Typescript not typecasting

Question

I'm trying to call a function from an object in my react state

//currentUser is of type User
this.state.currentUser.rankUp()

but it just gives an error this.state.currentUser.rankUp is not a function. If I create a new user object though it does let me call said function.Even if I try casting it as user: var user:User = this.state.currentUser as User then calling the function it gives the same error. I tried printing the type and even when casting it it returns as object not User

The only way I've managed to get it to work so far is user.rankUp = (new User("a")).rankUp; which feels like its incorrect even though it works.

EDIT:

User.rankUp is defined as:

rankUp(){
    this.rank +=1;
}

Should be this.state.currentUser.rankUp, () means its a function mate. — TGarrett
– TGarrett, Commented Aug 17, 2017 at 17:12
as Thing is a hint, not a conversion, it just tells TS you know what the type of the value will be (and, in this case, you were wrong). The solution you have is nearly correct, TypeScript won't convert a generic object (e.g. from JSON) to an instance without you newing one up. But you don't have to patch the one thing, try user = new User(user.whatever); instead. — jonrsharpe
– jonrsharpe, Commented Aug 17, 2017 at 17:13
You need to post more code. My assumption now is that the this.state.currentUser is not what you think it is. Do a console.log(this.state.currentUser) and look at the prototype of the object. Are you sure it's a User? We need to see more of your code to help you. — Cristi Mihai
– Cristi Mihai, Commented Aug 18, 2017 at 0:02

basarat · Accepted Answer · 2017-08-17 23:28:44Z

1

Even if I try casting it as user: var user:User = this.state.currentUser as User then calling the function it gives the same error.

TypeScript doesn't have type casting. It has type assertion : https://basarat.gitbooks.io/typescript/docs/types/type-assertion.html

The reason why it's not called "type casting" is that casting generally implies some sort of runtime support. However type assertions are purely a compile time construct and a way for you to provide hints to the compiler on how you want your code to be analyzed.

So to answer your question:

Typescript not typecasting

Because there is no type casting. 🌹

answered Aug 17, 2017 at 23:28

basarat

278k60 gold badges475 silver badges528 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Typescript not typecasting

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related