Part I: Implement ES2015 module system. (#2599)

JerryScript-DCO-1.0-Signed-off-by: Daniel Vince vinced@inf.u-szeged.hu

docs/05.PORT-API.md

@@ -83,6 +83,32 @@ information.
 void jerry_port_print_char (char c);
 ```
 
+### ES2015 Module system helper functions
+
+The import statement requires two specific functions for opening and closing files (the modules) port specific.
+
+```c
+/**
+ * Opens file with the given path and reads its source.
+ * @return the source of the file
+ */
+uint8_t *
+jerry_port_read_source (const char *file_name_p, /**< file name */
+                        size_t *out_size_p) /**< [out] read bytes */
+{
+  // open file from given path
+  // return its source
+} /* jerry_port_read_source */
+
+/**
+ * Release the previously opened file's content.
+ */
+void
+jerry_port_release_source (uint8_t *buffer_p) /**< buffer to free */
+{
+  free (buffer_p);
+} /* jerry_port_release_source */
+```
 
 ## Date
 

docs/14.MODULE-SYSTEM.md

@@ -0,0 +1,132 @@
+# ES6 module support for JerryScript
+
+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.
+The standard's relevant part can be found [here](https://www.ecma-international.org/ecma-262/6.0/#sec-modules).
+
+## General
+
+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.
+
+main.js
+
+```js
+import { secret_number } from "./module.js"
+
+print (secret_number);
+```
+
+module.js
+
+```js
+var secret_number = 42;
+
+export secret_number;
+```
+
+## Supported features
+
+* import variable or function
+  * add alias name to the imported variable (function)
+* export variable or function
+  * add alias name to the exported variable (function)
+
+### Example
+
+```js
+import {
+  engine,
+  version as v
+} from "./module.js"
+
+import { getFeatureDetails } from "./module_2.js"
+
+var version = "v3.1415";
+
+print("> main.js");
+
+print(">> Engine: " + engine);
+print(">> Version: " + v);
+
+print (">> " + getFeatureDetails());
+print (">> Script version: " + version);
+```
+
+```js
+// module.js
+var _engine = "JerryScript";
+export _engine as engine;
+
+export var version = "1.0 (e92ae0fb)";
+```
+
+```js
+// module_2.js
+var featureName = "EcmaScript 2015 modules";
+var year = 2018;
+
+export function getFeatureDetails() {
+  return "Feature name: " + featureName + " | developed in " + year;
+}
+```
+
+## Unsupported features
+
+* **snapshot**
+* errors from the imported scripts
+* redirection ( `export { a, b } from 'module.js'` )
+* default import and export
+  * `import b from 'module.js'`
+  * `export default b`,
+* whole module import statements
+  * `import * from 'module.js`
+  * `import { * as module } from 'module.js`
+* object freezing ( `Object.freeze (this)` )
+
+### Redirection
+
+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.
+
+```js
+import { a, b } from 'module.js'
+
+print (a + b);
+```
+
+```js
+// module.js
+export var a = 2;
+export { b } from 'module2.js'
+```
+
+```js
+// module2.js
+export var b = 40;
+```
+
+### Default imports and exports
+
+TODO: This part is going to be written in the next part of the patch.
+
+### Import the whole module
+
+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.
+
+```js
+import { * as m } from "./module.js"
+
+print (m.secret_number);
+print (m.getPrettifiedNumber());
+print (m.api.version);
+```
+
+```js
+// module.js
+var secret_number = 42;
+export secret_number;
+
+export function getPrettifiedNumber() {
+  return "*** " + secret_number + " ***";
+}
+
+export { ble as api } from "./ble.js";
+```