How do you JSON.stringify an ES6 Map?

Question

I'd like to start using ES6 Map instead of JS objects but I'm being held back because I can't figure out how to JSON.stringify() a Map. My keys are guaranteed to be strings and my values will always be listed. Do I really have to write a wrapper method to serialize?

Answer 1

Both JSON.stringify and JSON.parse support a second argument. replacer and reviver respectively. With replacer and reviver below it's possible to add support for native Map object, including deeply nested values

function replacer(key, value) {
  if(value instanceof Map) {
    return {
      dataType: 'Map',
      value: Array.from(value.entries()), // or with spread: value: [...value]
    };
  } else {
    return value;
  }
}

function reviver(key, value) {
  if(typeof value === 'object' && value !== null) {
    if (value.dataType === 'Map') {
      return new Map(value.value);
    }
  }
  return value;
}

Usage:

const originalValue = new Map([['a', 1]]);
const str = JSON.stringify(originalValue, replacer);
const newValue = JSON.parse(str, reviver);
console.log(originalValue, newValue);

Deep nesting with combination of Arrays, Objects and Maps

const originalValue = [
  new Map([['a', {
    b: {
      c: new Map([['d', 'text']])
    }
  }]])
];
const str = JSON.stringify(originalValue, replacer);
const newValue = JSON.parse(str, reviver);
console.log(originalValue, newValue);

Answer 2

You can't directly stringify the Map instance as it doesn't have any properties, but you can convert it to an array of tuples:

jsonText = JSON.stringify(Array.from(map.entries()));

For the reverse, use

map = new Map(JSON.parse(jsonText));

Answer 3

You can't.

The keys of a map can be anything, including objects. But JSON syntax only allows strings as keys. So it's impossible in a general case.

My keys are guaranteed to be strings and my values will always be lists

In this case, you can use a plain object. It will have these advantages:

• It will be able to be stringified to JSON.
• It will work on older browsers.
• It might be faster.

Answer 4

While there is no method provided by ecmascript yet, this can still be done using JSON.stringify if you map the Map to a JavaScript primitive. Here is the sample Map we'll use.

const map = new Map();
map.set('foo', 'bar');
map.set('baz', 'quz');

Going to an JavaScript Object

You can convert to JavaScript Object literal with the following helper function.

const mapToObj = m => {
  return Array.from(m).reduce((obj, [key, value]) => {
    obj[key] = value;
    return obj;
  }, {});
};
    
JSON.stringify(mapToObj(map)); // '{"foo":"bar","baz":"quz"}'

Going to a JavaScript Array of Objects

The helper function for this one would be even more compact

const mapToAoO = m => {
  return Array.from(m).map( ([k,v]) => {return {[k]:v}} );
};
    
JSON.stringify(mapToAoO(map)); // '[{"foo":"bar"},{"baz":"quz"}]'

Going to Array of Arrays

This is even easier, you can just use

JSON.stringify( Array.from(map) ); // '[["foo","bar"],["baz","quz"]]'

Answer 5

Using spread sytax Map can be serialized in one line:

JSON.stringify([...new Map()]);

and deserialize it with:

let map = new Map(JSON.parse(map));

Answer 6

Given your example is a simple use case in which keys are going to be simple types, I think this is the easiest way to JSON stringify a Map.

JSON.stringify(Object.fromEntries(map));

The way I think about the underlying data structure of a Map is as an array of key-value pairs (as arrays themselves). So, something like this:

const myMap = new Map([
     ["key1", "value1"],
     ["key2", "value2"],
     ["key3", "value3"]
]);

Because that underlying data structure is what we find in Object.entries, we can utilize the native JavaScript method of Object.fromEntries() on a Map as we would on an Array:

Object.fromEntries(myMap);

/*
{
     key1: "value1",
     key2: "value2",
     key3: "value3"
}
*/

And then all you're left with is using JSON.stringify() on the result of that.

