增量运算符返回NaN

我试图使用++运算符递增变量，但结果总是得到NaN，我不确定为什么。以下是我的代码：

var wordCounts = { };
var x = 0
var compare = "groove is in the heart";
        var words = compare.split(//);
        for(var i = 1; i < words.length; i++){
            if(words[i].length > 2){
                wordCounts["_" + words[i]]++;
            }
        }


alert(wordCounts.toSource());

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解决方案

wordCounts["_" + words[i]]的值最初是undefined，所以当您++它时，它会给您NaN。只需将您的代码更改为：

if (wordCounts["_" + words[i]]) {
    wordCounts["_" + words[i]]++;
} else {
    wordCounts["_" + words[i]] = 1;
}