typescript - How to define a return type that takes a Record, and returns a Record with the same properties, but with transforme

I have a function in which I transform an object's values, somewhat like this:

type InputValue = number | boolean;
type TransformValue<IV extends InputValue> = IV extends number ? "num" : "bool";

const transformObject = <
  Input extends Record<string, InputValue>,
  Key extends keyof Input,
>(
  input: Input,
): Record<K, TransformValue<Input[Key]>> {
  /* ... */
}

except that I would like to be able to encode in the type which property has what type of value.

So for example, if I have an object:

const inputObject: {
  keyA: number;
  keyB: boolean;
} = /* ... */;

I'd like the return type to be:

{
  keyA: "num";
  keyB: "bool";
}

instead of the current Record<"keyA" | "keyB", "num" | "bool">. Is that possible?

I have a function in which I transform an object's values, somewhat like this:

type InputValue = number | boolean;
type TransformValue<IV extends InputValue> = IV extends number ? "num" : "bool";

const transformObject = <
  Input extends Record<string, InputValue>,
  Key extends keyof Input,
>(
  input: Input,
): Record<K, TransformValue<Input[Key]>> {
  /* ... */
}

except that I would like to be able to encode in the type which property has what type of value.

So for example, if I have an object:

const inputObject: {
  keyA: number;
  keyB: boolean;
} = /* ... */;

I'd like the return type to be:

{
  keyA: "num";
  keyB: "bool";
}

instead of the current Record<"keyA" | "keyB", "num" | "bool">. Is that possible?

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Share Improve this question Follow edited Feb 1 at 15:02 jonrsharpe 122k3030 gold badges268268 silver badges476476 bronze badges asked Feb 1 at 14:45 VincentVincent 5,44577 gold badges4646 silver badges6363 bronze badges

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1 Answer 1

Sorted by: Reset to default 2

So, the transformObject function returns "num" or "bool" as the value for each key in the given object. You can achieve this as follows:

{ [Key in keyof Input]: TransformValue<Input[Key]> }

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type InputValue = number | boolean;
type TransformValue<IV extends InputValue> = IV extends number ? "num" : "bool";

const transformObject = <Input extends Record<string, InputValue>>(
  input: Input
): { [Key in keyof Input]: TransformValue<Input[Key]> } => {
  const result: any = {};
  for (const key in input) {
    result[key] = typeof input[key] === "number" ? "num" : "bool";
  }
  return result;
};

// Test case
const inputObject = {
  keyA: 42,
  keyB: true,
};
// { keyA: number; keyB: boolean; } --> { keyA: "num"; keyB: "bool"; }
const transformed = transformObject(inputObject);