The type '...' cannot be assigned to the type '...' for argument ts(2345)

type FruitName = 'apple' | 'banana';

interface Fruit {
    name: FruitName;
}

const apple = {
    'name': 'apple',
};

function test(fruit: Fruit) {
    console.log(fruit.name);
}

function main() {
    const name: FruitName = 'apple'; // this is permissible

    test(apple); // Error TS2345: Argument of type '{ name: string; }' is not suitable for the parameter of type 'Fruit'. Type 'FruitName'
                 // Types of property 'name' are inconsistent.
                 // Type 'string' cannot be assigned to type 'FruitName'.
}

main();

I am puzzled by why apple cannot be assigned to Fruit.

However, 'apple' can be assigned to name: FruitName.

What is the distinction between the two?

Answer №1

let fruit = { 'name': 'apple' as const }
is a valid way to define a constant object.

"apple" on its own is seen as type string, while "apple" as const is recognized as the "apple" string literal type. It's important to note that a string literal is a subtype of string, not the other way around.

let fruitString: string = "apple"
let fruitLiteral: "apple" = "apple" as const

fruitString = fruitLiteral // This works fine
fruitLiteral = fruitString // This will result in an error

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