How To Work With JSON In Node.js / JavaScript

Recently I started learning and working on Node.js platform. So, now I am again brushing up my long forgotten JavaScript skill. JSON is a common format you face every now and then in JavaScript, whether its client side or server side. As NodeJs being a server side platform, it has a little more capability than browser-based JavaScript, like writing to file system etc.

I am trying to aggregate most of the operations that we can usually perform related to JSON object. Though most of this tutorial will apply to both kind of JavaScript, few could be specific to Node.js.

String To JSON Object:

This is very much easier and straight forward as below:

var jsonString = "{\"key\":\"value\"}";
var jsonObj = JSON.parse(jsonString);
console.log(jsonObj.key);

As you can see, we are using the built-in global JSON Object to parse a string which has JSON Data. Also, it might be good idea to use “.trim()” method on the string, if you think there might be some chance of extra space etc in the JSON string. Otherwise, it won’t get parsed and you will face an unexpected error.

JSON Object To String:

As like the previous case, we can use the same global object’s ‘stringify’ method to convert a given json to string data. This can be done easily as like below:

var jsonObj = {'key':'value'};
console.log(JSON.stringify(jsonObj));

Treat User Defined Class Instance To JSON String:

If you are writing JavaScript OOP style and want to convert an object instance to JSON like string(with its attributes name/value as key/value), You still can use the same JSON object to string approach as below:

function MyClass(){
this.a = 'some value';
this.b = {
  'key': 'another json structure'
};
}

var instance = new MyClass();
console.log(JSON.stringify(instance));

However, you will need to be careful that you are declaring properties properly instead of declaring them as local variable. This stack-overflow thread might also help in understanding the differences easily.

Read JSON From File System In NodeJS:

At first I tried googling about it, I found a solution, which shows example with file system support of nodejs(fs module). But, I don’t really see any meaning of that at all, as we can simply do the same thing by:

var jsonObj = require("./path/to/myjsonfile.json");

Here, NodeJS automatically read the file, parse the content to a JSON object and assigns that to the left hand side variable. It’s as simple as that!

Add New Element To Existing JSON Object:

Say, you have an existing json object, which you want to modify to add new key/value pair(s). You can do that using either of the two ways as below:

var myJson = {'key':'value'};
//new element
myJson.key2 = 'value2';
//or
myJson[key3] = 'value3';

Delete An Element From A JSON Object:

Well, to delete an element from a JSON object, it can be done by using the ‘delete’ keyword. An example is given below:

var myJson = {'key':'value'};
delete myJson['key'];

Iterate Over A JSON Object:

Sometimes you will might need to traverse through each elements of the JSON object. This can be done in a for loop easily as like below:

var myJson = {'key':'value', 'key2':'value2'};
for(var myKey in myJson) {
   console.log("key:"+myKey+", value:"+myJson[myKey]);
}

However, the above code could give you error in case the value itself is a JSON object. So, you will might want to check whether the value is itself json or not and handle it thereby.

Check Key Existence:

If at some point we need to check whether a json object have a specific key, we can check that with below approach:

var myJson = {'key':'value', 'key2':'value2'};
if(myJson.hasOwnProperty('key2')){
     //do something if the key exist
}

Pretty Print JSON Object:

In debugging, we alway like to print data to console to verify if its OK. If you are trying to see if a large JSON has something you are expecting, then its very hard to locate if its printed in flat structure. In Such cases, what you need is pretty printing the JSON object. Here is the javascript code snippet that will do the trick:

JSON.stringify(myObj, null, 2);

Same applies if you are trying to write the json object in a file with pretty printed format.

Comparing Two JSON Objects:

If you need to compare two JSON objects to check if they are equal or not, it’s better to iterate over and compare each property. As a reference, please visit this stack-overflow thread.

Anything Else?

If you think anything is missing or something isn’t clear about using json data in node.js / JavaScript, please ask me know by commenting here. Happy coding 🙂