Does Express.js not `res.send` Sets?

Question

I have a set that I create in node.js and I am trying to send it to the client with Express.js using res.send or res.json but the set always appears empty on the client side. This is my code:

app.get('/test-endpoint', function(req, res) {
    let set1 = new Set();
    set1.add('SOME ITEM');
    console.log('Set 1:', set1);  // logs out set1 correctly in the terminal (Set 1 Set: { 'SOME ITEM' })

    res.json(set1);  // when this gets to the client side it is an empty set ({})
});

Why is this happening? Is it an Express.js issue?

Answer 1

Express response.json function uses JSON.stringify to send the data. And because JSON.stringify ignore all Symbols-keyed properties in addition to functions . That's why you receive an empty object on the client.

Check more info of JSON.stringify

 console.log(JSON.stringify({ prop: 5, foo() {}, [Symbol('s')]: 'symbol' })); // -> "{"prop":5}" console.log(JSON.stringify(new Set([1, 'two', null]))); // -> "{}"