Typescript: define function, which transforms an object, keeping keys

I need to define a function, which accepts an object of this type:

interface Source<A> {
    [index: string]: A
}

and transforms that object, keeping the keys, but replaces a values:

interface Target<B> {
    [index: string]: B
}

and I also want to keep typechecking for that case. This is example:

function transform(source) {
    var result = {}
    Object.keys(source).forEach((key) => {
        result[key] = source[key] + "prefix"
    })
}

var target = transform({
    "key1": 1,
    "key2": 2,
})

// now target has a {"key1": "1prefix", "key2": "2prefix"}

var three = target.key3 // I want to get type error here on compile-time