samples/engine/lambda_as_func.js

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// Simple example demonstrating how to expose 'function's
// from embedding code. Any lambda object exposed to engine
// can be called as 'function' in script.

var ScriptEngineManager = Java.type("javax.script.ScriptEngineManager");
// create manager
var manager = new ScriptEngineManager();
// create engine
var engine = manager.getEngineByName("js");

// Any lambda (@FunctionalInterface annotated type) object can be
// be exposed from script embedding code. Script can call
// it as a function
engine.put("upper", new java.util.function.Function() {
     apply: function(x) x.toUpperCase()
});

print(engine.eval("upper('hello')"));