I'm trying to conditionally set the return type of a function, based on a static value in the same type.
Given the following code, is it possible to have type checking work as I've specified in the comments?
IE, if const taskX: Task has the id: 'get-name', how can I make it so taskX.run() should return according to the type defined as Outputs['get-name']
type Outputs = {
"get-name": {
name: string;
};
"get-age": {
age: number;
};
"get-favourite-fruits": ("apple" | "strawberry" | "melon")[];
};
type Task = {
// This should always be a static property defined in `Outputs`
id: keyof Outputs;
// How can I make this conditional and return the type from `Outputs` based on `id`
run: () => never;
};
const taskOne: Task = {
id: "get-name",
// This should be valid
run: () => {
return { name: "frankenstein" };
},
};
const taskTwo: Task = {
id: "get-age",
// This should also be valid
run: () => {
return { age: 22 };
},
};
const taskThree: Task = {
id: "get-favourite-fruits",
// This should fail with
// Type '("apple" | "banana")[]' is not assignable to type
// '("apple" | "strawberry" | "melon")[]'
run: () => {
return ["apple", "banana"];
},
};
You want Task to be a discriminated union of the three different possible pairings of id and run. You can generate this programmatically from Outputs by mapping each property of Outputs to a value corresponding to this pairing, and then looking up all of the mapped properties to get the desired union:
type Task = {
[K in keyof Outputs]: { id: K, run: () => Outputs[K] }
}[keyof Outputs];
/* produces
type Task = {
id: "get-name";
run: () => {
name: string;
};
} | {
id: "get-age";
run: () => {
age: number;
};
} | {
id: "get-favourite-fruits";
run: () => ("apple" | "strawberry" | "melon")[];
}*/
Then the rest of your code works as desired. Valid tasks are accepted:
const taskOne: Task = {
id: "get-name",
run: () => {
return { name: "frankenstein" };
},
}; // okay
const taskTwo: Task = {
id: "get-age",
run: () => {
return { age: 22 };
},
}; // okay
And invalid tasks are rejected:
const taskThree: Task = {
id: "get-favourite-fruits",
run: () => { // error!
// Type '() => ("apple" | "banana")[]' is not assignable to type
// '(() => { name: string; }) | (() => { age: number; }) |
// (() => ("apple" | "strawberry" | "melon")[])
return ["apple", "banana"];
},
};
const taskFour: Task = { // error!
// ~~~~~~~~
// Type '{ id: "get-favourite-fruits"; run: () => { age: number; }; }'
// is not assignable to type 'Task'.
id: "get-favourite-fruits",
run: () => {
return { age: 123 };
},
};
The errors for invalid tasks might be a bit different from the particular one you were expecting. For taskThree the compiler complains that run()'s return is incompatible with every possible Task member; "not only isn't it the right array, but it isn't even one of the right types for get-name or get-age!" For taskFour the compiler sees that run() returns an {age: number} but the id is get-favorite-fruits and complains that the whole value is wrong without highlighting any particular property.
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