Node JS Load JSON array from file

Question

a json file has been created with pretty print like that

[
  {
    "name": "c",
    "content": 3,
    "_prototype": "item"
  },
  {
    "name": "d",
    "content": 4,
    "_prototype": "item"
  }
]

I can read the file with that

var fs = require('fs');
var arrayPath = './array.json';

function fsReadFileSynchToArray (filePath) {
    var data = JSON.parse(fs.readFileSync(filePath));
    console.log(data);
    return data;
}

var test  = arr.loadFile(arrayPath);
console.log(test);

but the output is in reverse order

[]
[ { name: 'c', content: 3, _prototype: 'item' },
  { name: 'd', content: 4, _prototype: 'item' },]

obviously the second output is shown as first. I used actually synchronous file read to avoid such an empty data stuff. Is there a way to really make sure the JSON file is completely read into an array before continuing ?

[update] arr is a module which uses

function loadFile(filePath){
    var arrLines = [];
    fs.stat(filePath, function(err, stat) {
        if(err == null) {
            arrLines = fsReadFileSynchToArray(filePath);
        } else if(err.code == 'ENOENT') {
            console.log('error: loading file ' + filePath + ' not found');
        } else {
            console.log('error: loading file', err.code);
        }
    });
    return arrLines;
}

to return the values

Possible duplicate of [How do I return the response from an asynchronous call?](http://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call) — VLAZ, Oct 11 '16 at 15:39
@vlaz — There's no callback function anywhere. `readFileSync` is not asyncronous. — Quentin, Oct 11 '16 at 15:41
I don't understand how either of the `console.log()` calls outputs an empty array. Nor do I understand how one of them has a trailing comma after the last object in it. — Quentin, Oct 11 '16 at 15:42
it clearly uses fs.readFileSync . see Node tutorials abut the two are completetly different — user3732793, Oct 11 '16 at 15:42
`arr.loadFile` — What is `arr`? You don't define it anywhere. Your code won't compile, you need a real [MCVE] — Quentin, Oct 11 '16 at 15:43
@user3732793 Yes you have, you're using it in your code. Just not for JSON. `var objOrArray = require('./path/to/file.json');` — Mike Cluck, Oct 11 '16 at 15:53

score 27 · Answer 1 · answered Oct 11 '16 at 15:47

As long as the file is inside your project folder (config file, i.e.) you can load it synchronously requiring it directly in NodeJS.

var test = require('./array.json');

And then the content will be loaded into your variable in the next executed sentence.

You can try to console.log it, and it will print:

[ { name: 'c', content: '3', _prototype: 'item' },
  { name: 'd', content: '4', _prototype: 'item' } ]

Right exactly in the order that the file had.

that did work nicely thanks. Thought that is what Ben mentioned — user3732793, Oct 11 '16 at 15:52

score 5 · Accepted Answer · answered Oct 11 '16 at 15:48

5

fs.stat is async, so your function is async.

You want fs.fstatSync instead.

answered Oct 11 '16 at 15:48

Quentin

914,110
126
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t.niese · Answer 3 · 2016-10-11T16:12:10.300

Testing if a file exists before loading is not recommended and is the reason why fs.exists is deprecated:

Using fs.exists() to check for the existence of a file before calling fs.open(), fs.readFile() or fs.writeFile() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file does not exist.

And the way you use fs.stats is just an equivalent to the deprecated fs.exists.

So you should always assume that the file does not exits when reading it.

For a sync call you should write:

var data;

try {
   data = JSON.parse(fs.readFileSync(filePath));
} catch ( err ) {
   // handle your file not found (or other error) here
}

or

var data;

try {
   data = require('./array.json');
} catch ( err ) {
   // handle your file not found (or other error) here
}

And if you do it the async way then check the error argument of your callback function, to handle the case if the file does not exist:

fs.readFile(filePath, (err, fileContent) => {
    var data;
    if( err ) {
      // handle your file not found (or other error) here
    } else {
      data = JSON.parse(fileContent);
    }
})

score 4 · Answer 4 · edited Mar 24 '20 at 14:57

4

const fs=require('fs');
var obj = JSON.parse(fs.readFileSync('file.json', 'utf8'));
console.log(obj);

edited Mar 24 '20 at 14:57

Nikos Hidalgo

3,666
9
25
39

answered Mar 24 '20 at 13:26

Rahul Baby

159
2
2
7

Node JS Load JSON array from file

4 Answers4