How do you JSON.stringify an ES6 Map?

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);

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));

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.