Read and Parse JSON asynchronously

JSON stands for JavaScript Object Notation. JSON is syntax for storing and exchanging data. It is a text, written with JavaScript Object Notation.

JavaScript can parse JSON file synchronously and asynchronously. The problem with using synchronous is when used in a webServer where all the requests will become blocked while the synchronous file read is running. For, this reason we generally use async version of the fs function in our code.

Accessing files in the NodeJS is done with the native module fs write gives you the functionality of read, write, along with many other tools. Because it is native module we do not need to dowload and install like other module we can simply import it as

const fs  = require('fs');

This is our JSON file ‘todos.json’

   {
    "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
        } 
    ]
}

So, to read the JSON file

const fs  = require('fs');
    fs.readFile('todos/json', 'utf8', (err, data) => {
        if(err) {
            console.log('Error While Reading JSON file', err);
        }else{
            console.log('File Read', data)
        }
    });

todos.json is the relative path of our json file. utf8 is optional parameters for encoding the file that we are reading. and (err, data) => {} is the callback function that runs after the file has been read.

After having the concept of reading the JSON file here is the complete example of parsing the JSON file

const fs = require('fs');

app.get('/todos', (req, res) => {

    fs.readFile('todos.json', 'utf-8', (err, data) => {
        if(err) {
            console.log('Error Occured', err)
        }
        try{
            const todos = JSON.parse(data);
            console.log('JSON parsed' + todos.todos[0].text);
            res.send('JSON Parsed ' +  todos.todos.map(todo => todo.done))
        }catch(err) {
            console.log('Error from catch Block', err );
        }
    });
});

Using the data from reading todos.json we create an object now we can access the property from the JSON. JSON.parse can throws an exception errors and halt our program if any invalid JSON string is passed so, such type of exception are caught in catch block.

todos.todos.[0].text returns the property(text) from the first array of objects todos.todos.map(todo => todo.done) returns the property(done) form all array of objects.

So, JSON is one of the common and important types of data while working with Node. Having the concept of read, write file as well as parsing the JSON file is important.

NodeJS, JSON