Objects type casting in TypeScript

Question

Type casting with enum works really well in TypeScript until you want to do it via util function. Here is an example:

enum MyTypes {
    FIRST = "FIRST",
    SECOND = "SECOND",
    THIRD = "THIRD"
}

type TFirst = {
    type: MyTypes.FIRST
    foo: string
}

type TSecond = {
    type: MyTypes.SECOND
    foo: string
}

type TThird = {
    type: MyTypes.THIRD
    bar: string
}

type TMyObject = TFirst | TSecond | TThird

const someFunction = (myObject: TMyObject) => {
    if (myObject.type === MyTypes.FIRST || myObject.type === MyTypes.SECOND) {
        // here typescript knows exactly that myObject is TFirst or TSecond
        console.log(myObject.foo)
    }
}

const isFirstOrSecondUtil = (myObject: TMyObject): boolean => {
    return myObject.type === MyTypes.FIRST || myObject.type === MyTypes.SECOND
}

const otherFunction = (myObject: TMyObject) => {
    if (isFirstOrSecondUtil(myObject)) {
        // typescript is aware that myObject is TMyObject, but does not know which type exactly
        console.log(myObject.foo)
    }
}

You can test it in TypeScript Playgroud .

As you can see on line 27 inside someFunction TypeScript is fully aware that myObject is of type TFirst or TSecond even though that function receives TMyObject . I am able to use myObject.foo without any problems since this property both of types have.

On the other hand I am using util function isFirstOrSecondUtil (which does the same checking as it is done in someFunction ) inside otherFunction and apparently type checking fails in this case. On line 38 TypeScript does not know which type exactly is myObject . I would expect to be able to console myObject.foo , but TypeScript fails with Property 'foo' does not exist on type 'TThird'.

Any suggestion how to do proper type casting via util function?

Answer 1

You can tell Typescript that the boolean returned indicates the type:

 const isFirstOrSecondUtil = (myObject: TMyObject): myObject is TFirst | TSecond => 
   myObject.type === MyTypes.FIRST || myObject.type === MyTypes.SECOND;

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