How to Convert to JSON in JavaScript

Use JSON.stringify() when JavaScript objects or arrays need to become a JSON string for APIs, local storage, logging, or file export. This is the standard way to serialize data.

What you’ll build or solve

You’ll learn how to convert to JSON in JavaScript using JSON.stringify(). You’ll also know how to format readable JSON and avoid unsupported values.

When this approach works best

This approach is the right choice when structured JavaScript data needs to leave memory as text.

Common real-world scenarios include:

• API POST bodies
• Local storage
• Config export
• Debug logging
• File downloads

This is a bad idea when the data should stay as a live object for immediate code logic.

Prerequisites

You only need:

• A JavaScript file or browser console
• A basic object or array
• Basic object syntax knowledge

Step-by-step instructions

Step 1: Pass the object into JSON.stringify()

Start with a JavaScript object.

const user = {
  name: "Alex",
  role: "admin"
};

Use JSON.stringify() to convert it into a JSON string.

const json = JSON.stringify(user);

console.log(json);

This also works for arrays.

const skills = ["HTML", "CSS", "JavaScript"];
const jsonSkills = JSON.stringify(skills);

For readable formatting, add spacing arguments.

const prettyJson = JSON.stringify(user, null, 2);
console.log(prettyJson);

What to look for:

• Input is a JavaScript object or array
• Output is a JSON string
• Great for APIs and local storage
• Pretty formatting helps debugging
• Functions and undefined are skipped

Examples you can copy

API request body

const body = JSON.stringify({
  email: "alex@example.com"
});

Save settings

localStorage.setItem("settings", JSON.stringify(settings));

Debug export

console.log(JSON.stringify(data, null, 2));

Common mistakes and how to fix them

Mistake 1: Forgetting to stringify before local storage

What the reader might do:

localStorage.setItem("user", user);

Why it breaks: local storage stores strings, so the object becomes [object Object].

Corrected approach:

localStorage.setItem("user", JSON.stringify(user));

Mistake 2: Expecting functions to survive

What the reader might do:

const user = {
  name: "Alex",
  greet() {}
};

Why it breaks: functions are omitted from JSON output.

Corrected approach:

Only serialize data, not behavior.

Mistake 3: Using the JSON string like an object

What the reader might do:

const json = JSON.stringify(user);
console.log(json.name);

Why it breaks: the result is a string.

Corrected approach:

Use the original object, or parse the string later.

Troubleshooting

If local storage shows [object Object], add JSON.stringify().

If properties disappear, check for functions or undefined.

If the output is hard to read, use the spacing argument.

If later code needs object access, parse the string back with JSON.parse().

Quick recap

• Use JSON.stringify() on objects or arrays
• The result is a JSON string
• Great for APIs and storage
• Use spacing for readable debug output
• Parse back later when object access is needed