问题

我在我的项目中使用了 Nashorn。我想从 JSON 中获取属性，但该属性可能没有值。

在 JavaScript 中，使用可选链并设置一个值，如果属性为 null；但在 Nashorn 中，当我使用 (?.) 时，我会得到这个错误：

    import org.springframework.stereotype.Service
    import javax.script.ScriptEngineManager
    import javax.script.Bindings
    
    @Service
    class SampleService {
    
        class Person(
                val firstName: String,
                val lastName: String,
                val child: Child?
        )
    
        class Child(
                val name: String,
                val age: Int
        )
    
        fun runScript() {
            val engine = ScriptEngineManager().getEngineByName("nashorn")
            val bindings: Bindings = engine.createBindings()
            val person = Person("John", "Smite", null)
            bindings["person"] = person
            try {
                val script = """
                   var childAge = person.child?.age || 0;
                   childAge; // 作为结果返回。
                """.trimIndent()
                val scriptResult: Any = engine.eval(script, bindings)
            } catch (e: Exception) {
                throw e
            }
    
        }
    }

我得到了这个错误：

javax.script.ScriptException: <eval>:1:28 预期一个操作数，但找到了 。
var childAge = person.child?.age || 0;
^ 在 <eval> 中位于行号 1，列号 28

我查看了这个链接，但我无法解决这个问题：
可选链 (?.)

我该如何修复这个错误？

<details>
<summary>英文:</summary>

I use nashorn in my project. I want get property from a json, but property may not have value.

In javascript, use optional chaining and set a value, if property is null; but in nashorn, when I use (?.), I get this error:

    import org.springframework.stereotype.Service
    import javax.script.ScriptEngineManager
    import javax.script.Bindings
    
    @Service
    class SampleService {
    
        class Person(
                val firstName: String,
                val lastName: String,
                val child: Child?
        )
    
        class Child(
                val name: String,
                val age: Int
        )
    
        fun runScript() {
            val engine = ScriptEngineManager().getEngineByName(&quot;nashorn&quot;)
            val bindings: Bindings = engine.createBindings()
            val person = Person(&quot;John&quot;, &quot;Smite&quot;, null)
            bindings[&quot;person&quot;] = person
            try {
                val script = &quot;&quot;&quot;
                   var childAge = person.child?.age ?? 0;
                   childAge; //return as result.
                &quot;&quot;&quot;.trimIndent()
                val scriptResult: Any = engine.eval(script, bindings)
            } catch (e: Exception) {
                throw e
            }
    
        }
    }

I get this error:

    javax.script.ScriptException: &lt;eval&gt;:1:28 Expected an operand but found .
    var childAge = person.child?.age ?? 0;
    ^ in &lt;eval&gt; at line number 1 at column number 28

I checked this link, but I could not solve the problem:
[Optional chaining (?.)][1]


How can I fix this error?


  [1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining

</details>


# 答案1
**得分**: 1

以下是翻译好的内容：

目前 Nashorn 的策略是遵循 ECMAScript 规范。当我们发布 JDK 8 时，我们将与 ECMAScript 5.1 保持一致。

Nashorn 引擎已在 JDK 11 中被弃用，作为 JEP 335 的一部分，并计划在未来的 JDK 发布中将其移除，作为 JEP 372 的一部分。

> **GraalVM JavaScript** 可以作为 Nashorn 引擎之前执行的代码的替代品。它提供了之前由 Nashorn 提供的所有功能，其中许多功能默认可用，一些在标志后面，一些需要对源代码进行轻微修改。

可选链是相当新的特性，ECMAScript 5.1 不支持这个功能。

我从 **nashorn** 迁移到了 **GraalVM JavaScript**，并将代码更改如下：

```kotlin
import com.oracle.truffle.js.scriptengine.GraalJSScriptEngine
import org.graalvm.polyglot.Context
import org.graalvm.polyglot.HostAccess
import org.springframework.stereotype.Service
import javax.script.ScriptEngine

@Service
class SampleService {

    data class Person(
            val firstName: String,
            val lastName: String,
            val child: Child?
    )

    data class Child(
            val name: String,
            val age: Int
    )

    fun runScript() {

        val person = Person("John", "Smite", null)

        val engine: ScriptEngine = GraalJSScriptEngine.create(null,
                Context.newBuilder("js")
                        .allowHostAccess(HostAccess.ALL)
                        .allowExperimentalOptions(true)
                        .option("js.ecmascript-version", "2020")
                        .option("js.nashorn-compat", "true"))
        engine.put("person", person)
        try {
            val script = """
                print(person.child?.name);
                //print undefined
                print(person.child?.name ?: 'not exist');
                //print not exist
            """.trimIndent()
            engine.eval(script)
        } catch (e: Exception) {
            throw e
        }
    }
}

import com.oracle.truffle.js.scriptengine.GraalJSScriptEngine
import org.graalvm.polyglot.Context
import org.graalvm.polyglot.HostAccess
import org.springframework.stereotype.Service
import javax.script.ScriptEngine


@Service
class SampleService {

    data class Person(
            val firstName: String,
            val lastName: String,
            val child: Child?
    )

    data class Child(
            val name: String,
            val age: Int
    )

    fun runScript() {

        val person = Person("John", "Smite", null)

        val engine: ScriptEngine = GraalJSScriptEngine.create(null,
                Context.newBuilder("js")
                        .allowHostAccess(HostAccess.ALL)
                        .allowExperimentalOptions(true)
                        .option("js.ecmascript-version", "2020")
                        .option("js.nashorn-compat", "true"))
        engine.put("person", person)
        try {
            val script = """
                print(person.child?.name);
                //print undefined
                print(person.child?.name ?? 'not exist');
                //print not exist
            """.trimIndent()
            engine.eval(script)
        } catch (e: Exception) {
            throw e
        }
    }
}