Let's say you have a file named app.js that contains the following.

let data = '{"my_string": "Hello World", "my_boolean": true, "my_integer" :42}';
let obj = JSON.parse(data);
let pretty_json = JSON.stringify(obj, null, 2)
console.log(pretty_json)
console.log(obj.my_string);
console.log(obj.my_boolean);
console.log(obj.my_integer);

If you use the node CLI to run app.js, the following should be displayed as stdout on your console.

~]$ node app.js
{"my_string":"Hello World","my_boolean":true,"my_integer":42}
Hello World
true
42

In this example, a new key/value pair is appended to the JSON object. The key is "foo" and the value is "bar".

let data = '{"my_string": "Hello World", "my_boolean": true, "my_integer" :42}';
let obj = JSON.parse(data);
obj.foo = 'bar'
let pretty_json = JSON.stringify(obj, null, 2)
console.log(pretty_json)

In this example, the values of keys in the JSON object are updated.

let data = '{"my_string": "Hello World", "my_boolean": true, "my_integer" :42}';
let obj = JSON.parse(data);

obj.my_string = 'Goodbye World'
obj.my_boolean = false
obj.my_integer = 67

let pretty_json = JSON.stringify(obj, null, 2)
console.log(pretty_json)