fix(redirectTo): fix TS type signature of redirectTo

relates to #3117

src/common/common.ts

@@ -24,7 +24,7 @@ export const noop = () => <any> undefined;
 
 export type Mapper<X, T> = (x: X, key?: (string|number)) => T;
 export interface TypedMap<T> { [key: string]: T; }
-export type Predicate<X> = (x: X) => boolean;
+export type Predicate<X> = (x?: X) => boolean;
 export type IInjectable = (Function|any[]);
 
 export interface Obj extends Object {

src/params/interface.ts

@@ -5,6 +5,14 @@ import {ParamType} from "./type";
  * Parameter values
  *
  * An object containing state parameter key/value pairs
+ *
+ * #### Example:
+ * ```js
+ * {
+ *   userId: 353474,
+ *   folderId: 'inbox'
+ * }
+ * ```
  */
 export interface RawParams {
   [key: string]: any;

src/state/interface.ts

@@ -1,5 +1,5 @@
 /** @module state */ /** for typedoc */
-import {ParamDeclaration} from "../params/interface";
+import { ParamDeclaration, RawParams } from "../params/interface";
 
 import {State} from "./stateObject";
 import {ViewContext} from "../view/interface";
@@ -66,6 +66,15 @@ export interface _ViewDeclaration {
   $context?: ViewContext;
 }
 
+/**
+ * The return value of a [[redirectTo]] function
+ *
+ * - string: a state name
+ * - TargetState: a target state, parameters, and options
+ * - object: an object with a state name and parameters
+ */
+export type RedirectToResult = string | TargetState | { state?: string, params?: RawParams };
+
 /**
  * The StateDeclaration object is used to define a state or nested state.
  *
@@ -431,10 +440,7 @@ export interface StateDeclaration {
    * })
    * ```
    */
-  redirectTo?: ( string |
-          (($transition$: Transition) => TargetState) |
-          { state: (string|StateDeclaration), params: { [key: string]: any }}
-  )
+  redirectTo?: RedirectToResult | Promise<RedirectToResult>
 
   /**
    * A Transition Hook called with the state is being entered.  See: [[IHookRegistry.onEnter]]