Assigning properties to an Object type in TypeScript

assigning-properties-in-typescript.ts

/**
 * # Assigning properties to an object type in TypeScript
 * 
 * When we want to assign some properties on a object type
 * we usually use the `&` operator:
 * 
 * type B = A & { someProperty: string }
 * 
 * It seems to work at first sight, but it doesn't behave exactly
 * as we would expect when we try to override properties that already
 * exist on type `A`. In other words it isn’t equivalent to Object.assign.
 * 
 * With `A & B`, properties of A aren’t overrided, but instead `&`
 * is also applied between each property of same key. let's see an example:
 **/

type X = { prop: number }
type Y = X & { prop: string }

// you would expect Y[‘prop’] to be of type `string`, but it isn’t,
// it is of type `number & string` which evaluates to `never`:

const y: Y = {
    prop: 'hello' // Typecheck Error! can’t assign string to never.
}

// Here is how to achieve assignment in TypeScript:

type Assign<A, B> = Omit<A, keyof B> & B;

// We first removes all properties of type B from the type A
// And then apply `& B` :)

type A = { prop: number, anotherProp: number }
type B = Assign<A, { prop: string }>

const b: B = {
    prop: 'hello', // it works!
    anotherProp: 10
}