Typescript fails to enforce the interface when assigning the output of Object.assign

One has the following two interfaces;

interface ComponentInterface {
    pollution: number,
    funds: number
}

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

Additionally, there is a function that generates objects based on these interfaces;

function generateObject(oConfig: ConfigInterface = {}): ComponentInterface {
    // Merging config with component defaults
    const objectComponent: ComponentInterface = Object.assign({
        pollution: 0
    }, oConfig);

    // ...
}

The function is called without any arguments like this;

generateObject();

Confusingly, no error is thrown by the compiler here. It seems that { pollution: 0 } is being assigned to a variable expecting a ComponentInterface.

typescript

Answer №1

The compiler is functioning correctly.
It does mention the following:

Type '{ pollution: number; } & ConfigInterface' is not compatible with type 'ComponentInterface'.
Property 'funds' might be optional in type '{ pollution: number; } & ConfigInterface' but it is required in type 'ComponentInterface'.

This aligns with the definition of the function:

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

Here, T represents { pollution: number; } and U represents ConfigInterface.

Additionally, you have the option to utilize Partial rather than redefining all properties:

type ConfigInterface = Partial<ComponentInterface>;

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