JSON.stringify()

The JSON.stringify() method converts a JavaScript value to a JSON string, optionally replacing values if a replacer function is specified or optionally including only the specified properties if a replacer array is specified.

Syntax

JSON.stringify(value)
JSON.stringify(value, replacer)
JSON.stringify(value, replacer, space)

Parameters

• value

  • : The value to convert to a JSON string.

• replacer optional

  • : A function that alters the behavior of the stringification process, or an array of strings or numbers naming properties of value that should be included in the output. If replacer is an array, all elements that are not strings or numbers (can be either primitives or wrapper objects), including Symbol values, are completely ignored. If replacer is anything other than a function or an array (e.g. null or not provided), all properties of the object are included in the resulting JSON string.

• space optional

  • : A string or number that's used to insert white space (including indentation, line break characters, etc.) into the output JSON string for readability purposes.

    If this is a number, it indicates the number of space characters to be used as indentation, clamped to 10 (that is, any number greater than 10 is treated as if it were 10). Values less than 1 indicate that no space should be used.

    If this is a string, the string (or the first 10 characters of the string, if it's longer than that) is inserted before every nested object or array.

    If space is anything other than a string or number (can be either a primitive or a wrapper object) — for example, is null or not provided — no white space is used.

Return value

A JSON string representing the given value, or undefined.

Exceptions

• TypeError
  • : Thrown if one of the following is true:
    • value contains a circular reference.
    • A BigInt value is encountered.

Description

JSON.stringify() converts a value to JSON notation representing it:

• Boolean, Number, String, and BigInt (obtainable via Object()) objects are converted to the corresponding primitive values during stringification, in accordance with the traditional conversion semantics. Symbol objects (obtainable via Object()) are treated as plain objects.

• Attempting to serialize BigInt values will throw. However, if the BigInt has a toJSON() method (through monkeypatching: BigInt.prototype.toJSON = ...), that method can provide the serialization result. This constraint ensures that a proper serialization (and, very likely, its accompanying deserialization) behavior is always explicitly provided by the user.

• undefined, Function, and Symbol values are not valid JSON values. If any such values are encountered during conversion, they are either omitted (when found in an object) or changed to null (when found in an array). JSON.stringify() can return undefined when passing in "pure" values like JSON.stringify(() => {}) or JSON.stringify(undefined).

• The numbers Infinity and NaN, as well as the value null, are all considered null. (But unlike the values in the previous point, they would never be omitted.)

• Arrays are serialized as arrays (enclosed by square brackets). Only array indices between 0 and length - 1 (inclusive) are serialized; other properties are ignored.

• For other objects:

  • All Symbol-keyed properties will be completely ignored, even when using the replacer parameter.

  • If the value has a toJSON() method, it's responsible to define what data will be serialized. Instead of the object being serialized, the value returned by the toJSON() method when called will be serialized. JSON.stringify() calls toJSON with one parameter, the key, which has the same semantic as the key parameter of the replacer function:

    • if this object is a property value, the property name
    • if it is in an array, the index in the array, as a string
    • if JSON.stringify() was directly called on this object, an empty string

    Date objects implement the toJSON() method which returns a string (the same as date.toISOString()). Thus, they will be stringified as strings.

  • Only enumerable own properties are visited. This means Map, Set, etc. will become "{}". You can use the replacer parameter to serialize them to something more useful.

    Properties are visited using the same algorithm as Object.keys(), which has a well-defined order and is stable across implementations. For example, JSON.stringify on the same object will always produce the same string, and JSON.parse(JSON.stringify(obj)) would produce an object with the same key ordering as the original (assuming the object is completely JSON-serializable).

The replacer parameter

The replacer parameter can be either a function or an array.

As an array, its elements indicate the names of the properties in the object that should be included in the resulting JSON string. Only string and number values are taken into account; symbol keys are ignored.

As a function, it takes two parameters: the key and the value being stringified. The object in which the key was found is provided as the replacer's this context.

The replacer function is called for the initial object being stringified as well, in which case the key is an empty string (""). It is then called for each property on the object or array being stringified. Array indices will be provided in its string form as key. The current property value will be replaced with the replacer's return value for stringification. This means:

• If you return a number, string, boolean, or null, that value is directly serialized and used as the property's value. (Returning a BigInt will throw as well.)
• If you return a Function, Symbol, or undefined, the property is not included in the output.
• If you return any other object, the object is recursively stringified, calling the replacer function on each property.

Note: When parsing JSON generated with replacer functions, you would likely want to use the reviver parameter to perform the reverse operation.

Typically, array elements' index would never shift (even when the element is an invalid value like a function, it will become null instead of omitted). Using the replacer function allows you to control the order of the array elements by returning a different array.

The space parameter

The space parameter may be used to control spacing in the final string.

• If it is a number, successive levels in the stringification will each be indented by this many space characters.
• If it is a string, successive levels will be indented by this string.

Each level of indentation will never be longer than 10. Number values of space are clamped to 10, and string values are truncated to 10 characters.