ErasedType Class

Packages > fluid-framework > ErasedType

Erased type which can be used to expose a opaque/erased version of a type without referencing the actual type.

Signature

/** @sealed */
export declare abstract class ErasedType<out Name = unknown>

Type Parameters

Remarks

This similar to the [type erasure](https://en.wikipedia.org/wiki/Type _erasure) pattern, but for erasing types at the package boundary.

This can be used to implement the TypeScript typing for the [handle](https://en.wikipedia.org/wiki/Handle _(computing)) pattern, allowing code outside of a package to have a reference/handle to something in the package in a type safe way without the package having to publicly export the types of the object. This should not be confused with the more specific IFluidHandle which is also named after this design pattern.

Recommended usage is to use interface instead of type so tooling (such as tsc and refactoring tools) uses the type name instead of expanding it.

Example

// public
export interface ErasedMyType extends ErasedType<"myPackage.MyType"> {}
// internal
export interface MyType {
	example: number;
}
// Usage
function extract(input: ErasedMyType): MyType {
	return input as unknown as MyType;
}
function erase(input: MyType): ErasedMyType {
	return input as unknown as ErasedMyType;
}

Do not use this class with instanceof: this will always be false at runtime, but the compiler may think it’s true in some cases.

Static Methods

Methods

Method Details

[Symbol.hasInstance]

Since this class is a compile time only type brand, instanceof will never work with it. This Symbol.hasInstance implementation ensures that instanceof will error if used, and in TypeScript 5.3 and newer will produce a compile time error if used.

Signature

static [Symbol.hasInstance](value: never): value is never;

Parameters

Returns

Return type: value is never

brand

Compile time only marker to make type checking more strict. This method will not exist at runtime and accessing it is invalid.

Signature

protected abstract brand(dummy: never): Name;

Parameters

Returns

Return type: Name