Compiling errors arise due to TypeScript 2.4 Generic Inference

Question

Compiling errors arise due to TypeScript 2.4 Generic Inference

Experiencing issues with existing TypeScript code breaking due to changes in generic inference.

For instance:

interface Task {
    num: number;
}
interface MyTask extends Task {
    description: string;
}
interface Job {}

type Executor<J> = <T extends Task>(job: J, task: T) => T;

const myExecutor: Executor<Job> = (job: Job, task: MyTask) => {
    if (task.num === 123) {
        return;
    }
    else {
        return;
    }
};

The error message during compilation reads as follows:

Error:(11, 7) TS2322:Type '(job: Job, task: MyTask) => {}' is not compatible with the type 'Executor<Job>'.
  The parameters 'task' and 'task' are conflicting.
    Type 'T' cannot be assigned to type 'MyTask'.
      Type 'Task' cannot be assigned to type 'MyTask'.
        The property 'description' is missing from type 'Task'.

typescript typescript2.4

Answer 1

Answer №1

interface AnotherAction {
    kind: 'AnotherActionKind'
    bar: number
}

declare const currentState: State
declare const anotherAction: AnotherAction

// the code below is considered valid based on the myReducer function's signature
myReducer(currentState, anotherAction)

However, if the function assigned to myReducer does not accept all types of actions, an error will occur.

It is unnecessary to make the second parameter generic in this case, as there is no relationship or constraint between the parameters/return value. Simply define it as follows:

type CustomReducer<T> = (state: T, action: Action) => T;

Answer 2

interface AnotherAction {
    kind: 'AnotherActionKind'
    bar: number
}

declare const currentState: State
declare const anotherAction: AnotherAction

// the code below is considered valid based on the myReducer function's signature
myReducer(currentState, anotherAction)

However, if the function assigned to myReducer does not accept all types of actions, an error will occur.

It is unnecessary to make the second parameter generic in this case, as there is no relationship or constraint between the parameters/return value. Simply define it as follows:

type CustomReducer<T> = (state: T, action: Action) => T;

Compiling errors arise due to TypeScript 2.4 Generic Inference

Answer №1

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