Import Attributes are an extension to the import syntax that allows specifying additional information to affect how the module is imported. This proposal, other than defining such syntax, also defines the first import attribtue: type, which is passed to the host to specify the expected type of the loaded module. See the explainer for more information.
+ Possible future extensions include: +
module attribute, to import an object representing the dependency's defer attribute, to import a module without evaluating it immediately.Not every import attribute needs to interact with host semantics, for example module and defer could be defined completely within ECMA-262.
Many productions operating on grammar are the same whether or not an
The host-defined abstract operation HostLoadImportedModule takes arguments referrer (a specifier (a String)moduleRequest (a
An example of when referrer can be a
<button type="button" onclick="import('./foo.mjs')">Click me</button>there will be no active script or module at the time the import()
An implementation of HostLoadImportedModule must conform to the following requirements:
+If this operation is called multiple times with the same (referrer, specifier) pair(referrer, moduleRequest.[[Specifier]], moduleRequest.[[Type]]) triple and it performs FinishLoadingImportedModule(referrer, specifiermoduleRequest, payload, result) where result is a normal completion, then it must perform FinishLoadingImportedModule(referrer, specifiermoduleRequest, payload, result) with the same result each time.
The abstract operation FinishLoadingImportedModule takes arguments referrer (a specifier (a String)moduleRequest (a
The description of the [[LoadedModules]] field of
A ModuleRequest Record represents the request to import a module with given import attributes. It consists of the following fields:
+| + Field Name + | ++ Value Type + | ++ Meaning + | +
|---|---|---|
| + [[Specifier]] + | ++ String + | ++ The module specifier + | +
| + [[Type]] + | +
+ a String, or |
+
+ The type attribute of this import
+ |
+
| + Field Name + | ++ Value Type + | ++ Meaning + | +
|---|---|---|
| + [[Status]] + | +
+ |
+
+ Initially |
+
| + [[EvaluationError]] + | +
+ An |
+
+ A completion of type |
+
| + [[DFSIndex]] + | +
+ |
+
+ Auxiliary field used during Link and Evaluate only.
+ If [[Status]] is |
+
| + [[DFSAncestorIndex]] + | +
+ |
+
+ Auxiliary field used during Link and Evaluate only. If [[Status]] is |
+
| + [[RequestedModules]] + | +
+ |
+
+ A type import attribute used by the module represented by this record to request the importation of a module. The |
+
An ImportEntry Record is a
| + Field Name + | ++ Value Type + | ++ Meaning + | +
|---|---|---|
| + [[ModuleRequest]] + | +
+ |
+
+ type import attribute of the |
+
| + [[ImportName]] + | ++ String + | +
+ The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value |
+
| + [[LocalName]] + | ++ String + | ++ The name that is used to locally access the imported value from within the importing module. + | +
The import attributes proposal is intended to give key information about how modules are interpreted to hosts. For the Web embedding and environments which aim to be similar to it, the string is interpreted as the "module type". This is not the primary way the module type is determined (which, on the Web, would be the MIME type, and in other environments may be the file extension), but rather a secondary check which is required to pass for the module graph to load.
+ +In the Web embedding, the following changes would be made to the HTML specification for import attributes:
+ +The module map is keyed by the absolute URL and the type. Initially no other import attributes are supported, so they are not present.
+- title: Import assertions + title: Import attributes status: proposal stage: 3 location: https://github.com/tc39/proposal-import-assertions @@ -12,8 +12,16 @@ contributors: Sven Sauleau, Myles Borins, Daniel Ehrenberg, Daniel Clark
See the explainer for information.
+Import Attributes are an extension to the import syntax that allows specifying additional information to affect how the module is imported. This proposal, other than defining such syntax, also defines the first import attribtue: `type`, which is passed to the host to specify the expected type of the loaded module. See the explainer for more information.
++ Possible future extensions include: +
Not every import attribute needs to interact with host semantics, for example `module` and `defer` could be defined completely within ECMA-262.
