承诺从功能NodeJS返回

Question

我想从函数中返回承诺

router.post('/diff', function (req, res) {
    let firstfile = req.body.file1
    let secondfile = req.body.file2
    let result = diff(firstfile, secondfile)
    return res.json({ result })
})

function diff(firstfile, secondfile) {
 Promise.all([readFile(firstfile), readFile(secondfile)]).then(data => {
        var file1 = data[0];
        var file2 = data[1];

        var diff = Diff.diffLines(file1, file2);
        // return diff
        return res.json({ "Changes to file1" : diff[1], "Changes to file 2": diff[2], "file1": file1, "file2": file2 });
    });
}

它返回空-那么我该如何返回字典？