Suppose there's an apiCall
function which returns a promise that resolves as an object that has a data
property, which can be either a Record
or an array of Record
s (like Axios). I'm trying to declare the types appropriately, but I'm getting an error.
Here's how I declare my types:
type ApiResponseType = {
data: Record<string, any>[] | Record<string, any>;
};
function apiCall(asArr: boolean): Promise<ApiResponseType> {
const d = {
foo: 'bar',
};
return Promise.resolve({
data: asArr ? [d] : d,
});
}
But when I use it like so:
(async()=>{
let a: {[key: string]: any} | null = null;
let b: {[key: string]: any}[] | null = null;
a = (await apiCall(false)).data;
b = (await apiCall(true)).data; // Type 'Record<string, any> | Record<string, any>[]' is not assignable to type '{[key: string]: any;}[] | null'. Type 'Record<string, any>' is missing the following properties from type '{[key: string]: any;}[]': length, pop, push, concat, and 26 more.
console.log({a, b});
})();
I get the following error:
Type 'Record<string, any> | Record<string, any>[]' is not assignable to type '{[key: string]: any;}[] | null'. Type 'Record<string, any>' is missing the following properties from type '{[key: string]: any;}[]': length, pop, push, concat, and 26 more.
I think it may have something to do with the array being treated as a Record<string, any>
This is because:
declare var a: Record<string, any>[]
declare var b: Record<string, any>
a = b // error
b = a // ok
Array of Records is assignable to Record but not vice - versa.
But let's go back to your example:
type Data = Record<string, any>[] | Record<string, any>;
type Nullable<T> = T | null
type ApiResponseType<T extends Data> = {
data: T
};
function apiCall(asArr: true): Promise<ApiResponseType<Record<string, any>[]>>
function apiCall(asArr: false): Promise<ApiResponseType<Record<string, any>>>
function apiCall(asArr: boolean) {
const d = {
foo: 'bar',
};
return Promise.resolve<ApiResponseType<Data>>({
data: asArr ? [d] : d,
});
}
type O = ReturnType<typeof apiCall>
(async () => {
let a: Nullable<Record<string, any>> = null;
let b: Nullable<Record<string, any>[]> = null;
a = (await apiCall(false)).data;
b = (await apiCall(true)).data;
// without LET bindings
{
const a = (await apiCall(false)).data; // Record<string, any>
const b = (await apiCall(true)).data; //Record<string, any>[]
}
})();
With help of overloadings, we can give some hint to compiler.
Please see this section: // without LET bindings
Compiler is able to figure out ReturnType of function
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