Filter certain values from the output of JSON.parse() method

Amit Merchant · August 5, 2021 ·

If you’ve ever worked with JSON objects in JavaScript, you most probably have reached the JSON.parse() method at some point or another which parses a JSON string and returns the JavaScript value or object described by the string.

For instance, let’s say, you have a JSON string like the following…

const User = `{
  "name": "Cherika",
  "age": 5,
  "is_active": false
}`

and if you want to transform it into a JavaScript object, you can do it using the JSON.parse() like so.

const userObject = JSON.parse(User);

// { name: 'Cherika', age: 5, is_active: false }

Now, what if you want to change the value of a certain key before the function returns the object, how can you do that?

Well, that is where an optional reviver function that you can pass to the JSON.parse() comes into play.

The reviver function

So, the way you can do this is you need to pass in a reviver function as a second argument to the JSON.parse() function.

Now, let’s say, you want to set the value of the is_active key to true before it returns the output in the previous example, you can do it like so.

const User = `{
  "name": "Cherika",
  "age": 5,
  "is_active": false
}`

const userObject = JSON.parse(User, (key, value) => {
  if (key === 'is_active') {
    return true;
  }

  return value;
});

// { name: 'Cherika', age: 5, is_active: true }

As you can tell, the reviver function will receive each key and value pair as the argument, and from there, you can transform the value based on the key.

Like in the example above, you can return the value as true when the key is is_active.

One thing to note here is, since the reviver function would go over each key-value pair, you need to make sure you are returning a value for each key of the object.

So, something like the following would result in an undefined object.

const userObject = JSON.parse(User, (key, value) => {
  if (key === 'is_active') {
    return true;
  }
});

// undefined

👋 Hi there! I'm Amit. I write articles about all things web development. If you like what I write and want me to continue doing the same, I would like you buy me some coffees. I'd highly appreciate that. Cheers!

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