Typescript - How to get a subset of properties from an object into a variable based on an interface - reactjs

Question

I have a function that receives an intersection of two types:
interface A {
propA: string;
}
interface B {
propB: number;
propC: boolean;
}
function C(props: A & B) {
}
Now, inside the function body I would like to get objects containing only the subset of properties from each of the interfaces; so I was wondering if Typescript had any utility to achieve that:
function C(props: A & B) {
const a = fancyPicker<A>(props);
const b = fancyPicker<B>(props);
console.log(a);
// prints "{ propA: "some string" }"
console.log(b);
// prints "{ propB: 42, propC: false }"
}

Answer 1

You're after a function that iterates over a known set of property names - this can't be done in pure TypeScript because TypeScript uses type-erasure, so the runtime script has no-knowledge of what the set of property names is.
But using a TypeScript compile-time extension known as a Custom Transformer, specifically ts-transformer-keys the TypeScript compiler will emit property-name lists that can be used.
Here's something that works, but isn't perfect because it doesn't use the types of each property - it only matches names:
import { keys } from 'ts-transformer-keys'; // <-- This module is where the magic happens.
type IndexableObject = {
[key: string]: any
};
/*
* The `IndexableObject` above is a hack. Note it allows `any` property type. Ideally it'd be something like this instead, but using `keyof` in a type indexer is not yet supported: https://github.com/microsoft/TypeScript/pull/26797
*
type IndexableObject<TOther> = {
[key: TKey extends keyof TOther]: PropertyType<TOther,TKey>
};
*/
function fancyPicker<TSubset extends object>(superset: IndexableObject): Partial<TSubset> {
const subsetPropertyNames = keys<TSubset>();
const ret: Partial<TSubset> = {
};
for (const subsetPropertyName of subsetPropertyNames) {
const propName: string = subsetPropertyName as string; // <-- This is also a hack because property keys/names are actually `string | number | symbol` - but this function assumes they're all named properties.
if (propName in superset) {
const value = superset[propName];
ret[subsetPropertyName] = value;
}
}
return ret;
}
Usage (using your example):
interface A {
propA: string;
}
interface B {
propB: number;
propC: boolean;
}
type ABIntersection = A & B;
type ABUnion = A | B;
function C(props: ABIntersection) {
const a = fancyPicker<A>(props);
const b = fancyPicker<B>(props);
console.log(a);
// prints "{ propA: "some string" }"
console.log(b);
// prints "{ propB: 42, propC: false }"
}
const testValue = { propA: "some string", propB: 42, propC: false, propD: "never see this" };
C(testValue);