Node: cannot parse a JSON via API

Question

I load a JSON from file as a string, try to parse it back to JSON and send as a response:

router.get('/todos', (req,res) =>{
    let todos = fs.readFile('todos.json', 'utf8',(err, data) =>{
        if (err) throw err

        res.send(JSON.parse(todos)) 
    })
})

my todos.json file:

{
    "todos": [
        {
            "id": 1,
            "text": "task number 1",
            "priority": 3,
            "done": false
        },
        {
            "id": 2,
            "text": "task number 2",
            "priority": 3,
            "done": false
        },
        {
            "id": 3,
            "text": "task number 3",
            "priority": 3,
            "done": false
        },
        {
            "id": 4,
            "text": "task number 4",
            "priority": 3,
            "done": false
        },
        {
            "id": 5,
            "text": "task number 5",
            "priority": 3,
            "done": false
        }
    ]
}

However I get this error:

undefined ^

SyntaxError: Unexpected token u in JSON at position 0 at JSON.parse () at fs.readFile (/Users/mgonline/Desktop/programs/assessment/node-todo/routes/api/v1/todo.js:11:17) at FSReqWrap.readFileAfterClose [as oncomplete] (internal/fs/read_file_context.js:53:3)

I don't know why but when I console.log this string fetched from file it seems like my program change it a bit by adding a commma after last object in array of collections:

{
    "id": 5,
    "text": "task number 5",
    "priority": 3,
    "done": false
},

Which supposedly rise this error. Should I rewrite my JSON different way in order to avoid collisions?

Try res.send(JSON.parse(data)) instead of res.send(JSON.parse(todos)) — Sebastian Kaczmarek
– Sebastian Kaczmarek, Commented Apr 5, 2020 at 14:06

Wyck · Accepted Answer · 2020-04-05 14:31:51Z

1

The problem

You are using fs.readFile incorrectly. fs.readFile will return undefined because readFile is asynchronous. The contents of the file actually arrive in the data parameter of the callback that you provided.

To Fix

Remove let todos = entirely and just call fs.readFile (because you don't need the return value of fs.readFile).

Then call JSON.parse(data) instead of JSON.parse(todos) (because that's where your file's contents arrive).

Before

router.get('/todos', (req,res) =>{
    let todos = fs.readFile('todos.json', 'utf8',(err, data) =>{
        if (err) throw err

        res.send(JSON.parse(todos)) 
    })
})

After

router.get('/todos', (req,res) =>{
    fs.readFile('todos.json', 'utf8',(err, data) =>{
        if (err) throw err

        res.send(JSON.parse(data)) 
    })
})

answered Apr 5, 2020 at 14:31

Wyck

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Comments

Nicky · Accepted Answer · 2020-04-05 14:09:38Z

1

I guess,The editor automatically adds a comma when saving. try to change the setting of IDE to removing the comma from the array element when the json file is saved

answered Apr 5, 2020 at 14:09

Nicky

765 bronze badges

Comments

Rayon · Accepted Answer · 2020-04-05 14:34:33Z

1

You have not defined todos in your application, it is undefined hence you are getting error as,

As corrected by @Sebastian Kaczmarek, todos is holding return value of fs.readFile. As fs.readFile returns "undefined", you are getting error.

Use data which is the name of the argument in your script which holds the content of the required file.

router.get('/todos', (req, res) => {
  fs.readFile('todos.json', 'utf8', (err, data) => {
    if (err) throw err
    res.send(JSON.parse(data));
  })
})

edited Apr 5, 2020 at 14:34

answered Apr 5, 2020 at 14:18

Rayon

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2 Comments

Sebastian Kaczmarek Over a year ago

Actually, he did define todos. He wrote let todos = fs.readFile(...) so it is defined but it's value is undefined. If it wasn't defined he would get ReferenceError

Rayon Over a year ago

@SebastianKaczmarek- Oops! My bad, I will edit it. I totally missed that. Thanks!

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