How to convert a Map to JSON in JavaScript

To convert a Map to JSON in JavaScript, call the Object.fromEntries(map), then pass the result to the JSON.stringify() method.

For example:

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const map = new Map([
  ['user1', 'John'],
  ['user2', 'Kate'],
  ['user3', 'Peter'],
]);

const json = JSON.stringify(Object.fromEntries(map));

// {"user1":"John","user2":"Kate","user3":"Peter"}
console.log(json);

We first transform the Map with Object.fromEntries(), because we can’t serialize a Map to a JSON string directly. The Object.fromEntries() method transforms any list of key-value pairs into an object:

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const map = new Map([
  ['user1', 'John'],
  ['user2', 'Kate'],
  ['user3', 'Peter'],
]);

const obj = Object.fromEntries(map);

// { user1: 'John', user2: 'Kate', user3: 'Peter' }
console.log(obj);

Convert JSON back to Map

To convert the JSON string back to a Map:

1. Parse the JSON string to an object with JSON.parse().
2. Call Object.entries() with this object as an argument.
3. Create a new Map object, passing the result of Object.entries() to the constructor.

For example:

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const map = new Map([
  ['user1', 'John'],
  ['user2', 'Kate'],
  ['user3', 'Peter'],
]);

const jsonFromMap = JSON.stringify(Object.fromEntries(map));

const obj = JSON.parse(jsonFromMap);
const mapFromObj = new Map(Object.entries(obj));

// Map(3) { 'user1' => 'John', 'user2' => 'Kate', 'user3' => 'Peter' }
console.log(mapFromObj);

We first convert the string to an object and then to an array, because we can’t parse a JSON string to a Map directly. The Object.entries() method takes an object and returns a list of key-value pairs that correspond to the key and value of each property of the object:

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const obj = {
  user1: 'John',
  user2: 'Kate',
  user3: 'Peter',
};

const arr = Object.entries(obj);

// [ [ 'user1', 'John' ], [ 'user2', 'Kate' ], [ 'user3', 'Peter' ] ]
console.log(arr);

The Map() constructor can take an iterable of key-value pairs to create the Map elements, so we pass the result of the Object.entries() directly to it.

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