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76 lines
2.8 KiB
TypeScript
76 lines
2.8 KiB
TypeScript
import {JsonPrimitive, JsonValue} from './basic';
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// Note: The return value has to be `any` and not `unknown` so it can match `void`.
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type NotJsonable = ((...args: any[]) => any) | undefined;
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/**
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Transform a type to one that is assignable to the `JsonValue` type.
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This includes:
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1. Transforming JSON `interface` to a `type` that is assignable to `JsonValue`.
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2. Transforming non-JSON value that is *jsonable* to a type that is assignable to `JsonValue`, where *jsonable* means the non-JSON value implements the `.toJSON()` method that returns a value that is assignable to `JsonValue`.
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@remarks
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An interface cannot be structurally compared to `JsonValue` because an interface can be re-opened to add properties that may not be satisfy `JsonValue`.
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@example
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```
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interface Geometry {
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type: 'Point' | 'Polygon';
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coordinates: [number, number];
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}
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const point: Geometry = {
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type: 'Point',
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coordinates: [1, 1]
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};
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const problemFn = (data: JsonValue) => {
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// Does something with data
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};
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problemFn(point); // Error: type Geometry is not assignable to parameter of type JsonValue because it is an interface
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const fixedFn = <T>(data: Jsonify<T>) => {
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// Does something with data
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};
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fixedFn(point); // Good: point is assignable. Jsonify<T> transforms Geometry into value assignable to JsonValue
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fixedFn(new Date()); // Error: As expected, Date is not assignable. Jsonify<T> cannot transforms Date into value assignable to JsonValue
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```
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Non-JSON values such as `Date` implement `.toJSON()`, so they can be transformed to a value assignable to `JsonValue`:
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@example
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```
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const time = {
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timeValue: new Date()
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};
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// `Jsonify<typeof time>` is equivalent to `{timeValue: string}`
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const timeJson = JSON.parse(JSON.stringify(time)) as Jsonify<typeof time>;
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```
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@link https://github.com/Microsoft/TypeScript/issues/1897#issuecomment-710744173
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@category JSON
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*/
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type Jsonify<T> =
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// Check if there are any non-JSONable types represented in the union.
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// Note: The use of tuples in this first condition side-steps distributive conditional types
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// (see https://github.com/microsoft/TypeScript/issues/29368#issuecomment-453529532)
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[Extract<T, NotJsonable>] extends [never]
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? T extends JsonPrimitive
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? T // Primitive is acceptable
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: T extends Array<infer U>
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? Array<Jsonify<U>> // It's an array: recursive call for its children
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: T extends object
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? T extends {toJSON(): infer J}
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? (() => J) extends (() => JsonValue) // Is J assignable to JsonValue?
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? J // Then T is Jsonable and its Jsonable value is J
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: never // Not Jsonable because its toJSON() method does not return JsonValue
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: {[P in keyof T]: Jsonify<Required<T>[P]>} // It's an object: recursive call for its children
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: never // Otherwise any other non-object is removed
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: never; // Otherwise non-JSONable type union was found not empty
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