Add typescript definitions for mergeProps module (#1756)

Author: lindskogen

src/connect/mergeProps.js

@@ -0,0 +1,53 @@
+import { Dispatch } from 'redux'
+import verifyPlainObject from '../utils/verifyPlainObject'
+
+type MergeProps<TStateProps, TDispatchProps, TOwnProps, TMergedProps> = (stateProps: TStateProps, dispatchProps: TDispatchProps, ownProps: TOwnProps) => TMergedProps
+
+export function defaultMergeProps<TStateProps, TDispatchProps, TOwnProps>(stateProps: TStateProps, dispatchProps: TDispatchProps, ownProps: TOwnProps) {
+  return { ...ownProps, ...stateProps, ...dispatchProps }
+}
+
+interface InitMergeOptions { 
+  displayName: string;
+  pure?: boolean;
+  areMergedPropsEqual: (a: any, b: any) => boolean;
+}
+
+export function wrapMergePropsFunc<TStateProps, TDispatchProps, TOwnProps, TMergedProps>(mergeProps: MergeProps<TStateProps, TDispatchProps, TOwnProps, TMergedProps>): (dispatch: Dispatch, options: InitMergeOptions) => MergeProps<TStateProps, TDispatchProps, TOwnProps, TMergedProps> {
+  return function initMergePropsProxy(
+    dispatch,
+    { displayName, pure, areMergedPropsEqual }
+  ) {
+    let hasRunOnce = false
+    let mergedProps: TMergedProps
+
+    return function mergePropsProxy(stateProps: TStateProps, dispatchProps: TDispatchProps, ownProps: TOwnProps) {
+      const nextMergedProps = mergeProps(stateProps, dispatchProps, ownProps)
+
+      if (hasRunOnce) {
+        if (!pure || !areMergedPropsEqual(nextMergedProps, mergedProps))
+          mergedProps = nextMergedProps
+      } else {
+        hasRunOnce = true
+        mergedProps = nextMergedProps
+
+        if (process.env.NODE_ENV !== 'production')
+          verifyPlainObject(mergedProps, displayName, 'mergeProps')
+      }
+
+      return mergedProps
+    }
+  }
+}
+
+export function whenMergePropsIsFunction<TStateProps, TDispatchProps, TOwnProps, TMergedProps>(mergeProps: MergeProps<TStateProps, TDispatchProps, TOwnProps, TMergedProps>) {
+  return typeof mergeProps === 'function'
+    ? wrapMergePropsFunc(mergeProps)
+    : undefined
+}
+
+export function whenMergePropsIsOmitted<TStateProps, TDispatchProps, TOwnProps, TMergedProps>(mergeProps?: MergeProps<TStateProps, TDispatchProps, TOwnProps, TMergedProps>) {
+  return !mergeProps ? () => defaultMergeProps : undefined
+}
+
+export default [whenMergePropsIsFunction, whenMergePropsIsOmitted] as const