要遍历JavaScript中的if/elseif/else语句，您可以使用ANTLR（ANother Tool for Language Recognition）来进行词法分析和语法分析。ANTLR是一个非常强大的工具，可用于构建语法解析器。

下面是一个示例ANTLR语法文件，用于解析JavaScript中的if/elseif/else语句：

grammar JavaScript;

ifStatement: 'if' '(' condition ')' statement ( 'else' statement )?;
condition: expression;
statement: block | ifStatement;
block: '{' statement* '}';

expression: // 定义表达式规则
  | '(' expression ')'                   #parenthesesExpression
  | expression '.' Identifier             #dotExpression
  | expression '[' expression ']'         #indexExpression
  | expression '(' expressionList? ')'    #functionCallExpression
  | expression ('++' | '--')              #postfixIncDecExpression
  | ('++' | '--') expression              #prefixIncDecExpression
  | ('+' | '-') expression                #unaryPlusMinusExpression
  | ('~' | '!') expression                #bitwiseNotExpression
  | expression ('*' | '/' | '%') expression   #multiplicativeExpression
  | expression ('+' | '-') expression          #additiveExpression
  | expression ('<<' | '>>' | '>>>') expression  #shiftExpression
  | expression ('<' | '>' | '<=' | '>=') expression  #relationalExpression
  | expression ('==' | '!=' | '===' | '!==') expression  #equalityExpression
  | expression ('&' | '^' | '|') expression    #bitwiseAndExpression
  | expression '&&' expression              #logicalAndExpression
  | expression '||' expression              #logicalOrExpression
  | expression '?' expression ':' expression   #ternaryExpression
  | expression (',' expression)+            #commaExpression
  | literalExpression                       #literalExpression
  | Identifier                              #identifierExpression;

expressionList: expression (',' expression)*;

literalExpression: 
  | BooleanLiteral
  | NumericLiteral
  | StringLiteral
  | NullLiteral;

BooleanLiteral: 'true' | 'false';
NumericLiteral: DecimalLiteral | HexIntegerLiteral;
DecimalLiteral: DecimalIntegerLiteral ('.' DecimalIntegerLiteral)? ExponentPart?;
DecimalIntegerLiteral: '0' | NonZeroDigit Digit*;
HexIntegerLiteral: '0x' HexDigit+;

StringLiteral: '"' StringCharacters? '"';
StringCharacters: StringCharacter+;
StringCharacter: ~["\n\r] | EscapeSequence;
EscapeSequence: '\\' [btnfr"'\\] | UnicodeEscapeSequence;
UnicodeEscapeSequence: '\\u' HexDigit HexDigit HexDigit HexDigit;

Identifier: IdentifierName - ReservedWord;
IdentifierName: IdentifierStart IdentifierPart*;
IdentifierStart: [a-zA-Z_$];
IdentifierPart: IdentifierStart | [0-9];

ReservedWord: // 省略保留字定义

WS: [ \t\r\n]+ -> skip;

以上示例中的语法规则描述了JavaScript中的各种语法结构，包括if/elseif/else语句。您可以根据自己的需要进行扩展和修改。

使用ANTLR生成解析器代码后，您可以使用ANTLR提供的API进行遍历和分析。下面是一个简单的示例，展示如何使用ANTLR解析JavaScript代码并遍历if/elseif/else语句：

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // 创建一个ANTLR的CharStream，从JavaScript代码中读取输入
        CharStream input = CharStreams.fromFileName("input.js");

        // 使用JavaScriptLexer对输入进行词法分析
        JavaScriptLexer lexer = new JavaScriptLexer(input);
        CommonTokenStream tokens = new CommonTokenStream(lexer);

        // 使用JavaScriptParser对词法分析结果进行语法分析
        JavaScriptParser parser = new JavaScriptParser(tokens);
        ParseTree tree = parser.program();

        // 创建一个自定义的ParseTreeWalker
        ParseTreeWalker walker = new ParseTreeWalker();

        // 注册针对ifStatement规则的监听器
        walker.walk(new JavaScriptListener(), tree);
    }

    public static class JavaScriptListener extends JavaScriptBaseListener {
        @Override
        public void enterIfStatement(JavaScriptParser.IfStatementContext ctx) {
            // 进入ifStatement规则时的处理逻辑
            System.out