Using closure-compiler to parse js file and extract the desired docs.

ChkFlags.java

package com.baidu.lego.server.utils;

import java.util.ArrayList;
import java.util.List;

import org.junit.Test;

import com.google.gson.JsonPrimitive;
import com.google.javascript.jscomp.Compiler;
import com.google.javascript.jscomp.CompilerOptions;
import com.google.javascript.jscomp.CompilerPass;
import com.google.javascript.jscomp.JsAst;
import com.google.javascript.jscomp.NodeTraversal;
import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.jscomp.SourceFile;
import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;

class FlagDefinitions {
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    private String name;
    private String description;
    private String type;
    private JsonPrimitive defaultValue;

    public JsonPrimitive getDefaultValue() {
        return defaultValue;
    }

    public void setDefaultValue(JsonPrimitive defaultValue) {
        this.defaultValue = defaultValue;
    }
}

class CheckFlagsPass extends AbstractPostOrderCallback implements CompilerPass {
    private List<FlagDefinitions> defs;

    public List<FlagDefinitions> getDefs() {
        return defs;
    }

    public CheckFlagsPass() {
        defs = new ArrayList<FlagDefinitions>();
    }

    @Override
    public void process(Node externs, Node root) {
    }

    @Override
    public void visit(NodeTraversal t, Node n, Node parent) {
        if (n.getType() == Token.NAME && n.getString().startsWith("FLAGS_")) {
            JSDocInfo doc = parent.getJSDocInfo();
            if (doc != null) {
                Node value = n.getLastChild();
                JsonPrimitive defaultValue = null;
                if (value.getType() == Token.NUMBER) {
                    defaultValue = new JsonPrimitive(value.getDouble());
                } else if (value.getType() == Token.STRING) {
                    defaultValue = new JsonPrimitive(value.getString());
                } else if (value.getType() == Token.TRUE) {
                    defaultValue = new JsonPrimitive(true);
                } else if (value.getType() == Token.FALSE) {
                    defaultValue = new JsonPrimitive(false);
                } else {
                    // Invalid @define type.
                    return;
                }

                FlagDefinitions def = new FlagDefinitions();
                def.setName(n.getString());
                def.setDescription(doc.getBlockDescription());
                def.setType(doc.getMarkers().iterator().next().getType()
                        .getItem().getString());
                def.setDefaultValue(defaultValue);
                defs.add(def);
            }
        }
    }

}

public class DynamicFlagsTest {
    @Test
    public void testGetJsDocInfo() {
        Compiler compiler = new Compiler();
        CompilerOptions options = new CompilerOptions();
        options.ideMode = true;
        compiler.initOptions(options);

        String fileName = "/Volumes/HDD/Users/leeight/local/app/ecom/lego/git/lego/lego-widgets/src/ad/flags.js";
        Node root = new JsAst(SourceFile.fromFile(fileName))
                .getAstRoot(compiler);
        CheckFlagsPass pass = new CheckFlagsPass();
        NodeTraversal.traverse(compiler, root, pass);

        System.out.println(JsonUtils.toPrettyPrintJson(pass.getDefs()));
    }
}