函数范围内的变量（Javascript）

Question

如果文件中的单词与要找到的单词相匹配，怎么计数总是为0？

getWordCount (wordToFind, files) {
        var count = 0;
        for (let i in files) {
            FS.readFile(files[i], 'utf8', function (err, data) {
                data.split(/\s+/).forEach(function (word) {
                    if (word == wordToFind) {
                        count = count + 1;
                    }
                });
            });
        }
        console.log('Word Count for word: ' + wordToFind + ' is: ' + count);