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.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"]]'