Abbreviation for ensuring type safety when combining properties from one object with another

Question

Abbreviation for ensuring type safety when combining properties from one object with another

Imagine we have an object x representing type X and an object y representing type Y.

Is there a way to combine (assign all properties) of y into x in such a way that if Y is not a proper subset of X, the compiler raises an error?

NOTE: While Object.assign(x, y) can technically achieve this, it won't raise an error if Y includes properties not found in X.

I am seeking a method similar to {...x} = y.

EDIT:

It's important to avoid reassignment to x since that would change the identity of x, which could be declared as const.

typescript

Answer 1

Answer №1

If you need a function that will raise an error if one type is not a subset of the other, you can use the following approach

// U is a subset of T
function merge<T extends U, U>(a:T, b:U):T{
  return {...a, ...b}
}

In case you want to overwrite 'a' with 'b', you can implement the function like this

function merge<T extends U, U>(a:T, b:U):T{
  for (const [key, value] of Object.entries(b)) {
    a[key as keyof U] = value
  }
  return a;
}

Here are some examples to demonstrate how this function operates

type A = {a:string, b:number, c:boolean}
type B = {a:number, b:number}
type C = {b:number, c:boolean}
type D = {a:string, d:string}

let a:A = {a:"Hi_a", b:1, c:true}
let b:B= {a:99, b:2}
let c:C = {b:3, c:false};
let d:D = {a:"Hi_d", d:"Low_d"}

let ab = merge(a,b) //compiler fail - Type B prop is incompatible
let ac = merge(a,c) //passes type C is a subset of type A
let ca = merge(c,a) //compiler fail - type A is a superset of type C
let ad = merge(a,d) // compiler fail - Type D has an extra property

Playground Link

Answer 2

If you need a function that will raise an error if one type is not a subset of the other, you can use the following approach

// U is a subset of T
function merge<T extends U, U>(a:T, b:U):T{
  return {...a, ...b}
}

In case you want to overwrite 'a' with 'b', you can implement the function like this

function merge<T extends U, U>(a:T, b:U):T{
  for (const [key, value] of Object.entries(b)) {
    a[key as keyof U] = value
  }
  return a;
}

Here are some examples to demonstrate how this function operates

type A = {a:string, b:number, c:boolean}
type B = {a:number, b:number}
type C = {b:number, c:boolean}
type D = {a:string, d:string}

let a:A = {a:"Hi_a", b:1, c:true}
let b:B= {a:99, b:2}
let c:C = {b:3, c:false};
let d:D = {a:"Hi_d", d:"Low_d"}

let ab = merge(a,b) //compiler fail - Type B prop is incompatible
let ac = merge(a,c) //passes type C is a subset of type A
let ca = merge(c,a) //compiler fail - type A is a superset of type C
let ad = merge(a,d) // compiler fail - Type D has an extra property

Playground Link

Abbreviation for ensuring type safety when combining properties from one object with another

Answer №1

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