How do I update a value in a json file and save it through node.js? I have the file content:
var file_content = fs.readFileSync(filename);
var content = JSON.parse(file_content);
var val1 = content.val1;
Now I want to change the value of val1
and save it to the file.
addition to the previous answer add file path directory for the write operation
fs.writeFile(path.join(__dirname,jsonPath), JSON.stringify(newFileData), function (err) {}
// read file and make object
let content = JSON.parse(fs.readFileSync('file.json', 'utf8'));
// edit or add property
content.expiry_date = 999999999999;
//write file
fs.writeFileSync('file.json', JSON.stringify(content));
//change the value in the in-memory object
content.val1 = 42;
//Serialize as JSON and Write it to a file
fs.writeFileSync(filename, JSON.stringify(content));
I would strongly recommend not to use synchronous (blocking) functions, as they hold other concurrent operations. Instead, use asynchronous fs.promises:
const fs = require('fs').promises
const setValue = (fn, value) =>
fs.readFile(fn)
.then(body => JSON.parse(body))
.then(json => {
// manipulate your data here
json.value = value
return json
})
.then(json => JSON.stringify(json))
.then(body => fs.writeFile(fn, body))
.catch(error => console.warn(error))
Remeber that setValue
returns a pending promise, you'll need to use .then function or, within async functions, the await operator.
// await operator
await setValue('temp.json', 1) // save "value": 1
await setValue('temp.json', 2) // then "value": 2
await setValue('temp.json', 3) // then "value": 3
// then-sequence
setValue('temp.json', 1) // save "value": 1
.then(() => setValue('temp.json', 2)) // then save "value": 2
.then(() => setValue('temp.json', 3)) // then save "value": 3
Doing this asynchronously is quite easy. It's particularly useful if you're concerned for blocking the thread (likely).
const fs = require('fs');
const fileName = './file.json';
const file = require(fileName);
file.key = "new value";
fs.writeFile(fileName, JSON.stringify(file), function writeJSON(err) {
if (err) return console.log(err);
console.log(JSON.stringify(file));
console.log('writing to ' + fileName);
});
The caveat is that json is written to the file on one line and not prettified. ex:
{
"key": "value"
}
will be...
{"key": "value"}
To avoid this, simply add these two extra arguments to JSON.stringify
JSON.stringify(file, null, 2)
null
- represents the replacer function. (in this case we don't want to alter the process)
2
- represents the spaces to indent.
For those looking to add an item to a json collection
function save(item, path = './collection.json'){
if (!fs.existsSync(path)) {
fs.writeFile(path, JSON.stringify([item]));
} else {
var data = fs.readFileSync(path, 'utf8');
var list = (data.length) ? JSON.parse(data): [];
if (list instanceof Array) list.push(item)
else list = [item]
fs.writeFileSync(path, JSON.stringify(list));
}
}
Save data after task completion
fs.readFile("./sample.json", 'utf8', function readFileCallback(err, data) {
if (err) {
console.log(err);
} else {
fs.writeFile("./sample.json", JSON.stringify(result), 'utf8', err => {
if (err) throw err;
console.log('File has been saved!');
});
}
});
Source: Stackoverflow.com