Typescript error: Cannot assign type to argument

Question

Typescript error: Cannot assign type to argument

Exploring the world of typescript (2.5.2) and looking for clarity on why the first call works but the second one encounters an error:

function printPerson(person: {firstName: string; lastName: string}): void{
    console.log(person.firstName + " " + person.lastName);
}

let geo = {firstName: "geo", lastName: "porz", sex: "M"};
printPerson(geo); //This works fine

// TS2345 Argument of type ... is not assignable to parameter of type ... 
printPerson({firstName: "geo", lastName: "porz", sex: "M"});

typescript

Answer 1

Answer №1

Although not a direct response to your inquiry, it seems like your question stems from using a Javascript coding style in Typescript. In the transition from Javascript, you may be accustomed to utilizing patterns such as {foo:"bar"} for passing data. However, in Typescript, specifying the types of variables using type or interface can greatly enhance code clarity.

interface Person { 
  firstName: string
  lastName: string
}

function printPerson(person: Person): void{
    console.log(person.firstName + " " + person.lastName);
}

So when creating a new person object, instead of relying on type inference, explicitly inform the compiler that geo is a Person:

let geo:Person = {firstName: "geo", lastName: "porz", sex: "M"};

As a result, you will encounter an error indicating that you cannot add the sex property to Person because it has not been declared beforehand.

Answer 2

Although not a direct response to your inquiry, it seems like your question stems from using a Javascript coding style in Typescript. In the transition from Javascript, you may be accustomed to utilizing patterns such as {foo:"bar"} for passing data. However, in Typescript, specifying the types of variables using type or interface can greatly enhance code clarity.

interface Person { 
  firstName: string
  lastName: string
}

function printPerson(person: Person): void{
    console.log(person.firstName + " " + person.lastName);
}

So when creating a new person object, instead of relying on type inference, explicitly inform the compiler that geo is a Person:

let geo:Person = {firstName: "geo", lastName: "porz", sex: "M"};

As a result, you will encounter an error indicating that you cannot add the sex property to Person because it has not been declared beforehand.

Typescript error: Cannot assign type to argument

Answer №1

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