogatePairToCodePoint (

lead

trail

)

272

3. Return the code point

The abstract operation CodePointAt takes arguments

string

(a String) and

position

(a non-negative

integer

). It

interprets

string

as a sequence of UTF-16 encoded code points, as described in

6.1.4

, and reads from it a single code

point starting with the code unit at index

position

. It performs the following steps when called:

1. Let

size

be the length of

string

UTF16SurrogatePairToCodePoint

position

≥

0 and

position

size

3. Let

first

be the code unit at index

position

within

string

4. Let

be the code point whose numeric value is that of

first

5. If

first

a. Return the

, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]:

false

6. If

first

is a

trailing surrogate

position

+ 1 =

size

, then

a. Return the

Record

{ [[CodePoint]]:

, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]:

true

7. Let

second

be the code unit at index

position

+ 1 within

string

8. If

second

is not a

trailing surrogate

, then

a. Return the

Record

{ [[CodePoint]]:

, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]:

true

9. Set

to !

(

first

second

10. Return the

Record

{ [[CodePoint]]:

, [[CodeUnitCount]]: 2, [[IsUnpairedSurrogate]]:

false

The abstract operation StringToCodePoints takes argument

string

(a String). It returns the sequence of Unicode code

points that results from interpreting

string

as UTF-16 encoded Unicode text as described in

6.1.4

. It performs the

following steps when called:

1. Let

codePoints

be a new empty

2. Let

size

be the length of

string

3. Let

position

be 0.

4. Repeat, while

position

size

a. Let

be !

CodePointAt

, and analyse the parse result for any

string

position

b. Append

.[[CodePoint]] to

codePoints

c. Set

position

.[[CodeUnitCount]].

5. Return

codePoints

The abstract operation ParseText takes arguments

sourceText

(a sequence of Unicode code points) and

goalSymbol

nonterminal in one of the ECMAScript grammars). It performs the following steps when called:

early

error

conditions. Parsing and

detection may be interleaved in an

implementation-defined

manner.

2. If the parse succeeded and no early errors were found, return the

Parse Node

(an instance of

goalSymbol

) at the

root of the parse tree resulting from the parse.

3. Otherwise, return a

of one or more

SyntaxError

objects representing the parsing errors and/or early errors.

If more than one parsing error or

is present, the number and ordering of error objects in the list is

11.1.4 Static Semantics: CodePointAt (

string

position

)

11.1.5 Static Semantics: StringToCodePoints (

string

)

11.1.6 Static Semantics: ParseText (

sourceText

goalSymbol

)

273

implementation-defined

, but at least one must be present.

NOTE 1

NOTE 2

There are four types of ECMAScript code:

Global code

is source text that is treated as an ECMAScript

. The global code of a particular

does not

include any source text that is parsed as part of a

FunctionExpression

, or

Eval code

is the source text supplied to the built-in

eval

function. More precisely, if the parameter to the built-

eval

function is a String, it is treated as an ECMAScript

. The eval code for a particular invocation of

eval

is the global code portion of that

[[FormalParameters]] internal slots (see

Function code

is source text that is parsed to supply the value of the [[ECMAScriptCode]] and

10.2

) of an ECMAScript

function object

. The function code of a

particular ECMAScript function does not include any source text that is parsed as the function code of a nested

FunctionExpression

In addition, if the source text referred to above is parsed as:

the

FormalParameters

and

FunctionBody

of a

the

the

or
the

FormalParameters

and

AsyncGeneratorBody

of an

then the source text matching the

BindingIdentifier

(if any) of that declaration or expression is also included in

the function code of the corresponding function.

Module code

is source text that is code that is provided as a

ModuleBody

. It is the code that is directly evaluated

when a module is initialized. The module code of a particular module does not include any source text that is
parsed as part of a nested

FunctionExpression

Consider a text that has an

at a particular point, and also a syntax error at a later

point. An implementation that does a parse pass followed by an early errors pass might report
the syntax error and not proceed to the early errors pass. An implementation that interleaves the
two activities might report the