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With inspiration from other libraries
There is a newer version: 2.12.15
/*
 * Copyright (c) 2012-2017 The ANTLR Project. All rights reserved.
 * Use of this file is governed by the BSD 3-clause license that
 * can be found in the LICENSE.txt file in the project root.
 */

package org.antlr.v4.codegen.target;

import org.antlr.v4.codegen.CodeGenerator;
import org.antlr.v4.codegen.Target;
import org.antlr.v4.codegen.UnicodeEscapes;
import org.antlr.v4.tool.ast.GrammarAST;
import org.stringtemplate.v4.STGroup;
import org.stringtemplate.v4.StringRenderer;

import java.util.Arrays;
import java.util.HashSet;
import java.util.Locale;
import java.util.Set;

/**
 *
 * @author Eric Vergnaud
 */
public class JavaScriptTarget extends Target {

	/** Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar */
	protected static final String[] javaScriptKeywords = {
		"break", "case", "class", "catch", "const", "continue", "debugger",
		"default", "delete", "do", "else", "export", "extends", "finally", "for",
		"function", "if", "import", "in", "instanceof", "let", "new", "return",
		"super", "switch", "this", "throw", "try", "typeof", "var", "void",
		"while", "with", "yield",
		//future reserved
		"enum", "await", "implements", "package", "protected", "static",
		"interface", "private", "public",
		//future reserved in older standards
		"abstract", "boolean", "byte", "char", "double", "final", "float",
		"goto", "int", "long", "native", "short", "synchronized", "transient",
		"volatile",
		//literals
		"null", "true", "false"
	};

	/** Avoid grammar symbols in this set to prevent conflicts in gen'd code. */
	protected final Set badWords = new HashSet();

	public JavaScriptTarget(CodeGenerator gen) {
		super(gen, "JavaScript");
	}

    @Override
    public String getVersion() {
        return "4.7.2";
    }

    public Set getBadWords() {
		if (badWords.isEmpty()) {
			addBadWords();
		}

		return badWords;
	}

	protected void addBadWords() {
		badWords.addAll(Arrays.asList(javaScriptKeywords));
		badWords.add("rule");
		badWords.add("parserRule");
	}

	@Override
	public String encodeIntAsCharEscape(int v) {
		if (v < Character.MIN_VALUE || v > Character.MAX_VALUE) {
			throw new IllegalArgumentException(String.format("Cannot encode the specified value: %d", v));
		}

		if (v >= 0 && v < targetCharValueEscape.length && targetCharValueEscape[v] != null) {
			return targetCharValueEscape[v];
		}

		if (v >= 0x20 && v < 127) {
			return String.valueOf((char)v);
		}

		String hex = Integer.toHexString(v|0x10000).substring(1,5);
		return "\\u"+hex;
	}

	@Override
	public int getSerializedATNSegmentLimit() {
		return 2 ^ 31;
	}

	@Override
	public int getInlineTestSetWordSize() {
		return 32;
	}

	@Override
	protected boolean visibleGrammarSymbolCausesIssueInGeneratedCode(GrammarAST idNode) {
		return getBadWords().contains(idNode.getText());
	}

	@Override
	protected STGroup loadTemplates() {
		STGroup result = super.loadTemplates();
		result.registerRenderer(String.class, new JavaStringRenderer(), true);
		return result;
	}

	protected static class JavaStringRenderer extends StringRenderer {

		@Override
		public String toString(Object o, String formatString, Locale locale) {
			if ("java-escape".equals(formatString)) {
				// 5C is the hex code for the \ itself
				return ((String)o).replace("\\u", "\\u005Cu");
			}

			return super.toString(o, formatString, locale);
		}

	}

	@Override
	public boolean wantsBaseListener() {
		return false;
	}

	@Override
	public boolean wantsBaseVisitor() {
		return false;
	}

	@Override
	public boolean supportsOverloadedMethods() {
		return false;
	}

	@Override
	protected void appendUnicodeEscapedCodePoint(int codePoint, StringBuilder sb) {
		// JavaScript and Java share the same escaping style.
		UnicodeEscapes.appendJavaStyleEscapedCodePoint(codePoint, sb);
	}
}