How to copy values of only a subset of object properties to another existing object in TypeScript

Question

Lets say I have an existing object obj1 = {a: 'A', b: 'B', c: 'C', d: 'D'}

I have another object something like obj2 = {b: 'B2', c: 'C2', d: 'D2', e: 'E2'}

Now I want to selectively copy the values of only a few properties from obj2 to obj1, for rest of the properties in obj1, I want them unchanged.

Now, say I want to copy of the values of properties b and d. I'd want to do something like

obj1 = {a: 'A', b: 'B', c: 'C', d: 'D'};
obj2 = {b: 'B2', c: 'C2', d: 'D2', e: 'E2'};

copyPropertyvalues(
    obj2, // source
    obj1, // destination
    [obj2.b, obj2.d] // properties to copy >> see clarification below
); // should result in obj1 = {a: 'A', b: 'B2', c: 'C', d: 'D2'}

How do I write this method? Note that I also do not want to provide the list of properties as strings, as I'd want compile time safety as much as possible.

So basic asks here are:

1. Copy only a few property values from an object
2. Copy to an existing object & not create a new one
3. Retain other property values in destination object
4. Avoid strings for property names (like 'b', 'd') and leverage TypeScript safety as much as possible

Clarification based on comment:

When I say obj2.b or so in (pseudo) code example

• I do not mean literally obj2.b
• Rather some way to check at compile time that b is actually a property on obj2's type

Answer 1

My suggestion for copyPropertyvalues() would be something like this:

function copyPropertyvalues<T, K extends keyof T>(s: Pick<T, K>, d: T, ks: K[]) {
    ks.forEach(k => d[k] = s[k]);
    return d;
}

The function is generic in the type T of the destination object, and the type K of the key names you're going to copy from the source to the destination object. I assume that you want to require that the properties you're copying from the source should be the same type as the properties already in the destination; for example, you wouldn't copy the "a" property from a source of type {a: number} to a destination of type {a: string}, since you'd be writing a number to a string and violating the type of the destination.

Note that we don't need the source object to be exactly T; it only has to agree with T at all the keys in K. That is, we only say that it must be Pick<T, K> using the Pick<T, K> utility type.

And note that we are definitely passing in an array of key strings, which the compiler will infer not just as string, but specifically as a union of string literal types which are a subset of keyof T (using the keyof type operator to get a union of the known keys of T). So if you pass in ["b", "d"], the compiler will infer that as being of type Array<"b" | "d"> and not just Array<string>. This will give you the type safety you're looking for, without trying to worry about something like nameof which does not exist in JavaScript (and will not exist in TypeScript until it does, see microsoft/TypeScript#1579).

---

Okay, let's try it out:

copyPropertyvalues(
    obj2, // source
    obj1, // destination
    ["b", "d"] // properties to copy
);
console.log(obj1)
/* {
  "a": "A",
  "b": "B2",
  "c": "C",
  "d": "D2"
}  */

Looks like it behaves how you want. And if you put the wrong keys in there you get compile time errors:

copyPropertyvalues(
    obj2, // error!
    obj1,
    ["a"]
)

copyPropertyvalues(
    obj2,// error!
    obj1,
    ["z"]
)

Playground link to code