How do you JSON.stringify an ES6 Map?

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?

Answers:

Answer

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) {
  const originalObject = this[key];
  if(originalObject instanceof Map) {
    return {
      dataType: 'Map',
      value: Array.from(originalObject.entries()), // or with spread: value: [...originalObject]
    };
  } 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

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

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

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

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

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

While there is no method provided by ecmascript yet, this can still be done using JSON.stingify 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

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));

Tags

Recent Questions

Top Questions

Home Tags Terms of Service Privacy Policy DMCA Contact Us

©2020 All rights reserved.