Free JSON Developer Tools
2026-02-26 · 12 min read · By AllJSONTools
Paste broken JSON and fix it instantly with AI — plain-English explanations included.
JavaScript has two built-in methods for working with JSON. They ship in every browser and Node.js runtime — no libraries required.
// Parse a JSON string into a JavaScript object
const json = '{"name": "Jane", "age": 28}';
const user = JSON.parse(json);
console.log(user.name); // "Jane"
// Convert a JavaScript object into a JSON string
const obj = { name: "Jane", age: 28 };
const str = JSON.stringify(obj);
console.log(str); // '{"name":"Jane","age":28}'That covers 90% of use cases. The rest of this guide covers the edge cases, gotchas, and advanced patterns you’ll hit in production.
JSON.parse() throws a SyntaxError if the input is not valid JSON. If you don’t catch it, your entire function crashes.
function safeParse(jsonString) {
try {
return { data: JSON.parse(jsonString), error: null };
} catch (err) {
return { data: null, error: err.message };
}
}
const result = safeParse('{"broken": }');
if (result.error) {
console.error("Invalid JSON:", result.error);
} else {
console.log(result.data);
}This is especially important when parsing user input, API responses, or data from localStorage. Use the JSON Formatter to quickly validate and debug malformed JSON strings.
JSON.parse() accepts an optional second argument — a reviver function that transforms each key-value pair during parsing. The most common use: converting date strings into Date objects.
const json = '{"name": "Jane", "createdAt": "2026-02-25T10:30:00Z"}';
const user = JSON.parse(json, (key, value) => {
// Convert ISO date strings to Date objects
if (typeof value === "string" && /^d{4}-d{2}-d{2}T/.test(value)) {
return new Date(value);
}
return value;
});
console.log(user.createdAt instanceof Date); // true
console.log(user.createdAt.getFullYear()); // 2026JSON.stringify() accepts two optional arguments: a replacer (to filter or transform values) and a space parameter (for pretty-printing).
const user = {
name: "Jane",
password: "secret123",
role: "admin",
createdAt: new Date("2026-02-25"),
};
// Pretty-print with 2-space indentation
console.log(JSON.stringify(user, null, 2));
// Filter out sensitive fields with a replacer
const safe = JSON.stringify(user, (key, value) => {
if (key === "password") return undefined; // omit
return value;
}, 2);
console.log(safe);
// {
// "name": "Jane",
// "role": "admin",
// "createdAt": "2026-02-25T00:00:00.000Z"
// }
// Or pass an array of allowed keys
const filtered = JSON.stringify(user, ["name", "role"]);
console.log(filtered); // '{"name":"Jane","role":"admin"}'The fetch() API returns a Response object with a built-in .json() method that parses the body as JSON.
async function getUser(id) {
const response = await fetch(`https://api.example.com/users/${id}`);
// Always check the status before parsing
if (!response.ok) {
throw new Error(`HTTP ${response.status}: ${response.statusText}`);
}
const data = await response.json();
return data;
}
// POST with a JSON body
async function createUser(user) {
const response = await fetch("https://api.example.com/users", {
method: "POST",
headers: { "Content-Type": "application/json" },
body: JSON.stringify(user),
});
if (!response.ok) {
const error = await response.json();
throw new Error(error.message);
}
return response.json();
}Note: response.json() returns a Promise. It can also throw if the response body is not valid JSON, so handle that case in production code.
A common trick is using JSON.parse(JSON.stringify(obj)) to deep-clone an object. It works, but has limitations:
const original = { name: "Jane", scores: [10, 20, 30] };
const clone = JSON.parse(JSON.stringify(original));
clone.scores.push(40);
console.log(original.scores); // [10, 20, 30] — not affected
// But it loses non-JSON types:
const problematic = {
date: new Date(), // becomes a string
regex: /abc/, // becomes {}
func: () => {}, // gets dropped
undef: undefined, // gets dropped
nan: NaN, // becomes null
infinity: Infinity, // becomes null
};For modern JavaScript, prefer structuredClone(obj) which handles Date, Map, Set, ArrayBuffer, and more. The JSON trick is still useful when you specifically want a JSON-safe snapshot.
JSON.parse() returns any, which defeats TypeScript’s type system. Use a validation library like Zod to parse and validate in one step:
import { z } from "zod";
const UserSchema = z.object({
name: z.string(),
email: z.string().email(),
age: z.number().int().positive(),
});
type User = z.infer<typeof UserSchema>;
function parseUser(json: string): User {
const raw = JSON.parse(json);
return UserSchema.parse(raw); // throws ZodError if invalid
}
// Safe version
function safeParseUser(json: string) {
try {
const raw = JSON.parse(json);
return UserSchema.safeParse(raw);
} catch {
return { success: false, error: "Invalid JSON" };
}
}Generate Zod schemas from your API responses automatically with the JSON to Zod converter, or create TypeScript interfaces with JSON to TypeScript.
JSON.parse() loads the entire string into memory and blocks the main thread while parsing. For large payloads (10MB+), consider these strategies:
Streaming parsers — Libraries like stream-json (Node.js) parse JSON incrementally without loading it all at once.
Web Workers — Move JSON.parse() to a Web Worker so parsing doesn’t block the UI thread.
Paginate at the API level — The best fix for large JSON is not to send it. Use pagination and field selection to keep payloads small.
When you need to extract specific values from complex, nested JSON without writing loops, use JSONPath — a query language for JSON similar to XPath for XML.
import { JSONPath } from "jsonpath-plus";
const data = {
store: {
books: [
{ title: "Dune", price: 12.99 },
{ title: "Neuromancer", price: 9.99 },
{ title: "Snow Crash", price: 14.99 },
]
}
};
// Get all book titles
const titles = JSONPath({ path: "$.store.books[*].title", json: data });
console.log(titles); // ["Dune", "Neuromancer", "Snow Crash"]
// Get books under $13
const cheap = JSONPath({ path: "$.store.books[?(@.price < 13)]", json: data });Test JSONPath expressions interactively with the JSONPath Query tool, and check the JSONPath Cheatsheet for the full syntax reference.
Double-parsing — Calling JSON.parse() on something that’s already an object. This happens when an API response is auto-parsed by the HTTP client.
Trailing commas — Valid in JavaScript but invalid in JSON. {"a": 1,} will throw.
Single quotes — JSON requires double quotes. {'a': 1} is not valid JSON.
Circular references — JSON.stringify() throws on circular objects. Use a replacer or a library to handle them.
Losing precision on large numbers — JavaScript numbers are 64-bit floats. Integers larger than 2^53 - 1 lose precision when parsed. Use BigInt or keep them as strings.
When you hit a parsing error, paste your JSON into the JSON Formatter — it will pinpoint the exact line and character where the syntax breaks.
Paste broken JSON and fix it instantly with AI — plain-English explanations included.
Learn how to safely parse, validate, and handle JSON API responses in production. Covers Zod, JSON Schema, error handling strategies, and type-safe clients.
A quick-reference guide to JSONPath syntax with practical examples. Learn how to select, filter, and extract data from complex JSON structures.
Learn what JSON Schema is, how it works, and how to use it for API validation, form generation, and data documentation. Includes examples and common patterns.