javascript - 当迭代嵌套在嵌套在另一个数组中的对象中的数组时，jQuery click 方法返回未定义

Question

为什么for in循环遍历第一个answer对象数组后click方法返回undefined？

var i = 0;
var j = 0;

var allQuestions = [{question:"What is the color of the sky?",answer:["blue","yellow",  
                "red"],correctAnswer:["blue",1]},{question:"What is the opposite of  
                 up?",answer:["down","sideways", "circle"],correctAnswer:["down",1]},
                {question:"What is the first number?",answer:["1","5", "7"],correctAnswer:
                ["1",1]}];

$(document).ready(function() {  

    function changeQuestion() {
        $("#radios").empty();
        for( answers in allQuestions[i].answer) {
            var radioBtn = $('<input type="radio" class="radios" name="btnAnswers" value="'+ 
                allQuestions[i].answer[j] + '" /><label for ="secondbtn">'
                + allQuestions[i].answer[j] + '</label>');
            radioBtn.appendTo('#radios');
            alert(allQuestions[i].answer[j]);
            j++;
        }
        i++;
        return true;
    };

    $("#nextbtn").click(function(){
        changeQuestion();
    });
});

This question's fiddle

Answer 1

由于对象的 .answer 属性是一个数组，您可以循环使用

for(var j=0; allQuestions[i].answer.length; j++) {...}

代替

for(answers in allQuestions[i].answer) {...}

完整代码为:

$(document).ready(function() {
    var i = 0;
    function changeQuestion() {
        $("#radios").empty();
        var obj = allQuestions[i];
        for(var j=0; j<obj.answer.length; j++) {
            $('<input type="radio" class="radios" name="btnAnswers" value="' + obj.answer[j] + '" /><label for ="secondbtn">' + obj.answer[j] + '</label>').appendTo('#radios');
        }
        i++;
    };
    $("#nextbtn").click(changeQuestion);
});

https://jsfiddle.net/zu9e5poh/10/