TypeScript type assertions

type-assertions.ts

// https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#type-assertions
// not to be confused with an "assertion function"
// https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions

type ObjectType1 = {
	prop1: string
}

type ObjectType2 = {
	prop1: string
	prop2: string
}

const obj1: ObjectType1 = {
	prop1: 'string prop for obj1'
}

const obj2: ObjectType2 = {
	prop1: 'string prop for obj2',
	prop2: 'string prop for obj2'
}

// it's ok to assign an ObjectType2 to an ObjectType1 but not the other way around
let obj3: ObjectType1 = obj2
let obj4: ObjectType2 = obj1

// suppress the error with a type assertion (cast)
// both styles do the same thing but the pointy brackets don't work with JSX
obj4 = obj1 as ObjectType2
obj4 = <ObjectType2>obj1

// accessing an unknown property might work but TypeScript still complains
console.log(obj3.prop2)

// suppress the error with a type assertion
console.log((obj3 as ObjectType2).prop2)
console.log((<ObjectType2>obj3).prop2)

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