Many productions operating on grammar are the same whether or not an |AssertClause|/second |ImportCall| parameter is included; the new parameter is ignored. In this section, only the semantically significant changes are included, and the PR to merge into the main specification would fill in the straightforward details.
Many productions operating on grammar are the same whether or not an |AttributesClause|/second |ImportCall| parameter is included; the new parameter is ignored. In this section, only the semantically significant changes are included, and the PR to merge into the main specification would fill in the straightforward details.
If this operation is called multiple times with the same (_referrer_, _specifier_) pair(_referrer_, _moduleRequest_.[[Specifier]], _moduleRequest_.[[Assertions]]) triple and it performs FinishLoadingImportedModule(_referrer_, _specifier__moduleRequest_, _payload_, _result_) where _result_ is a normal completion, then it must perform FinishLoadingImportedModule(_referrer_, _specifier__moduleRequest_, _payload_, _result_) with the same _result_ each time.
It is recommended but not required that implementations additionally conform to the following stronger constraint:
-If this operation is called multiple times with the same (_referrer_, _specifier_) pair(_referrer_, _moduleRequest_.[[Specifier]], _moduleRequest_.[[Type]]) triple and it performs FinishLoadingImportedModule(_referrer_, _specifier__moduleRequest_, _payload_, _result_) where _result_ is a normal completion, then it must perform FinishLoadingImportedModule(_referrer_, _specifier__moduleRequest_, _payload_, _result_) with the same _result_ each time.
The actual process performed is host-defined, but typically consists of performing whatever I/O operations are necessary to load the appropriate Module Record. Multiple different (_referrer_, _specifier__moduleRequest_.[[Specifier]]) pairs may map to the same Module Record instance. The actual mapping semantic is host-defined but typically a normalization process is applied to _specifier__moduleRequest_.[[Specifier]] as part of the mapping process. A typical normalization process would include actions such as expansion of relative and abbreviated path specifiers.
The above text implies that is recommended but not required that hosts do not use _moduleRequest_.[[Assertions]] as part of the module cache key. In either case, an exception thrown from an import with a given assertion list does not rule out success of another import with the same specifier but a different assertion list.
-Assertions do not affect the contents of the module. Future follow-up proposals may relax this restriction with "evaluator attributes" that would change the contents of the module. There are three possible ways to handle multiple imports of the same module with "evaluator attributes":
-It's possible that one of these three options may make sense for a module load, on a case-by-case basis by attribute, but it's worth careful thought before making this choice.
-The description of the [[LoadedModules]] field of Realm Record, Script Record, and Cyclic Module Record should be updated to include the [[Assertions]] field.
+The description of the [[LoadedModules]] field of Realm Record, Script Record, and Cyclic Module Record should be updated to include the [[Type]] field.
The abstract operation AssertionsEqual takes arguments _left_ and _right_ (two Lists of Records { [[Key]]: a String, [[Value]]: a String }), and returns a Boolean.
- -- HostGetSupportedImportAssertions is a host-defined abstract operation that allows host environments to specify which import assertions they support. - Only assertions with supported keys will be provided to the host. -
- -The implementation of HostGetSupportedImportAssertions must conform to the following requrements:
- -The default implementation of HostGetSupportedImportAssertions is to return an empty List.
- -A ModuleRequest Record represents the request to import a module with given import assertions. It consists of the following fields:
+A ModuleRequest Record represents the request to import a module with given import attributes. It consists of the following fields:
| - [[Assertions]] + [[Type]] | - a List of Records { [[Key]]: a String, [[Value]]: a String } + a String, or ~empty~ | - The import assertions + The `type` attribute of this import | - A List of all the |ModuleSpecifier| strings and import assertions used by the module represented by this record to request the importation of a module. The List is source code occurrence ordered. + A List of all the |ModuleSpecifier| strings with the corresponding `type` import attribute used by the module represented by this record to request the importation of a module. The List is source code occurrence ordered. | @@ -431,7 +341,7 @@
|