As the first argument passed in to connect , mapStateToProps is used for selecting the part of the data from the store that the connected component needs. It's frequently referred to as just mapState for short. It is called every time the store state changes.

When mapDispatchToProps is called?

The mapDispatchToProps function will be called with dispatch as the first argument. You will normally make use of this by returning new functions that call dispatch() inside themselves, and either pass in a plain action object directly or pass in the result of an action creator.

What is the difference between mapStateToProps and mapDispatchToProps?

mapStateToProps is a function that you would use to provide the store data to your component, whereas mapDispatchToProps is something that you will use to provide the action creators as props to your component.