What is the difference between a JavaScript array and a JSON array?

A JavaScript array is a live, in-memory object you can push to, map over, and mutate. A JSON array is text — a string that follows the JSON grammar, used to store or transmit that data. JSON.stringify() turns the live array into the text form; JSON.parse() turns the text back into a live array. They look similar because JSON's array syntax was based on JavaScript's, but one is data in memory and the other is a string.

Why did some of my array items become null in the JSON?

JSON has no way to represent undefined, functions, or symbols. When one of those appears as an array element, JSON.stringify() replaces it with null so the array positions stay aligned. (Inside an object, such properties are dropped entirely instead.) NaN and Infinity also become null. If you see unexpected nulls, one of these non-serialisable values is the cause.

How do I turn the array into a downloadable .json file?

Pretty-print it with JSON.stringify(arr, null, 2), wrap the string in a Blob of type application/json, create an object URL with URL.createObjectURL(blob), and click a temporary anchor with a download attribute. Or skip the code: paste the array output into JSON Formatter Pro and use its Download button — everything stays in your browser.

How to Convert an Array to JSON in JavaScript (JSON.stringify)

To convert an array to JSON in JavaScript, pass it to JSON.stringify(). It accepts any array — of strings, numbers, booleans, objects, or nested arrays — and returns a JSON-formatted string. For example, JSON.stringify(["a", "b"]) returns '["a","b"]'. Add a third argument, JSON.stringify(value, null, 2), to pretty-print it with two-space indentation.

const fruits = ["apple", "banana", "cherry"];
const json = JSON.stringify(fruits);

console.log(json);
// '["apple","banana","cherry"]'  (a string)

The result is a string, not an array — that is the whole point. You now have a portable text form you can send in a request body, write to a file, or store in a database column, and later restore with JSON.parse().

Convert an array of objects to JSON

The same call works for an array of objects — the most common real-world case, such as rows from an API or a database query. Every object is serialised in order:

const users = [
  { id: 1, name: "Ada" },
  { id: 2, name: "Linus" },
];

JSON.stringify(users);
// '[{"id":1,"name":"Ada"},{"id":2,"name":"Linus"}]'

Pretty-print the JSON output

JSON.stringify() takes two optional arguments: a replacer and a space value. Pass null for the replacer and a number (or a string like a tab) for the space to get readable, indented output:

JSON.stringify(users, null, 2);
// [
//   {
//     "id": 1,
//     "name": "Ada"
//   },
//   ...
// ]

That indentation is exactly what a formatter does. If you would rather paste the array's output and beautify or validate it visually, the JSON formatter and JSON editor do it in your browser, with nothing uploaded.

Common gotchas when stringifying arrays

JSON is a smaller data model than JavaScript, so a few values do not survive the round trip. These are the ones that surprise people:

JSON.stringify([undefined, function () {}, Symbol("x")]);
// '[null,null,null]'   -> non-serialisable items become null

JSON.stringify([new Date(0)]);
// '["1970-01-01T00:00:00.000Z"]'   -> Date becomes an ISO string

JSON.stringify([NaN, Infinity]);
// '[null,null]'

const a = {};
a.self = a;
JSON.stringify([a]);
// TypeError: Converting circular structure to JSON

undefined, functions, symbols — become null inside an array (and are dropped inside an object).
Date — serialised to an ISO 8601 string via its toJSON() method.
NaN and Infinity — become null.
BigInt — throws a TypeError; convert it to a string or number first.
Circular references — throw a TypeError; break the cycle before stringifying.

Turn the JSON string back into an array

The inverse of JSON.stringify() is JSON.parse(), which reads the text and returns a live array you can work with again:

const json = '["apple","banana"]';
const arr = JSON.parse(json);

Array.isArray(arr); // true
arr[0];             // "apple"