Daniel Vince
133 lines
2.9 KiB
Markdown
133 lines
2.9 KiB
Markdown
# ES6 module support for JerryScript
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The module system allows users to write import and export statements in scripts. Therefore the logic of the application could be separated in custom modules.
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The standard's relevant part can be found [here](https://www.ecma-international.org/ecma-262/6.0/#sec-modules).
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## General
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If the main script contains import statements, then Jerry opens and runs the appropriate scripts before the main script (as the standard says). The script's and the module's extension is `.js`, custom extensions are unnecessary.
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main.js
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```js
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import { secret_number } from "./module.js"
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print (secret_number);
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```
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module.js
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```js
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var secret_number = 42;
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export secret_number;
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```
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## Supported features
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* import variable or function
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* add alias name to the imported variable (function)
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* export variable or function
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* add alias name to the exported variable (function)
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### Example
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```js
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import {
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engine,
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version as v
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} from "./module.js"
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import { getFeatureDetails } from "./module_2.js"
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var version = "v3.1415";
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print("> main.js");
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print(">> Engine: " + engine);
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print(">> Version: " + v);
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print (">> " + getFeatureDetails());
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print (">> Script version: " + version);
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```
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```js
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// module.js
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var _engine = "JerryScript";
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export _engine as engine;
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export var version = "1.0 (e92ae0fb)";
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```
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```js
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// module_2.js
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var featureName = "EcmaScript 2015 modules";
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var year = 2018;
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export function getFeatureDetails() {
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return "Feature name: " + featureName + " | developed in " + year;
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}
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```
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## Unsupported features
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* **snapshot**
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* errors from the imported scripts
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* redirection ( `export { a, b } from 'module.js'` )
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* default import and export
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* `import b from 'module.js'`
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* `export default b`,
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* whole module import statements
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* `import * from 'module.js`
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* `import { * as module } from 'module.js`
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* object freezing ( `Object.freeze (this)` )
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### Redirection
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An export statement can import variables from a custom module and export it directly from the current script. This statement is called redirection. In this case the `export { b } from 'module2.js'` works as the `b` was imported before then exported as a local variable.
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```js
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import { a, b } from 'module.js'
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print (a + b);
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```
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```js
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// module.js
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export var a = 2;
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export { b } from 'module2.js'
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```
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```js
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// module2.js
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export var b = 40;
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```
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### Default imports and exports
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TODO: This part is going to be written in the next part of the patch.
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### Import the whole module
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The whole module can be imported. In this case the `m` object would contain the exported parts of the module. If the import is not aliased, the `global object` would contain the exports.
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```js
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import { * as m } from "./module.js"
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print (m.secret_number);
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print (m.getPrettifiedNumber());
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print (m.api.version);
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```
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```js
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// module.js
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var secret_number = 42;
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export secret_number;
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export function getPrettifiedNumber() {
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return "*** " + secret_number + " ***";
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}
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export { ble as api } from "./ble.js";
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```
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