Answer 7

Correctly round-tripping serialization

Just copy this and use it. Or use the npm package.

const serialize = (value) => JSON.stringify(value, stringifyReplacer);
const deserialize = (text) => JSON.parse(text, parseReviver);

// License: CC0
function stringifyReplacer(key, value) {
  if (typeof value === "object" && value !== null) {
    if (value instanceof Map) {
      return {
        _meta: { type: "map" },
        value: Array.from(value.entries()),
      };
    } else if (value instanceof Set) { // bonus feature!
      return {
        _meta: { type: "set" },
        value: Array.from(value.values()),
      };
    } else if ("_meta" in value) {
      // Escape "_meta" properties
      return {
        ...value,
        _meta: {
          type: "escaped-meta",
          value: value["_meta"],
        },
      };
    }
  }
  return value;
}

function parseReviver(key, value) {
  if (typeof value === "object" && value !== null) {
    if ("_meta" in value) {
      if (value._meta.type === "map") {
        return new Map(value.value);
      } else if (value._meta.type === "set") {
        return new Set(value.value);
      } else if (value._meta.type === "escaped-meta") {
        // Un-escape the "_meta" property
        return {
          ...value,
          _meta: value._meta.value,
        };
      } else {
        console.warn("Unexpected meta", value._meta);
      }
    }
  }
  return value;
}

Performance?

There is a version that is equally high quality, but has better performance (tested in Chrome and Firefox). If that matters to you, then please check it out!

https://stackoverflow.com/a/79016027/3492994

Why is this hard?

It should be possible to input any kind of data, get valid JSON, and from there correctly reconstruct the input.

This means dealing with

• Maps that have objects as keys new Map([ [{cat:1}, "value"] ]). This means that any answer which uses Object.fromEntries is probably wrong.
• Maps that have nested maps new Map([ ["key", new Map([ ["nested key", "nested value"] ])] ]). A lot of answers sidestep this by only answering the question and not dealing with anything beyond that.
• Mixing objects and maps {"key": new Map([ ["nested key", "nested value"] ]) }.

and on top of those difficulties, the serialisation format must be unambiguous. Otherwise one cannot always reconstruct the input. The top answer has one failing test case, see below.

Hence, I wrote this improved version. It uses _meta instead of dataType, to make conflicts rarer and if a conflict does happen, it actually unambiguously handles it. Hopefully the code is also simple enough to easily be extended to handle other containers.

My answer does, however, not attempt to handle exceedingly cursed cases, such as a map with object properties.

A test case for my answer, which demonstrates a few edge cases

const originalValue = [
  new Map([['a', {
    b: {
      _meta: { __meta: "cat" },
      c: new Map([['d', 'text']])
    }
  }]]),
 { _meta: { type: "map" }}
];

console.log(originalValue);
let text = JSON.stringify(originalValue, stringifyReplacer);
console.log(text);
console.log(JSON.parse(text, parseReviver));

Accepted answer not round-tripping

The accepted answer is really lovely. However, it does not round trip when an object with a dataType property is passed it it. That can make it dangerous to use in certain circumstances, such as

1. JSON.stringify(data, acceptedAnswerReplacer) and send it over the network.
2. Naive network handler automatically JSON decodes it. From this point on forth, you cannot safely use the accepted answer with the decoded data, since doing so would cause lots of sneaky issues.

This answer uses a slightly more complex scheme to fix such issues.

// Test case for the accepted answer
const originalValue = { dataType: "Map" };
const str = JSON.stringify(originalValue, replacer);
const newValue = JSON.parse(str, reviver);
console.log(originalValue, str, newValue); 
// > Object { dataType: "Map" } , Map(0)
// Notice how the input was changed into something different

Answer 8

A Better Solution

    // somewhere...
    class Klass extends Map {

        toJSON() {
            var object = { };
            for (let [key, value] of this) object[key] = value;
            return object;
        }

    }

    // somewhere else...
    import { Klass as Map } from '@core/utilities/ds/map';  // <--wherever "somewhere" is

    var map = new Map();
    map.set('a', 1);
    map.set('b', { datum: true });
    map.set('c', [ 1,2,3 ]);
    map.set( 'd', new Map([ ['e', true] ]) );

    var json = JSON.stringify(map, null, '\t');
    console.log('>', json);

Output

    > {
        "a": 1,
        "b": {
            "datum": true
        },
        "c": [
            1,
            2,
            3
        ],
        "d": {
            "e": true
        }
    }

Hope that is less cringey than the answers above.

