type Information = Record<string, string | number | boolean | null>;
Presenting my data structure, however, I am unable to implement it correctly.
I aim to generate an error if no properties are added to an object.
const newObj: Information = {} // should trigger a type error
I experimented with various TypeScript utility types but could not achieve the desired outcome.
In TypeScript, there isn't a specific data type called ValidData that represents "an object of type Data with at least one property". Therefore, you cannot define variables like const obj1: ValidData = {⋯}. Even though you would want to write:
const obj1: ValidData = { okay: 123 }; // okay
const obj2: ValidData = { okay: 456, ok: 789 }; // okay
const obj3: ValidData = {}; // error!
Unfortunately, there is no built-in ValidData type in TypeScript that fits your requirements.
However, you can create a generic type called ValidData<T> that acts as a constraint on a type T, ensuring that T extends ValidData<T> if and only if T has at least one property and is of type Data. Here's an example:
type ValidData<T extends Data> =
T & (keyof T extends never ? { needAtLeastOneProperty: "oops" } : unknown);
This conditional type checks if the type T has any known keys by comparing it to the impossible never type. Depending on whether T has keys or not, it creates the appropriate ValidData<T>.
type Okay = ValidData<{ okay: number }>;
// type Okay = { okay: number; }
type AlsoOkay = ValidData<{ okay: number, ok: number }>;
// type AlsoOkay = { okay: number, ok: number };
type NotOkay = ValidData<{}>;
// type NotOkay = { needAtLeastOneProperty: "oops"; }
To avoid redundancy, you can use a generic helper function that infers the type T for you:
const asValidData = <T extends Data>(t: ValidData<T>) => t;
Then, you can utilize this function to validate objects as follows:
const obj1 = asValidData({ okay: 123 }); // okay
const obj2 = asValidData({ okay: 456, ok: 789 }); // okay
const obj3 = asValidData({}); // error!
// ----------------------------------> ~~
// Argument of type '{}' is not assignable to
// parameter of type '{ needAtLeastOneProperty: "oops"; }'.
This approach ensures that invalid empty objects are rejected with clear error messages for developers to understand.
While we're using const obj1 = asValidData({⋯}) instead of the desired const obj1: ValidData = {⋯}, the difference is minimal when considering functionality.
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