Typescript not enforcing interface when assigning result of Object.assign

Question

I have the following interfaces;

interface ComponentInterface {
    pollution: number,
    funds: number
}

interface ConfigInterface {
    pollution?: number,
    funds?: number
}

And then I have a function that creates objects based on the interfaces;

function create(oConfig: ConfigInterface = {}): ComponentInterface {
    // Merge the config with component defaults
    const oComponent: ComponentInterface = Object.assign({
        pollution: 0
    }, oConfig);

    // ...
}

And I'm calling the function without any arguments like;

create();

I don't understand why my compiler isn't throwing an error here. AFAIK I'm assigning { pollution: 0 } to a variable that's expecting a ComponentInterface.

Nitzan Tomer · Accepted Answer · 2017-05-09 22:09:07Z

1

The compiler is working well.
It does complain saying:

Type '{ pollution: number; } & ConfigInterface' is not assignable to type 'ComponentInterface'.
Property 'funds' is optional in type '{ pollution: number; } & ConfigInterface' but required in type 'ComponentInterface'.

Which is what you'd expect considering the signature of the function:

interface ObjectConstructor {
    assign<T, U>(target: T, source: U): T & U;
    ...
}

Where T is { pollution: number; } and U is ConfigInterface.

Also, You can also use Partial instead of redefining all props:

type ConfigInterface = Partial<ComponentInterface>;

edited May 9, 2017 at 22:09

answered May 9, 2017 at 21:49

Nitzan Tomer

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2 Comments

Meirion Hughes Over a year ago

I had the same.

michael Over a year ago

I looks at though it's just my compiler that isn't flagging it then! (atom-typescript 11.0.3)

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