Answer 9

Stringify a Map instance (objects as keys are OK):

JSON.stringify([...map])

or

JSON.stringify(Array.from(map))

or

JSON.stringify(Array.from(map.entries()))

output format:

// [["key1","value1"],["key2","value2"]]

Answer 10

Just want to share my version for both Map and Set JSON.stringify only. I'm sorting them, useful for debugging...

function replacer(key, value) {
    if (value instanceof Map) {
        const reducer = (obj, mapKey) => {
            obj[mapKey] = value.get(mapKey);
            return obj;
        };
        return [...value.keys()].sort().reduce(reducer, {});
    } else if (value instanceof Set) {
        return [...value].sort();
    }
    return value;
}

Usage:

const map = new Map();
const numbers= new Set()
numbers.add(3);
numbers.add(2);
numbers.add(3);
numbers.add(1);
const chars= new Set()
chars.add('b')
chars.add('a')
chars.add('a')
map.set("numbers",numbers)
map.set("chars",chars)

console.log(JSON.stringify(map, replacer, 2));

Result:

{
  "chars": [
    "a",
    "b"
  ],
  "numbers": [
    1,
    2,
    3
  ]
}

Answer 11

The very simple way.

  const map = new Map();
  map.set('Key1', "Value1");
  map.set('Key2', "Value2");
  console.log(Object.fromEntries(map));

` Output:-

{"Key1": "Value1","Key2": "Value2"}

Answer 12

Below solution works even if you have nested Maps

function stringifyMap(myMap) {
    function selfIterator(map) {
        return Array.from(map).reduce((acc, [key, value]) => {
            if (value instanceof Map) {
                acc[key] = selfIterator(value);
            } else {
                acc[key] = value;
            }

            return acc;
        }, {})
    }

    const res = selfIterator(myMap)
    return JSON.stringify(res);
}

Answer 13

You cannot call JSON.stringify on Map or Set.

You will need to convert:

• the Map into a primitive Object, using Object.fromEntries, or
• the Set into a primitive Array, using the spread operator [...]

…before calling JSON.stringify

Map

const
  obj = { 'Key1': 'Value1', 'Key2': 'Value2' },
  map = new Map(Object.entries(obj));

map.set('Key3', 'Value3'); // Add a new entry

// Does NOT show the key-value pairs
console.log('Map:', JSON.stringify(map));

// Shows the key-value pairs
console.log(JSON.stringify(Object.fromEntries(map), null, 2));

.as-console-wrapper { top: 0; max-height: 100% !important; }

Set

const
  arr = ['Value1', 'Value2'],
  set = new Set(arr);

set.add('Value3'); // Add a new item

// Does NOT show the values
console.log('Set:', JSON.stringify(set));

// Show the values
console.log(JSON.stringify([...set], null, 2));

.as-console-wrapper { top: 0; max-height: 100% !important; }

toJSON method

If you want to call JSON.stringify on a class object, you will need to override the toJSON method to return your instance data.

class Cat {
  constructor(options = {}) {
    this.name = options.name ?? '';
    this.age = options.age ?? 0;
  }
  toString() {
    return `[Cat name="${this.name}", age="${this.age}"]`
  }
  toJSON() {
    return { name: this.name, age: this.age };
  }
  static fromObject(obj) {
    const { name, age } = obj ?? {};
    return new Cat({ name, age });
  }
}

