JSON Stringify Online: Convert JavaScript Objects to JSON Strings
JSON stringify converts a JavaScript value into a JSON string. Understanding how it works — and how to control its output — is fundamental for any JavaScript developer.
What JSON.stringify() Does
JSON.stringify(value) takes a JavaScript value — object, array, string, number, boolean, or null — and returns a JSON-formatted string. This process is called serialization: converting a runtime data structure into a portable text representation.
The reverse operation is JSON.parse(), which takes a JSON string and returns the corresponding JavaScript value. Together, stringify and parse enable JSON data to be stored (in localStorage, a database, a file) and later restored to its original structure.
The Three Parameters of JSON.stringify
JSON.stringify(value, replacer, space) accepts two optional parameters. The replacer is either a function that transforms values during serialization or an array of key names to include. The space parameter controls indentation: a number sets spaces per level, a string is used as the indent character.
Omitting both optional parameters produces compact JSON with no whitespace — the default for production use. Passing 2 as the space argument produces formatted, readable output. Passing a replacer array lets you serialize only specific keys, which is useful for security (omitting sensitive fields) or size optimization.
Values That JSON.stringify Cannot Serialize
JSON.stringify silently handles non-serializable values: undefined and function values in object properties are omitted entirely. undefined in array positions is converted to null. Symbol keys are ignored. This behavior is often surprising and can cause data loss if you are not aware of it.
To verify that JSON.stringify produces what you expect, compare its output to the original object. Any missing keys indicate non-serializable values. For Date objects, JSON.stringify calls .toISOString() automatically, producing a string representation rather than serializing the Date as an object.
Using an Online Stringify Tool
An online JSON stringify tool accepts a JavaScript object literal (not necessarily valid JSON) and produces the equivalent JSON string. This is useful when you have an object defined in JavaScript syntax — with single quotes, trailing commas, or unquoted keys — and need the properly quoted, escaped JSON version.
The tool also handles escaping: if your object contains string values with special characters (newlines, tabs, quotes, backslashes), JSON.stringify escapes them correctly. Seeing the escaped output helps you understand what the serialized value actually looks like in transmission.
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Open JSON Stringify Onlinearrow_forwardFrequently Asked Questions
Why does JSON.stringify return undefined instead of a string?
JSON.stringify returns undefined (not the string "undefined") when the value cannot be serialized to JSON — this includes functions, Symbols, and the undefined value itself at the top level.
How do I stringify a JavaScript Map or Set?
JSON.stringify does not support Map or Set natively — it outputs {} for both. Convert to array first: JSON.stringify([...myMap]) or JSON.stringify([...mySet]), or write a custom replacer.
Can I stringify a class instance with JSON.stringify?
JSON.stringify serializes all enumerable own properties of an object, including class instances. Define a toJSON() method on the class to control what gets serialized.