The challenge of generics in Typescript: destructuring and spreading

I am facing an issue with destructing parameters and creating a new object of the same type in Typescript.

The following code functions properly:

function customFunc<T extends { attribute: string }>(parameter: T): T {
  const { ...rest } = parameter;
  return { ...rest };
}

However, this code does not work as expected:

function customFunc<T extends { attribute: string }>(parameter: T): T {
  const { attribute, ...rest } = parameter;
  return { attribute, ...rest };
}

An error message is displayed:

Type '{ attribute: string; } & Pick<T, Exclude<keyof T, "attribute">>' is not assignable to type 'T'

I would appreciate it if someone could explain what is causing the issue here.

Answer №1

Here's a simplified example illustrating the issue.

function func2<T extends { a: string, b: string }>(param: T)
    : { a: string, b: string } {
    const { a, ...rest } = param;
    const x = rest.b; // Error message says 'b' does not exist on type 'Pick<T, Exclude<keyof T, "a">>'.
    return { a, ...rest }; // Error message indicates that property 'b' is missing in type '{ a: string; } & Pick<T, Exclude<keyof T, "a">>' 
                           // but required in type '{ a: string; b: string; }'.
}

This seems to be linked to the recent addition of "Generic spread expressions in object literals" in version 3.2: https://github.com/Microsoft/TypeScript/pull/28234

This appears to be a bug in TypeScript.

Answer №2

After conducting further research based on information from this GitHub thread, it has been found that the syntax is now supported with a small adjustment:

function func<T extends { attr: string }>(param: T): T {
  const { attr, ...rest } = param;
  return { attr, ...rest } as T;
}

The assertion as T is currently required according to the details provided in this comment

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