/*
 * JSON Set adds the missing methods:
 * - toJSON
 * - toString
 */
class JSONSet extends Set {
  constructor(values) {
    super(values)
  }
  toString() {
    return super
      .toString()
      .replace(']', ` ${[...this].map(v => v.toString())
      .join(', ')}]`);
  }
  toJSON() {
    return [...this];
  }
}

const cats = new JSONSet([
  Cat.fromObject({ name: 'Furball', age: 2 }),
  Cat.fromObject({ name: 'Artemis', age: 5 })
]);

console.log(cats.toString());
console.log(JSON.stringify(cats, null, 2));

.as-console-wrapper { top: 0; max-height: 100% !important; }

Answer 14

The following method will convert a Map to a JSON string:

public static getJSONObj(): string {
    return JSON.stringify(Object.fromEntries(map));
}

Example:

const x = new Map();
x.set("SomeBool", true);
x.set("number1", 1);
x.set("anObj", { name: "joe", age: 22, isAlive: true });

const json = getJSONObj(x);

// Output:
// '{"SomeBool":true,"number1":1,"anObj":{"name":"joe","age":222,"isAlive":true}}'

Answer 15

Even Better than Correctly round-tripping serialization

The top-voted answer is not really the correct one, as has been already described in detail by Stefnotch in their answer here.

I'm providing my own drop-in solution that can be easily copy-pasted into any project (and can be used freely with a reference to this answer somewhere in the comments):

/**
 * Provides a `JSON.stringify` replacer function that supports Map and Set object serialization.
 *
 * This replacer function fully supports Map and Set nesting as well as mixed values (including
 * regular objects) as opposed to the popular solution recommended at MDN and described in more
 * detail here: https://stackoverflow.com/q/29085197.
 *
 * Map and Set objects are serialized as plain JSON arrays where the first element is a special
 * token string (`@Map` or `@Set`, respectively), and all subsequent elements are either `[key,
 * value]` arrays (with the Map contents) or just `value` sequences (with the Set contents). To
 * avoid ambiguity, regular arrays with the first element equal to one of the token strings will
 * have the `@Array` token string prepended to their contents when serialized.
 *
 * The reverse #jsonMapSetReviver function will undo these operations when parsing the resulting
 * JSON data, so it must always be used when deserializing data serialized with #jsonMapSetReplacer
 * (even if there are no Map or Set objects, because there may be arrays with `@Array` prepended).
 *
 * This implementation seems to be faster than other alternatives almost in all cases (taking its
 * functionality into account). It is also published at https://stackoverflow.com/a/79016027/3579458
 * (there are some tests).
 */

function jsonMapSetReplacer (_, value_)
{
  if (typeof (value_) === 'object')
  {
    if (value_ instanceof Map)
    {
      value_ = Array.from (value_);
      value_.unshift ('@Map');
    }
    else if (value_ instanceof Set)
    {
      value_ = Array.from (value_);
      value_.unshift ('@Set');
    }
    else if (Array.isArray (value_) && value_.length > 0 &&
      (value_ [0] === '@Map' || value_ [0] === '@Set' || value_ [0] === '@Array'))
    {
      value_ = value_.slice ();
      value_.unshift ('@Array');
    }
  }

  return value_;
}


/**
 * Provides a `JSON.parse` reviver function that supports Map and Set object deserialization.
 *
 * Must be used to deserialize JSON data serialized using #jsonMapSetReplacer.
 */

function jsonMapSetReviver (_, value_)
{
  if (Array.isArray (value_) && value_.length > 0)
  {
    let isMap, isSet;
    if ((isMap = value_ [0] === '@Map') || (isSet = value_ [0] === '@Set') || value_ [0] === '@Array')
    {
      value_.shift ();
      if (isMap)
        value_ = new Map (value_);
      else if (isSet)
        value_ = new Set (value_);
    }
  }

  return value_;
}

This solution has the following benefits over other variants presented here:

1. It supports nested Map and Set objects and should properly support all kinds of mixing (maps with objects, objects with maps etc).

