im new to JS and i have an problem

let dict = data.foo;
dict.hello = "world";
console.log(dict);

data.foo is {"num":["1","2"]}

results are {"num":["1","2"]}

Tried doing

let dict = data.foo;
dict["hello"] = "world";
console.log(dict);

but same output…
Excepted {"num":["1","2"], "hello":"world"}

Thanks.