Fixes #698, create transform accepts funcs with default args

CHANGELOG.md

@@ -166,6 +166,7 @@ See [#640](https://github.com/mobxjs/mobx/issues/640)
 * Passing a function to `observable` will now create a boxed observable refering to that function
 * Deprecated `whyRun` (as it seems hardly used, please let us know if it should stay)
 * Fixed #603: exceptions in transaction breaks future reactions
+* Fixed #698: createTransformer should support default arguments
 * The following deprecated methods have been removed:
   * `autorunUntil`
   * `trackTransitions`

src/api/createtransformer.ts

@@ -5,7 +5,7 @@ import {globalState} from "../core/globalstate";
 export type ITransformer<A, B> = (object: A) => B;
 
 export function createTransformer<A, B>(transformer: ITransformer<A, B>, onCleanup?: (resultObject: B | undefined, sourceObject?: A) => void): ITransformer<A, B> {
-	invariant(typeof transformer === "function" && transformer.length === 1, "createTransformer expects a function that accepts one argument");
+	invariant(typeof transformer === "function" && transformer.length < 2, "createTransformer expects a function that accepts one argument");
 
 	// Memoizes: object id -> reactive view that applies transformer to the object
 	let objectCache: {[id: number]: ComputedValue<B>} = {};