2. It doesn't use relatively "heavy" object machinery to encapsulate Map and Set (and yet it protects from possible conflicts in values matching encapsulation tags) — just plain arrays. This should make it faster both to serialize and deserialize (some tests, "Array Based 2" is the latest version, "Map Based" is Stefnotch's solution).

3. It doesn't use the spread operator that is known to be much slower than other primitives when it's needed to prepend items to the beginning of the array (look for tests in the comments to other answers) — it only uses slice, shift and unshift modifying the original array in place where possible instead of creating a copy (e.g. when deserializing). This should also give some speed boost (and use memory more efficiently), especially on large data sets.

4. The output overhead due to encapsulation and escaping should be very small — it is used only when absolutely necessary (and it will be just a single additional short array element in all cases). For non-Map and non-Set objects and for almost all "normal" arrays there will be no overhead at all, compared to the standard JSON serialization.

UPDATE

I removed support for serializing undefined values. I found out that JSON.parse doesn't play well with undefined in arrays: when it gets undefined from the reviver, it simply deletes the respective element (effectively creating a hole) instead of just storing undefined there. Not what one wants.

Answer 16

JSON.stringify(errorCodes, (_, value) =>
  value instanceof Map || value instanceof Set ? Array.from(value) : value
);

Answer 17

Whatever you do, don't do this.

Map.prototype.toJSON = function() {
    return Object.fromEntries(this);
};

const map = new Map();
map.set('a', 1);
map.set('b', { datum: true });
map.set(false, [1, 2, 3]);
map.set('d', new Map([['e', true]]) );

const json = JSON.stringify(map);

console.log(json);

Answer 18

I really don't know why there are so many long awesers here. This short version solved my problem:

const data = new Map()
data.set('visible', true)
data.set('child', new Map())
data.get('child').set('visible', false)

const str = JSON.stringify(data, (_, v) => v instanceof Map ? Object.fromEntries(v) : v)
// '{"visible":true,"child":{"visible":false}}'

const recovered = JSON.parse(str, (_, v) => typeof v === 'object' ? new Map(Object.entries(v)) : v)
// Map(2) { 'visible' => true, 'child' => Map(1) { 'visible' => false } }

Answer 19

Recursive JSON stringify for MAP

You can use the second argument "replacer" of JSON.stringify :

JSON.stringify(value, replacer)

replacer (Optional) :

• A function that alters the behavior of the stringification process, or an array of strings and numbers that specifies properties of value to be included in the output.
  [Read more on mdn...]

If you detect a map with instanceof Map, you will apply Object.fromEntries(value) like this :

function JSON_MapStringify(obj) {
  return JSON.stringify(obj, function (key, value) {
    if (value instanceof Map) {
      return Object.fromEntries(value)
    }
    return value
  })
}

Demo

For:

{
   "foo": Map (1) {
      "o": Map(1) {
         "k": 2
      }
   },
   "bar": 1
}

See the result:

default (JSON.stringify)  : {"foo":{},"bar":1}
custom (JSON_MapStringify): {"foo":{"o":{"k":2}},"bar":1}

const sub = new Map()
sub.set('k', 2)
const main = new Map()
main.set('o', sub)

const obj = { foo: main, bar: 1}

console.log('default (JSON.stringify)  :', JSON.stringify(obj))

function JSON_MapStringify(obj) {
  return JSON.stringify(obj, function (key, value) {
    if (value instanceof Map) {
      return Object.fromEntries(value)
    }
    return value
  })
}
console.log('custom (JSON_MapStringify):', JSON_MapStringify(obj))

Answer 20

Although there would be some scenarios where if you were the creator of the map you would write your code in a separate 'src' file and save a copy as a .txt file and, if written concisely enough, could easily be read in, deciphered, and added to server-side.

The new file would then be saved as a .js and a reference to it sent back from the server. The file would then reconstruct itself perfectly once read back in as JS. The beauty being that no hacky iterating or parsing is required for reconstruction.