Typescript type confusion. Variable doesn't mind type

Question

If i have:

var greeter: Greeter = new Sausage();

And both the Greeter class and Sausage class have the same functions and properties in them, the variable greeter will quite happily be filled up with a Sausage... How can i stop this?

For example, the following code compiles fine :(

class Greeter {
    greeting: string;
    constructor(message: string) {
        this.greeting = message;
    }
    greet() {
        return "Hello, " + this.greeting;
    }
}

class Sausage {
    size: number;
    name: string;
    greeting: string;

    constructor() {
        this.size = this.SausageLogic(); 
    }

    private SausageLogic(): number {
        return this.size * 3;
    }
    greet() {
        return "Hello, ";
    }
}

var greeter: Greeter = new Sausage();

Ryan Cavanaugh · Accepted Answer · 2015-01-22 02:49:36Z

5

TypeScript uses a structural type system. Even though you didn't say Sausage extends Greeter, Sausage is still a subtype of Greeter because it has at least all the same members that Greeter does. Notice that anything you can do with a Greeter (access its greeting property or invoke its greet method) is legal to do with a Sausage.

If you were to add a method foo to Greeter, or remove the greeting property from Sausage, for example, that last line of code would become an error. Any private member is also enough to cause a structural mismatch (unless Sausage was explicitly derived from Greeter).

answered Jan 22, 2015 at 2:49

Ryan Cavanaugh

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5 Comments

xmojmr Over a year ago

@Jimmyt1988 See also typescriptlang.org: Handbook → Type Compatibility

Jimmyt1988 Over a year ago

What is the benefit of this loose type structural nonsense... Or is it a compromise because of JavaScript's failings?

Jimmyt1988 Over a year ago

Is there any built in way to keep this type check from failing compilation... I see many situations where this could cause issues... Other than manually adding a unique id as a property to each class thus showing error. If not... Is there another language I can use that does check this type and show errors? Is the typescript compiler open source so I can change it?

xmojmr Over a year ago

@Jimmyt1988 yes to your last question (see github.com/Microsoft/TypeScript/issues/274)

Jimmyt1988 Over a year ago

@xmojmr - Mega helpful, I hope it comes into fruition

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