title: Reflect.isTemplateObject
stage: 2
copyright: false
contributors: Mike Samuel, Krzysztof Kotowicz, Jordan Harband, Daniel Ehrenberg

Ordinary and Exotic Objects Behaviours

Built-in Exotic Object Internal Methods and Slots

Array Exotic Objects

ArrayCreate ( _length_: a non-negative integer, optional _proto_: an Object, ): either a normal completion containing an Array exotic object or a throw completion

To enable the new API, this proposal modifies ArrayCreate to add an internal slot to each array.

1. If _length_ > 2³² - 1, throw a *RangeError* exception. 1. If _proto_ is not present, set _proto_ to %Array.prototype%. 1. Let _A_ be MakeBasicObject(« [[Prototype]], [[Extensible]], [[TemplateObject]] »). 1. Set _A_.[[Prototype]] to _proto_. 1. Set _A_.[[TemplateObject]] to *false*. 1. Set _A_.[[DefineOwnProperty]] as specified in . 1. Perform ! OrdinaryDefineOwnProperty(_A_, *"length"*, PropertyDescriptor { [[Value]]: 𝔽(_length_), [[Writable]]: *true*, [[Enumerable]]: *false*, [[Configurable]]: *false* }). 1. Return _A_.

ECMAScript Language: Expressions

Primary Expression

Template Literals

GetTemplateObject ( _templateLiteral_: a Parse Node, ): an Array

To enable the new API, this proposal modifies GetTemplateObject to change the value of the new internal slot.

1. Let _realm_ be the current Realm Record. 1. Let _templateRegistry_ be _realm_.[[TemplateMap]]. 1. For each element _e_ of _templateRegistry_, do 1. If _e_.[[Site]] is the same Parse Node as _templateLiteral_, then 1. Return _e_.[[Array]]. 1. Let _rawStrings_ be the TemplateStrings of _templateLiteral_ with argument *true*. 1. Assert: _rawStrings_ is a List of Strings. 1. Let _cookedStrings_ be the TemplateStrings of _templateLiteral_ with argument *false*. 1. Let _count_ be the number of elements in the List _cookedStrings_. 1. Assert: _count_ ≤ 2³² - 1. 1. Let _template_ be ! ArrayCreate(_count_). 1. Set _template_.[[TemplateObject]] to *true*. 1. Let _rawObj_ be ! ArrayCreate(_count_). 1. Let _index_ be 0. 1. Repeat, while _index_ < _count_, 1. Let _prop_ be ! ToString(𝔽(_index_)). 1. Let _cookedValue_ be _cookedStrings_[_index_]. 1. Perform ! DefinePropertyOrThrow(_template_, _prop_, PropertyDescriptor { [[Value]]: _cookedValue_, [[Writable]]: *false*, [[Enumerable]]: *true*, [[Configurable]]: *false* }). 1. Let _rawValue_ be the String value _rawStrings_[_index_]. 1. Perform ! DefinePropertyOrThrow(_rawObj_, _prop_, PropertyDescriptor { [[Value]]: _rawValue_, [[Writable]]: *false*, [[Enumerable]]: *true*, [[Configurable]]: *false* }). 1. Set _index_ to _index_ + 1. 1. Perform ! SetIntegrityLevel(_rawObj_, ~frozen~). 1. Perform ! DefinePropertyOrThrow(_template_, *"raw"*, PropertyDescriptor { [[Value]]: _rawObj_, [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *false* }). 1. Perform ! SetIntegrityLevel(_template_, ~frozen~). 1. Append the Record { [[Site]]: _templateLiteral_, [[Array]]: _template_ } to _realm_.[[TemplateMap]]. 1. Return _template_.

The creation of a template object cannot result in an abrupt completion.

Each |TemplateLiteral| in the program code of a realm is associated with a unique template object that is used in the evaluation of tagged Templates (). The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated. Whether template objects are created lazily upon first evaluation of the |TemplateLiteral| or eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.

Future editions of this specification may define additional non-enumerable properties of template objects.

Reflection

The Reflect Object

Reflect.isTemplateObject ( _value_ )

When the `isTemplateObject` method is called with argument _value_ the following steps are taken:

1. If ? IsArray(_value_) is *true* and _value_.[[TemplateObject]] is *true*, then 1. Return *true*. 1. Return *false*.

IsTemplateObject is realm-agnostic. Since template objects are frozen before escaping GetTemplateObject, testing (IsTemplateObject(_x_) and _x_.[[Prototype]] is the _realm_'s %Array.prototype%) is sufficient to determine whether an _x_ is a template object in a particular _realm_.

In user code, `Reflect.isTemplateObject(x) && x instanceof Array` is an equivalent test, assuming no changes to builtins.