JavaScript Sources Processing

The JavaScript code is not executed directly, it is first translated in Java code and compiled with the Java application code. This transpilation is done by the JavaScript Add-on Processor. This processor uses the Java Nashorn library (extracted from jre1.8.0_92) to parse the Javascript files.

The operations performed by this processor are summarized in this diagram:

MicroEJ JavaScript Processor Overview
  • Parsing: all JavaScript source files located in the folder src/main/js and src/test/js are parsed by the Nashorn library to provide a JavaScript AST.
  • JS Validation: validation on the JavaScript AST to detect unsupported language features (for example eval).
  • Conversion preparation: before actually converting the JavaScript AST to a Java AST, a preparation operation is done to initialize all the lexical environments (done by JsIrVisitor).
  • Conversion: conversion of the JavaScript AST to a Java AST.
  • Java AST cleanup/optim: post-conversion step to cleanup and optimize the Java AST. The following operations are done: - fix imports - remove dead code - remove literal statements
  • Java sources generation: generation of the Java sources from the Java AST.

Foreign Function Interface

As said in the section Calling Java from JavaScript, a JavaScript code can manipulate Java objects and call methods on Java objects. This chapter describes how does the call to methods on Java objets work.

Let getValue() a Java method called from JavaScript on a Java object. As long as the type of the object is not known at compile-time in the JavaScript code, all the types containing a method with the same signature are searched in the classpath. Then the JavaScript pre-processor generates a JsFfi class and a method that dynamically tries to find the type of the receiver object. So, when the getValue() method is called from JavaScript, this generated method is called.


Calling a method whose name is very common could result in a delay while calling it, and some useless methods embedded.

This example shares a Java Date of the current time:

JsRuntime.JS_GLOBAL_OBJECT.put("getCurrentDate", JsRuntime.createFunction(new JsClosure() {
        public Object invoke(Object thisBinding, Object... arguments) {
                return Calendar.getInstance().getTime();
}), false);

The JavaScript can then use this Date to print the current time:

var date = getCurrentDate()
var time = date.getTime()
print("Current time: ", time)

In this case, the generated method in JsFfi looks like:

public static Object ffi_getTime_0(Object function, @ej.annotation.Nullable Object this_) {
        try {
                if (this_ instanceof JsObject || this_ instanceof String)
                        return JsRuntime.functionCall(((Reference) function).getValue(), this_);
                if (this_ instanceof Calendar) {
                        return ((Calendar) this_).getTime();
                if (this_ instanceof Date) {
                        return new Double(((Date) this_).getTime());
        } catch (JsErrorWrapper e) {
                throw e;
        } catch (Throwable t) {
                throw new JsErrorWrapper(new JsObjectError.TypeError("A Java exception has been thrown in generated FFI code of getTime"), t);
        throw new JsErrorWrapper(new JsObjectError.TypeError("getTime"));