在Scala / Java中将undefined传递给Nashorn Javascript [英] Passing undefined to Nashorn Javascript in Scala/Java

问题描述

我需要在Scala / Java的Javascript中评估这个函数

I need to evaluate this function in Javascript from Scala/Java

function hello(a, b) {
    return a+b;
}

我做了这个基本代码：

val factory = new ScriptEngineManager(null)
val engine = factory.getEngineByName("JavaScript")

val body =
  """
    |function hello(a, b) {
    |    return a+b;
    |}
  """.stripMargin
engine match {
  case engine: Invocable =>
    engine.eval(body)
    println(engine.invokeFunction("hello", null, 1: java.lang.Double))
}

对于参数a，我传递的是null，结果得到1.0。如果我破解了我的javascript（我不想做这个）而且我做到了：

For the parameter a I'm passing a null and I get a 1.0 as a result. If I hack my javascript (I DON'T WONT TO DO THIS) and I make it:

function hello(a, b) {
    if (a === null) {
        a = undefined;
    }
    return a+b;
}

我得到预期的NaN。

正确的解决方案是将undefined传递给invokeFunction：
我该怎么做？

The correct solution would be passing an undefined to the invokeFunction: How do I do this?

推荐答案

ScriptEngine.eval将undefined转换为null - 因为Java中没有undefined的概念。如果你想获得内部nashorn未定义对象并传递它（为什么？），你可以这样做：

ScriptEngine.eval translates undefined to null - as there is no concept of undefined in Java. If you do want to get internal nashorn undefined object and pass it around (why?), you can do something like this:

import javax.script.*;

public class Test {
    private static Object undefined;

    public static void main(String[] args) throws Exception {
        ScriptEngineManager m = new ScriptEngineManager();
        ScriptEngine e = m.getEngineByName("nashorn");
        e.eval("Packages.Test.setUndefined(undefined)");
        e.eval("function func(a, b) { return a + b; }");

        Object val = ((Invocable)e).invokeFunction("func", undefined, 44);
        if (val instanceof Double && ((Double)val).isNaN()) {
            System.out.println("got NaN as expected");
        }
    }

    public static void setUndefined(Object obj) {
        undefined = obj;
    }
}

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