undefined in JSON.stringify(x)... Well, I didn't expect that.

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x = {a:1, b:2, c:3}
x.b = undefined

console.log(JSON.stringify(x))
>   {"a":1,"c":3}
//No more "b" !!!

//But "b" is still on the object...
Object.keys(x)
>   ["a", "b", "c"]
console.log(x)
>   {a: 1, b: undefined, c: 3}

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