javascript - Modify a JSON file with a JS function

I have a json array in a .json file. This file is linked with a node.js file on the server side. This file is handling user comments with a post request. For now, I push a line in the object "comments".

This is my Json file:

[{
    "id": 0,

    "test_id": "Password test",
    "pass": 1,
    "fail": 5,
    "time": 0.03,
    "pass_fail": 20,
    "comments":[ {
        "comment": "",
        "commentuser": ""
    }] 
},

{
    "id": 1,
    "test_id": "Password test",
    "pass": 1,
    "fail": 5,
    "time": 0.03,
    "pass_fail": 20,
    "comments": [{
        "comment": "",
        "commentuser": ""

    }] 
},
{
    "id": 2,
    "test_id": "Form testing",
    "pass": 10,
    "fail": 0,
    "time": 0.09,
    "pass_fail": 100,
    "comments": [{

        "comment": "",
        "commentuser": ""
    }] 
},
{
    "id": 3,
    "test_id": "Login Test 2",
    "pass": 15,
    "fail": 0,
    "time": 0.04,

    "pass_fail": 100,
    "comments":[ {
        "comment": "",
        "commentuser": ""
    }] 
},
{
    "id": 4,
    "test_id": "Pen Test",
    "pass": 119,

    "fail": 2,
    "time": 0.04,
    "pass_fail": 0,
    "comments": [{
        "comment": "",
        "commentuser": ""
    }] 
},
{
    "id": 5,

    "test_id": "Pen Test 2",
    "pass": 19,
    "fail": 22,
    "time": 0.08,
    "pass_fail": 1,
    "comments":[ {
        "comment": "",
        "commentuser": ""
    }] 
},

{
    "id": 6,
    "test_id": "Pen Test 3",
    "pass": 119,
    "fail": 2,
    "time": 0.04,
    "pass_fail": 40,
    "comments":[ {
        "comment": "",
        "commentuser": ""

    } ]
},
{
    "id": 7,
    "test_id": "Login Test 4",
    "pass": 119,
    "fail": 2,
    "time": 0.04,
    "pass_fail": 0,
    "comments":[ {

        "comment": "",
        "commentuser": ""
    } ]
},
{
    "id": 8,
    "test_id": "Pen Test 4",
    "pass": 119,
    "fail": 210,
    "time": 0.04,

    "pass_fail": 80,
    "comments":[ {
        "comment": "",
        "commentuser": ""
    }]
},
{
    "id": 9,
    "test_id": "Pen Test 5",
    "pass": 12,

    "fail": 2,
    "time": 0.04,
    "pass_fail": 0,
    "comments":[ {
        "comment": "",
        "commentuser": ""
    } ]
}
]

When a comment is posted the object comments is pushed.
And this is my post request with the push:

app.post('/test/:id/comment', urlencodedParser, function (req, res) {
    console.log("User Name : " + req.body.name);
    console.log("User Comment: " + req.body.comment);
    if (req.body.name && req.body.comment) {
        console.log('Your comment was posted !');
        res.sendFile(__dirname + '/HTML/comment-success.html');

        testing[req.params.id].comments.push({
            comment: req.body.comment,
            commentuser: req.body.name
        })

I want to have the post request such that JS write a new line in the Json file.
Such that the comment is still there once i turn off the server and then on again.
Something like document.write.
How can I implement that ?

Thanks in advance!