useEffect( () => {
        let allCryptoFuturesSymbols = {}
        getOrderRestrictions().then((result) => {
            let listOfSymbols = result.symbols
            for (let num in listOfSymbols) {
                let elt = listOfSymbols[num]
                const symbol = elt["symbol"]
                Object.defineProperties(allCryptoFuturesSymbols, {
                    [symbol]: {
                        value: symbol,
                        enumerable: true
                    }
                });
        })
        console.log(allCryptoFuturesSymbols)
        console.log(JSON.stringify(allCryptoFuturesSymbols))
    }, [])

The output of the first log is correct and expected, and the second is an empty object: {}. If you stringify a manually-defined object there is no problem, but the object created via this for-loop does not behave the same way.