获取单词中重复次数最多的字母的数量

我正在尝试计算单词中重复次数最多的字母的数量

function GreatestCount(str)
{
    var count = {}

    for (var i = 0 ; i<str.length;i++)
    {
        var char = str[i];
        count[char] = (count[char] || 0) + 1;

    }

     //get the largest number for the letter counts
    var max = 0;

    for (var c in count) {
        if (count[c] > max) max = count[c];
    }

    return max
}

有人可以向我解释为什么吗

count[char] = (count[char] || 0) + 1;// this works

count[char] += 1 // this does not work 

Because

count[char] += 1

等于

count[char] = count[char] + 1

第一次运行代码时，count[char] is undefined所以它几乎与

undefined + 1 // which is NaN

工作版本通过安全地添加来规避这种情况0 using ||操作员。