JSON stringify a Set

Question

How would one JSON.stringify() a Set?

Things that did not work in Chromium 43:

var s = new Set(['foo', 'bar']);  JSON.stringify(s); // -> "{}" JSON.stringify(s.values()); // -> "{}" JSON.stringify(s.keys()); // -> "{}" 

I would expect to get something similar to that of a serialized array.

JSON.stringify(["foo", "bar"]); // -> "["foo","bar"]" 

Answer 1

JSON.stringify doesn't directly work with sets because the data stored in the set is not stored as properties.

But you can convert the set to an array. Then you will be able to stringify it properly.

Any of the following will do the trick:

JSON.stringify([...s]); JSON.stringify([...s.keys()]); JSON.stringify([...s.values()]); JSON.stringify(Array.from(s)); JSON.stringify(Array.from(s.keys())); JSON.stringify(Array.from(s.values())); 

Answer 2

You can pass a "replacer" function to JSON.stringify:

const fooBar = {   foo: new Set([1, 2, 3]),   bar: new Set([4, 5, 6]) };  JSON.stringify(   fooBar,   (_key, value) => (value instanceof Set ? [...value] : value) ); 

Result:

"{"foo":[1,2,3],"bar":[4,5,6]}" 

toJSON is a legacy artifact, and a better approach is to use a custom replacer, see https://github.com/DavidBruant/Map-Set.prototype.toJSON/issues/16